diff mbox series

[1/1] ubifs: support authentication, for ro mount, when no key is given

Message ID 20200625155927.28430-2-torben.hohn@linutronix.de
State Changes Requested
Headers show
Series ubifs: support authentication without hmac | expand

Commit Message

Torben Hohn June 25, 2020, 3:59 p.m. UTC 
Ubifs authentication requires a hmac key, even when a
filesystem is mounted read-only.

Split c->authenticated into _rw and _ro.

Also implement ubifs_init_authentication_read_only(),
which only allocates the structures needed for validating
the hashes. That is called, when no auth_key_name is specified,
only auth_hash_name. It sets c->authenticated_ro to true.

Then implement ubifs_authenticated_read() and
ubifs_authenticated_write(). And change all occurences of
ubifs_authenticated() to the respective read or write version.

ubifs_authenticated_write() verifies, that it is not called
when only c->authenticated_ro is active. (The WARN_ON should
probably be BUG_ON(), though)

Signed-off-by: Torben Hohn <torben.hohn@linutronix.de>
---
 fs/ubifs/auth.c    | 69 ++++++++++++++++++++++++++++++++++++++++++----
 fs/ubifs/gc.c      |  2 +-
 fs/ubifs/journal.c | 12 ++++----
 fs/ubifs/lpt.c     |  4 +--
 fs/ubifs/master.c  |  2 +-
 fs/ubifs/replay.c  |  2 +-
 fs/ubifs/sb.c      | 16 +++++++----
 fs/ubifs/super.c   | 21 ++++++++++----
 fs/ubifs/ubifs.h   | 48 +++++++++++++++++++++-----------
 9 files changed, 133 insertions(+), 43 deletions(-)

Comments

Sascha Hauer June 26, 2020, 4:31 a.m. UTC | #1 
Hi Torben,

On Thu, Jun 25, 2020 at 05:59:27PM +0200, Torben Hohn wrote:
> diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
> index 7fc2f3f07c16..ec95f1f50e5e 100644
> --- a/fs/ubifs/super.c
> +++ b/fs/ubifs/super.c
> @@ -1291,6 +1291,17 @@ static int mount_ubifs(struct ubifs_info *c)
>  			err = -EINVAL;
>  			goto out_free;
>  		}
> +	} else if (c->auth_hash_name) {
> +		if (IS_ENABLED(CONFIG_UBIFS_FS_AUTHENTICATION)) {
> +			err = ubifs_init_authentication_read_only(c);
> +			if (err)
> +				goto out_free;
> +		} else {
> +			ubifs_err(c, "auth_hash_name, but UBIFS is built without"
> +				  " authentication support");
> +			err = -EINVAL;
> +			goto out_free;
> +		}
>  	}

In case we don't have a key available for HMAC and can only verify the
FS is correctly signed then we have to be sure that we are mounting
readonly. This means the above needs an additional check for
c->ro_mount.

Once we can be sure that UBIFS is in readonly mode when we can't do HMAC
then there's no point in adding a ubifs_authenticated_write(), because
the places where you call it will never be hit in a readonly mounted
filesystem.

Regards,
 Sascha
Torben Hohn June 26, 2020, 7:27 a.m. UTC | #2 
On Fri, Jun 26, 2020 at 06:31:20AM +0200, Sascha Hauer wrote:
> Hi Torben,
> 
> On Thu, Jun 25, 2020 at 05:59:27PM +0200, Torben Hohn wrote:
> > diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
> > index 7fc2f3f07c16..ec95f1f50e5e 100644
> > --- a/fs/ubifs/super.c
> > +++ b/fs/ubifs/super.c
> > @@ -1291,6 +1291,17 @@ static int mount_ubifs(struct ubifs_info *c)
> >  			err = -EINVAL;
> >  			goto out_free;
> >  		}
> > +	} else if (c->auth_hash_name) {
> > +		if (IS_ENABLED(CONFIG_UBIFS_FS_AUTHENTICATION)) {
> > +			err = ubifs_init_authentication_read_only(c);
> > +			if (err)
> > +				goto out_free;
> > +		} else {
> > +			ubifs_err(c, "auth_hash_name, but UBIFS is built without"
> > +				  " authentication support");
> > +			err = -EINVAL;
> > +			goto out_free;
> > +		}
> >  	}
> 
> In case we don't have a key available for HMAC and can only verify the
> FS is correctly signed then we have to be sure that we are mounting
> readonly. This means the above needs an additional check for
> c->ro_mount.

Indeed, i had that check in authenticate_sb_node() in an earlier
version, and forgot to add it here.

Will do.

> 
> Once we can be sure that UBIFS is in readonly mode when we can't do HMAC
> then there's no point in adding a ubifs_authenticated_write(), because
> the places where you call it will never be hit in a readonly mounted
> filesystem.

The point is making sure, that it really is never hit in a readonly
filesystem. Now, and in the future. If we miss one point, we might
trigger the hmac code with an empty hmac. Although it might just crash.


Maybe we could generate a random key, to lessen the impact of such
an error. But i doubt that i have enough entropy to make that more than
a fig leaf.


Another topic:

Richard raised the point on irc, that in a dirty filesystem, the journal
would be replayed even in ro mode.

i dont see that happening.

--------------------------------------------------------------------------
static int mount_ubifs(struct ubifs_info *c)
{
	[...]

	if (!c->ro_mount) {

	   [...]

	} else if (c->need_recovery) {
		err = ubifs_recover_size(c, false);
		if (err)
			goto out_orphans;
	} else {
		/*
		 * Even if we mount read-only, we have to set space in GC LEB
		 * to proper value because this affects UBIFS free space
		 * reporting. We do not want to have a situation when
		 * re-mounting from R/O to R/W changes amount of free space.
		 */
		err = take_gc_lnum(c);
		if (err)
			goto out_orphans;
	}

	[...]

}
--------------------------------------------------------------------------

ubifs_recover_size() with in_place = false, does not seem to write.



> 
> Regards,
>  Sascha
> 
> -- 
> Pengutronix e.K.                           |                             |
> Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
> 31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
> Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |
Richard Weinberger June 26, 2020, 7:53 a.m. UTC | #3 
----- Ursprüngliche Mail -----
> Another topic:
> 
> Richard raised the point on irc, that in a dirty filesystem, the journal
> would be replayed even in ro mode.
> 
> i dont see that happening.

I think there is a confusion. Replay is not the same as recovery.
Of course it does not recover, this needs writing.
But it has to replay the journal, otherwise the index tree can point to already
garbage collected LEBs.

Thanks,
//richard
Sascha Hauer June 26, 2020, 8:10 a.m. UTC | #4 
On Fri, Jun 26, 2020 at 09:27:14AM +0200, Torben Hohn wrote:
> On Fri, Jun 26, 2020 at 06:31:20AM +0200, Sascha Hauer wrote:
> > Hi Torben,
> > 
> > On Thu, Jun 25, 2020 at 05:59:27PM +0200, Torben Hohn wrote:
> > > diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
> > > index 7fc2f3f07c16..ec95f1f50e5e 100644
> > > --- a/fs/ubifs/super.c
> > > +++ b/fs/ubifs/super.c
> > > @@ -1291,6 +1291,17 @@ static int mount_ubifs(struct ubifs_info *c)
> > >  			err = -EINVAL;
> > >  			goto out_free;
> > >  		}
> > > +	} else if (c->auth_hash_name) {
> > > +		if (IS_ENABLED(CONFIG_UBIFS_FS_AUTHENTICATION)) {
> > > +			err = ubifs_init_authentication_read_only(c);
> > > +			if (err)
> > > +				goto out_free;
> > > +		} else {
> > > +			ubifs_err(c, "auth_hash_name, but UBIFS is built without"
> > > +				  " authentication support");
> > > +			err = -EINVAL;
> > > +			goto out_free;
> > > +		}
> > >  	}
> > 
> > In case we don't have a key available for HMAC and can only verify the
> > FS is correctly signed then we have to be sure that we are mounting
> > readonly. This means the above needs an additional check for
> > c->ro_mount.
> 
> Indeed, i had that check in authenticate_sb_node() in an earlier
> version, and forgot to add it here.
> 
> Will do.
> 
> > 
> > Once we can be sure that UBIFS is in readonly mode when we can't do HMAC
> > then there's no point in adding a ubifs_authenticated_write(), because
> > the places where you call it will never be hit in a readonly mounted
> > filesystem.
> 
> The point is making sure, that it really is never hit in a readonly
> filesystem. Now, and in the future. If we miss one point, we might
> trigger the hmac code with an empty hmac. Although it might just crash.

If that's your point then you can add a ubifs_assert(c, c->ro_mount) at
those places. This has the advantage that it triggers not only in
authenticated mode, but also in unauthenticated mode. Please add this
assertion explicitly and not indirectly in ubifs_authenticated_write().
This function has a strange semantics, the name suggests that it returns
the status of authenticated write. It's quite unexpected to me that it
triggers a warning when called with only readonly authentication
available.

Regards,
 Sascha
Torben Hohn June 26, 2020, 9:39 a.m. UTC | #5 
On Fri, Jun 26, 2020 at 10:10:28AM +0200, Sascha Hauer wrote:
> On Fri, Jun 26, 2020 at 09:27:14AM +0200, Torben Hohn wrote:
> > On Fri, Jun 26, 2020 at 06:31:20AM +0200, Sascha Hauer wrote:
> > > Hi Torben,
> > > 
> > > On Thu, Jun 25, 2020 at 05:59:27PM +0200, Torben Hohn wrote:
> > > > diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
> > > > index 7fc2f3f07c16..ec95f1f50e5e 100644
> > > > --- a/fs/ubifs/super.c
> > > > +++ b/fs/ubifs/super.c
> > > > @@ -1291,6 +1291,17 @@ static int mount_ubifs(struct ubifs_info *c)
> > > >  			err = -EINVAL;
> > > >  			goto out_free;
> > > >  		}
> > > > +	} else if (c->auth_hash_name) {
> > > > +		if (IS_ENABLED(CONFIG_UBIFS_FS_AUTHENTICATION)) {
> > > > +			err = ubifs_init_authentication_read_only(c);
> > > > +			if (err)
> > > > +				goto out_free;
> > > > +		} else {
> > > > +			ubifs_err(c, "auth_hash_name, but UBIFS is built without"
> > > > +				  " authentication support");
> > > > +			err = -EINVAL;
> > > > +			goto out_free;
> > > > +		}
> > > >  	}
> > > 
> > > In case we don't have a key available for HMAC and can only verify the
> > > FS is correctly signed then we have to be sure that we are mounting
> > > readonly. This means the above needs an additional check for
> > > c->ro_mount.
> > 
> > Indeed, i had that check in authenticate_sb_node() in an earlier
> > version, and forgot to add it here.
> > 
> > Will do.
> > 
> > > 
> > > Once we can be sure that UBIFS is in readonly mode when we can't do HMAC
> > > then there's no point in adding a ubifs_authenticated_write(), because
> > > the places where you call it will never be hit in a readonly mounted
> > > filesystem.
> > 
> > The point is making sure, that it really is never hit in a readonly
> > filesystem. Now, and in the future. If we miss one point, we might
> > trigger the hmac code with an empty hmac. Although it might just crash.
> 
> If that's your point then you can add a ubifs_assert(c, c->ro_mount) at
> those places. This has the advantage that it triggers not only in
> authenticated mode, but also in unauthenticated mode. Please add this
> assertion explicitly and not indirectly in ubifs_authenticated_write().
> This function has a strange semantics, the name suggests that it returns
> the status of authenticated write. It's quite unexpected to me that it
> triggers a warning when called with only readonly authentication
> available.

Ok. will do.


> 
> Regards,
>  Sascha
> 
> -- 
> Pengutronix e.K.                           |                             |
> Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
> 31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
> Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |
diff mbox series

Patch

diff --git a/fs/ubifs/auth.c b/fs/ubifs/auth.c
index cc5c0abfd536..5429a05f537e 100644
--- a/fs/ubifs/auth.c
+++ b/fs/ubifs/auth.c
@@ -94,7 +94,7 @@  static struct shash_desc *ubifs_get_desc(const struct ubifs_info *c,
 	struct shash_desc *desc;
 	int err;
 
-	if (!ubifs_authenticated(c))
+	if (!ubifs_authenticated_read(c))
 		return NULL;
 
 	desc = kmalloc(sizeof(*desc) + crypto_shash_descsize(tfm), GFP_KERNEL);
@@ -248,6 +248,61 @@  int ubifs_sb_verify_signature(struct ubifs_info *c,
 	return err;
 }
 
+/**
+ * ubifs_init_authentication_read_only - init only the read_only parts
+ *
+ * @c: UBIFS file-system description object
+ *
+ * This function returns 0 for success or a negative error code otherwise.
+ */
+
+int ubifs_init_authentication_read_only(struct ubifs_info *c)
+{
+	int err;
+
+	if (!c->auth_hash_name) {
+		ubifs_err(c, "authentication hash name needed with authentication");
+		return -EINVAL;
+	}
+
+	c->auth_hash_algo = match_string(hash_algo_name, HASH_ALGO__LAST,
+					 c->auth_hash_name);
+	if ((int)c->auth_hash_algo < 0) {
+		ubifs_err(c, "Unknown hash algo %s specified",
+			  c->auth_hash_name);
+		return -EINVAL;
+	}
+
+	c->hash_tfm = crypto_alloc_shash(c->auth_hash_name, 0, 0);
+	if (IS_ERR(c->hash_tfm)) {
+		err = PTR_ERR(c->hash_tfm);
+		ubifs_err(c, "Can not allocate %s: %d",
+			  c->auth_hash_name, err);
+		goto out;
+	}
+
+	c->hash_len = crypto_shash_digestsize(c->hash_tfm);
+	if (c->hash_len > UBIFS_HASH_ARR_SZ) {
+		ubifs_err(c, "hash %s is bigger than maximum allowed hash size (%d > %d)",
+			  c->auth_hash_name, c->hash_len, UBIFS_HASH_ARR_SZ);
+		err = -EINVAL;
+		goto out_free_hash;
+	}
+
+	c->authenticated_ro = true;
+
+	c->log_hash = ubifs_hash_get_desc(c);
+	if (IS_ERR(c->log_hash))
+		goto out_free_hash;
+
+	err = 0;
+out_free_hash:
+	if (err)
+		crypto_free_shash(c->hash_tfm);
+out:
+	return err;
+}
+
 /**
  * ubifs_init_authentication - initialize UBIFS authentication support
  * @c: UBIFS file-system description object
@@ -335,7 +390,7 @@  int ubifs_init_authentication(struct ubifs_info *c)
 	if (err)
 		goto out_free_hmac;
 
-	c->authenticated = true;
+	c->authenticated_rw = true;
 
 	c->log_hash = ubifs_hash_get_desc(c);
 	if (IS_ERR(c->log_hash))
@@ -364,11 +419,15 @@  int ubifs_init_authentication(struct ubifs_info *c)
  */
 void __ubifs_exit_authentication(struct ubifs_info *c)
 {
-	if (!ubifs_authenticated(c))
+	if (!ubifs_authenticated_read(c))
 		return;
 
-	crypto_free_shash(c->hmac_tfm);
 	crypto_free_shash(c->hash_tfm);
+
+	if (!c->authenticated_rw)
+		return;
+
+	crypto_free_shash(c->hmac_tfm);
 	kfree(c->log_hash);
 }
 
@@ -511,7 +570,7 @@  int ubifs_hmac_wkm(struct ubifs_info *c, u8 *hmac)
 	int err;
 	const char well_known_message[] = "UBIFS";
 
-	if (!ubifs_authenticated(c))
+	if (!ubifs_authenticated_write(c))
 		return 0;
 
 	shash->tfm = c->hmac_tfm;
diff --git a/fs/ubifs/gc.c b/fs/ubifs/gc.c
index 62cb3db44e6e..8616e8ba97d6 100644
--- a/fs/ubifs/gc.c
+++ b/fs/ubifs/gc.c
@@ -410,7 +410,7 @@  static int move_nodes(struct ubifs_info *c, struct ubifs_scan_leb *sleb)
 			moved = 1;
 		}
 
-		if (ubifs_authenticated(c) && moved) {
+		if (ubifs_authenticated_write(c) && moved) {
 			struct ubifs_auth_node *auth;
 
 			auth = kmalloc(ubifs_auth_node_sz(c), GFP_NOFS);
diff --git a/fs/ubifs/journal.c b/fs/ubifs/journal.c
index e5ec1afe1c66..da13e5eb93e1 100644
--- a/fs/ubifs/journal.c
+++ b/fs/ubifs/journal.c
@@ -80,7 +80,7 @@  static inline void zero_trun_node_unused(struct ubifs_trun_node *trun)
 
 static void ubifs_add_auth_dirt(struct ubifs_info *c, int lnum)
 {
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_write(c))
 		ubifs_add_dirt(c, lnum, ubifs_auth_node_sz(c));
 }
 
@@ -278,7 +278,7 @@  static int write_head(struct ubifs_info *c, int jhead, void *buf, int len,
 	dbg_jnl("jhead %s, LEB %d:%d, len %d",
 		dbg_jhead(jhead), *lnum, *offs, len);
 
-	if (ubifs_authenticated(c)) {
+	if (ubifs_authenticated_write(c)) {
 		err = ubifs_hash_nodes(c, buf, len, c->jheads[jhead].log_hash);
 		if (err)
 			return err;
@@ -572,7 +572,7 @@  int ubifs_jnl_update(struct ubifs_info *c, const struct inode *dir,
 
 	len = aligned_dlen + aligned_ilen + UBIFS_INO_NODE_SZ;
 	/* Make sure to also account for extended attributes */
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_write(c))
 		len += ALIGN(host_ui->data_len, 8) + ubifs_auth_node_sz(c);
 	else
 		len += host_ui->data_len;
@@ -778,7 +778,7 @@  int ubifs_jnl_write_data(struct ubifs_info *c, const struct inode *inode,
 	}
 
 	dlen = UBIFS_DATA_NODE_SZ + out_len;
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_write(c))
 		write_len = ALIGN(dlen, 8) + auth_len;
 	else
 		write_len = dlen;
@@ -860,7 +860,7 @@  int ubifs_jnl_write_inode(struct ubifs_info *c, const struct inode *inode)
 		write_len += UBIFS_INO_NODE_SZ * ui->xattr_cnt;
 	}
 
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_write(c))
 		write_len += ALIGN(ilen, 8) + ubifs_auth_node_sz(c);
 	else
 		write_len += ilen;
@@ -1572,7 +1572,7 @@  int ubifs_jnl_truncate(struct ubifs_info *c, const struct inode *inode,
 	/* Must make reservation before allocating sequence numbers */
 	len = UBIFS_TRUN_NODE_SZ + UBIFS_INO_NODE_SZ;
 
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_write(c))
 		len += ALIGN(dlen, 8) + ubifs_auth_node_sz(c);
 	else
 		len += dlen;
diff --git a/fs/ubifs/lpt.c b/fs/ubifs/lpt.c
index e21abf250951..f3a7136518d1 100644
--- a/fs/ubifs/lpt.c
+++ b/fs/ubifs/lpt.c
@@ -1660,7 +1660,7 @@  int ubifs_lpt_calc_hash(struct ubifs_info *c, u8 *hash)
 	void *buf;
 	int err;
 
-	if (!ubifs_authenticated(c))
+	if (!ubifs_authenticated_read(c))
 		return 0;
 
 	if (!c->nroot) {
@@ -1750,7 +1750,7 @@  static int lpt_check_hash(struct ubifs_info *c)
 	int err;
 	u8 hash[UBIFS_HASH_ARR_SZ];
 
-	if (!ubifs_authenticated(c))
+	if (!ubifs_authenticated_read(c))
 		return 0;
 
 	err = ubifs_lpt_calc_hash(c, hash);
diff --git a/fs/ubifs/master.c b/fs/ubifs/master.c
index 911d0555b9f2..57304e3f7016 100644
--- a/fs/ubifs/master.c
+++ b/fs/ubifs/master.c
@@ -129,7 +129,7 @@  static int scan_for_master(struct ubifs_info *c)
 	c->mst_offs = offs;
 	ubifs_scan_destroy(sleb);
 
-	if (!ubifs_authenticated(c))
+	if (!ubifs_authenticated_read(c))
 		return 0;
 
 	if (ubifs_hmac_zero(c, c->mst_node->hmac)) {
diff --git a/fs/ubifs/replay.c b/fs/ubifs/replay.c
index b69ffac7e415..6c1b8739359c 100644
--- a/fs/ubifs/replay.c
+++ b/fs/ubifs/replay.c
@@ -595,7 +595,7 @@  static int authenticate_sleb(struct ubifs_info *c, struct ubifs_scan_leb *sleb,
 	u8 hash[UBIFS_HASH_ARR_SZ];
 	u8 hmac[UBIFS_HMAC_ARR_SZ];
 
-	if (!ubifs_authenticated(c))
+	if (!ubifs_authenticated_write(c))
 		return sleb->nodes_cnt;
 
 	list_for_each_entry(snod, &sleb->nodes, list) {
diff --git a/fs/ubifs/sb.c b/fs/ubifs/sb.c
index 4b4b65b48c57..52396a92f8af 100644
--- a/fs/ubifs/sb.c
+++ b/fs/ubifs/sb.c
@@ -176,7 +176,7 @@  static int create_default_filesystem(struct ubifs_info *c)
 		sup_flags |= UBIFS_FLG_BIGLPT;
 	sup_flags |= UBIFS_FLG_DOUBLE_HASH;
 
-	if (ubifs_authenticated(c)) {
+	if (ubifs_authenticated_write(c)) {
 		sup_flags |= UBIFS_FLG_AUTHENTICATION;
 		sup->hash_algo = cpu_to_le16(c->auth_hash_algo);
 		err = ubifs_hmac_wkm(c, sup->hmac_wkm);
@@ -542,20 +542,20 @@  static int authenticate_sb_node(struct ubifs_info *c,
 	int hash_algo;
 	int err;
 
-	if (c->authenticated && !authenticated) {
+	if (ubifs_authenticated_read(c) && !authenticated) {
 		ubifs_err(c, "authenticated FS forced, but found FS without authentication");
 		return -EINVAL;
 	}
 
-	if (!c->authenticated && authenticated) {
-		ubifs_err(c, "authenticated FS found, but no key given");
+	if (!ubifs_authenticated_read(c) && authenticated) {
+		ubifs_err(c, "authenticated FS found, but no key nor hash name given");
 		return -EINVAL;
 	}
 
 	ubifs_msg(c, "Mounting in %sauthenticated mode",
-		  c->authenticated ? "" : "un");
+		  c->authenticated_rw ? "RW " : (c->authenticated_ro ? "RO " :"un"));
 
-	if (!c->authenticated)
+	if (!ubifs_authenticated_read(c))
 		return 0;
 
 	if (!IS_ENABLED(CONFIG_UBIFS_FS_AUTHENTICATION))
@@ -583,6 +583,10 @@  static int authenticate_sb_node(struct ubifs_info *c,
 	if (ubifs_hmac_zero(c, sup->hmac)) {
 		err = ubifs_sb_verify_signature(c, sup);
 	} else {
+		if (!ubifs_authenticated_write(c)) {
+			ubifs_err(c, "HMAC authenticated FS found, but no key given");
+			return -EINVAL;
+		}
 		err = ubifs_hmac_wkm(c, hmac_wkm);
 		if (err)
 			return err;
diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
index 7fc2f3f07c16..ec95f1f50e5e 100644
--- a/fs/ubifs/super.c
+++ b/fs/ubifs/super.c
@@ -1291,6 +1291,17 @@  static int mount_ubifs(struct ubifs_info *c)
 			err = -EINVAL;
 			goto out_free;
 		}
+	} else if (c->auth_hash_name) {
+		if (IS_ENABLED(CONFIG_UBIFS_FS_AUTHENTICATION)) {
+			err = ubifs_init_authentication_read_only(c);
+			if (err)
+				goto out_free;
+		} else {
+			ubifs_err(c, "auth_hash_name, but UBIFS is built without"
+				  " authentication support");
+			err = -EINVAL;
+			goto out_free;
+		}
 	}
 
 	err = ubifs_read_superblock(c);
@@ -1383,7 +1394,7 @@  static int mount_ubifs(struct ubifs_info *c)
 	 * in the superblock, we can update the offline signed
 	 * superblock with a HMAC version,
 	 */
-	if (ubifs_authenticated(c) && ubifs_hmac_zero(c, c->sup_node->hmac)) {
+	if (c->authenticated_rw && ubifs_hmac_zero(c, c->sup_node->hmac)) {
 		err = ubifs_hmac_wkm(c, c->sup_node->hmac_wkm);
 		if (err)
 			goto out_lpt;
@@ -1430,7 +1441,7 @@  static int mount_ubifs(struct ubifs_info *c)
 		}
 
 		if (c->need_recovery) {
-			if (!ubifs_authenticated(c)) {
+			if (!ubifs_authenticated_write(c)) {
 				err = ubifs_recover_size(c, true);
 				if (err)
 					goto out_orphans;
@@ -1440,7 +1451,7 @@  static int mount_ubifs(struct ubifs_info *c)
 			if (err)
 				goto out_orphans;
 
-			if (ubifs_authenticated(c)) {
+			if (ubifs_authenticated_write(c)) {
 				err = ubifs_recover_size(c, false);
 				if (err)
 					goto out_orphans;
@@ -1686,7 +1697,7 @@  static int ubifs_remount_rw(struct ubifs_info *c)
 		err = ubifs_write_rcvrd_mst_node(c);
 		if (err)
 			goto out;
-		if (!ubifs_authenticated(c)) {
+		if (!ubifs_authenticated_write(c)) {
 			err = ubifs_recover_size(c, true);
 			if (err)
 				goto out;
@@ -1771,7 +1782,7 @@  static int ubifs_remount_rw(struct ubifs_info *c)
 		if (err)
 			goto out;
 
-		if (ubifs_authenticated(c)) {
+		if (ubifs_authenticated_write(c)) {
 			err = ubifs_recover_size(c, false);
 			if (err)
 				goto out;
diff --git a/fs/ubifs/ubifs.h b/fs/ubifs/ubifs.h
index bff682309fbe..0789c859a148 100644
--- a/fs/ubifs/ubifs.h
+++ b/fs/ubifs/ubifs.h
@@ -1040,7 +1040,8 @@  struct ubifs_debug_info;
  * @default_compr: default compression algorithm (%UBIFS_COMPR_LZO, etc)
  * @rw_incompat: the media is not R/W compatible
  * @assert_action: action to take when a ubifs_assert() fails
- * @authenticated: flag indigating the FS is mounted in authenticated mode
+ * @authenticated_rw: flag indigating the FS is mounted in authenticated mode
+ * @authenticated_ro: flag indigating the FS is mounted in read-only authenticated mode (without HMAC)
  *
  * @tnc_mutex: protects the Tree Node Cache (TNC), @zroot, @cnext, @enext, and
  *             @calc_idx_sz
@@ -1293,7 +1294,8 @@  struct ubifs_info {
 	unsigned int default_compr:2;
 	unsigned int rw_incompat:1;
 	unsigned int assert_action:2;
-	unsigned int authenticated:1;
+	unsigned int authenticated_rw:1;
+	unsigned int authenticated_ro:1;
 	unsigned int superblock_need_write:1;
 
 	struct mutex tnc_mutex;
@@ -1506,21 +1508,34 @@  extern const struct inode_operations ubifs_symlink_inode_operations;
 extern struct ubifs_compressor *ubifs_compressors[UBIFS_COMPR_TYPES_CNT];
 
 /* auth.c */
-static inline int ubifs_authenticated(const struct ubifs_info *c)
+static inline int ubifs_authenticated_read(const struct ubifs_info *c)
 {
-	return (IS_ENABLED(CONFIG_UBIFS_FS_AUTHENTICATION)) && c->authenticated;
+	return (IS_ENABLED(CONFIG_UBIFS_FS_AUTHENTICATION)) && (c->authenticated_rw || c->authenticated_ro);
+}
+
+static inline int ubifs_authenticated_write(const struct ubifs_info *c)
+{
+	/*
+	 * Lets make sure, that ubifs does not try to write
+	 * when authenticate_ro is active.
+	 *
+	 * Because this will certeinly be an error.
+	 */
+	WARN_ON(c->authenticated_ro && !c->authenticated_rw);
+
+	return (IS_ENABLED(CONFIG_UBIFS_FS_AUTHENTICATION)) && c->authenticated_rw;
 }
 
 struct shash_desc *__ubifs_hash_get_desc(const struct ubifs_info *c);
 static inline struct shash_desc *ubifs_hash_get_desc(const struct ubifs_info *c)
 {
-	return ubifs_authenticated(c) ? __ubifs_hash_get_desc(c) : NULL;
+	return ubifs_authenticated_read(c) ? __ubifs_hash_get_desc(c) : NULL;
 }
 
 static inline int ubifs_shash_init(const struct ubifs_info *c,
 				   struct shash_desc *desc)
 {
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_read(c))
 		return crypto_shash_init(desc);
 	else
 		return 0;
@@ -1532,7 +1547,7 @@  static inline int ubifs_shash_update(const struct ubifs_info *c,
 {
 	int err = 0;
 
-	if (ubifs_authenticated(c)) {
+	if (ubifs_authenticated_read(c)) {
 		err = crypto_shash_update(desc, buf, len);
 		if (err < 0)
 			return err;
@@ -1544,7 +1559,7 @@  static inline int ubifs_shash_update(const struct ubifs_info *c,
 static inline int ubifs_shash_final(const struct ubifs_info *c,
 				    struct shash_desc *desc, u8 *out)
 {
-	return ubifs_authenticated(c) ? crypto_shash_final(desc, out) : 0;
+	return ubifs_authenticated_read(c) ? crypto_shash_final(desc, out) : 0;
 }
 
 int __ubifs_node_calc_hash(const struct ubifs_info *c, const void *buf,
@@ -1552,7 +1567,7 @@  int __ubifs_node_calc_hash(const struct ubifs_info *c, const void *buf,
 static inline int ubifs_node_calc_hash(const struct ubifs_info *c,
 					const void *buf, u8 *hash)
 {
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_read(c))
 		return __ubifs_node_calc_hash(c, buf, hash);
 	else
 		return 0;
@@ -1599,17 +1614,18 @@  int __ubifs_node_check_hash(const struct ubifs_info *c, const void *buf,
 static inline int ubifs_node_check_hash(const struct ubifs_info *c,
 					const void *buf, const u8 *expected)
 {
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_read(c))
 		return __ubifs_node_check_hash(c, buf, expected);
 	else
 		return 0;
 }
 
+int ubifs_init_authentication_read_only(struct ubifs_info *c);
 int ubifs_init_authentication(struct ubifs_info *c);
 void __ubifs_exit_authentication(struct ubifs_info *c);
 static inline void ubifs_exit_authentication(struct ubifs_info *c)
 {
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_read(c))
 		__ubifs_exit_authentication(c);
 }
 
@@ -1638,7 +1654,7 @@  static inline u8 *ubifs_branch_hash(struct ubifs_info *c,
 static inline void ubifs_copy_hash(const struct ubifs_info *c, const u8 *from,
 				   u8 *to)
 {
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_read(c))
 		memcpy(to, from, c->hash_len);
 }
 
@@ -1647,7 +1663,7 @@  int __ubifs_node_insert_hmac(const struct ubifs_info *c, void *buf,
 static inline int ubifs_node_insert_hmac(const struct ubifs_info *c, void *buf,
 					  int len, int ofs_hmac)
 {
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_write(c))
 		return __ubifs_node_insert_hmac(c, buf, len, ofs_hmac);
 	else
 		return 0;
@@ -1658,7 +1674,7 @@  int __ubifs_node_verify_hmac(const struct ubifs_info *c, const void *buf,
 static inline int ubifs_node_verify_hmac(const struct ubifs_info *c,
 					 const void *buf, int len, int ofs_hmac)
 {
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_write(c))
 		return __ubifs_node_verify_hmac(c, buf, len, ofs_hmac);
 	else
 		return 0;
@@ -1674,7 +1690,7 @@  static inline int ubifs_node_verify_hmac(const struct ubifs_info *c,
  */
 static inline int ubifs_auth_node_sz(const struct ubifs_info *c)
 {
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_write(c))
 		return sizeof(struct ubifs_auth_node) + c->hmac_desc_len;
 	else
 		return 0;
@@ -1691,7 +1707,7 @@  static inline int ubifs_shash_copy_state(const struct ubifs_info *c,
 					   struct shash_desc *src,
 					   struct shash_desc *target)
 {
-	if (ubifs_authenticated(c))
+	if (ubifs_authenticated_write(c))
 		return __ubifs_shash_copy_state(c, src, target);
 	else
 		return 0;