Patchwork [v2] ext4: Prevent stack overrun in ext4_file_open when recording last known mountpoint

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Submitter Darrick J. Wong
Date Sept. 16, 2011, 6:40 p.m.
Message ID <20110916184005.GH12086@tux1.beaverton.ibm.com>
Download mbox | patch
Permalink /patch/115020/
State Superseded
Headers show

Comments

Darrick J. Wong - Sept. 16, 2011, 6:40 p.m.
In ext4_file_open, the filesystem records the mountpoint of the first file that
is opened after mounting the filesystem.  It does this by allocating a 64-byte
stack buffer, calling d_path() to grab the mount point through which this file
was accessed, and then memcpy()ing 64 bytes into the superblock's
s_last_mounted field, starting from the return value of d_path(), which is
stored as "cp".  However, if cp > buf (which it frequently is since path
components are prepended starting at the end of buf) then we can end up copying
stack data into the superblock.

Writing stack variables into the superblock doesn't sound like a great idea, so
use strlcpy instead.  Andi Kleen suggested using strlcpy instead of strncpy.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
---

 fs/ext4/file.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

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Darrick J. Wong - Sept. 30, 2011, 7:46 p.m.
On Fri, Sep 16, 2011 at 11:40:05AM -0700, Darrick J. Wong wrote:
> In ext4_file_open, the filesystem records the mountpoint of the first file that
> is opened after mounting the filesystem.  It does this by allocating a 64-byte
> stack buffer, calling d_path() to grab the mount point through which this file
> was accessed, and then memcpy()ing 64 bytes into the superblock's
> s_last_mounted field, starting from the return value of d_path(), which is
> stored as "cp".  However, if cp > buf (which it frequently is since path
> components are prepended starting at the end of buf) then we can end up copying
> stack data into the superblock.
> 
> Writing stack variables into the superblock doesn't sound like a great idea, so
> use strlcpy instead.  Andi Kleen suggested using strlcpy instead of strncpy.

Ok, it's been a couple of weeks.... any thoughts, Ted?

--D
> 
> Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
> ---
> 
>  fs/ext4/file.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index e4095e9..9781099 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -181,8 +181,8 @@ static int ext4_file_open(struct inode * inode, struct file * filp)
>  		path.dentry = mnt->mnt_root;
>  		cp = d_path(&path, buf, sizeof(buf));
>  		if (!IS_ERR(cp)) {
> -			memcpy(sbi->s_es->s_last_mounted, cp,
> -			       sizeof(sbi->s_es->s_last_mounted));
> +			strlcpy(sbi->s_es->s_last_mounted, cp,
> +				sizeof(sbi->s_es->s_last_mounted));
>  			ext4_mark_super_dirty(sb);
>  		}
>  	}
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Lukas Czerner - Oct. 4, 2011, 11:30 a.m.
On Fri, 30 Sep 2011, Darrick J. Wong wrote:

> On Fri, Sep 16, 2011 at 11:40:05AM -0700, Darrick J. Wong wrote:
> > In ext4_file_open, the filesystem records the mountpoint of the first file that
> > is opened after mounting the filesystem.  It does this by allocating a 64-byte
> > stack buffer, calling d_path() to grab the mount point through which this file
> > was accessed, and then memcpy()ing 64 bytes into the superblock's
> > s_last_mounted field, starting from the return value of d_path(), which is
> > stored as "cp".  However, if cp > buf (which it frequently is since path
> > components are prepended starting at the end of buf) then we can end up copying
> > stack data into the superblock.
> > 
> > Writing stack variables into the superblock doesn't sound like a great idea, so
> > use strlcpy instead.  Andi Kleen suggested using strlcpy instead of strncpy.
> 
> Ok, it's been a couple of weeks.... any thoughts, Ted?
> 
> --D

Makes sense.

Reviewed-by: Lukas Czerner <lczerner@redhat.com>

> > 
> > Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
> > ---
> > 
> >  fs/ext4/file.c |    4 ++--
> >  1 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> > index e4095e9..9781099 100644
> > --- a/fs/ext4/file.c
> > +++ b/fs/ext4/file.c
> > @@ -181,8 +181,8 @@ static int ext4_file_open(struct inode * inode, struct file * filp)
> >  		path.dentry = mnt->mnt_root;
> >  		cp = d_path(&path, buf, sizeof(buf));
> >  		if (!IS_ERR(cp)) {
> > -			memcpy(sbi->s_es->s_last_mounted, cp,
> > -			       sizeof(sbi->s_es->s_last_mounted));
> > +			strlcpy(sbi->s_es->s_last_mounted, cp,
> > +				sizeof(sbi->s_es->s_last_mounted));
> >  			ext4_mark_super_dirty(sb);
> >  		}
> >  	}
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > 
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Patch

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index e4095e9..9781099 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -181,8 +181,8 @@  static int ext4_file_open(struct inode * inode, struct file * filp)
 		path.dentry = mnt->mnt_root;
 		cp = d_path(&path, buf, sizeof(buf));
 		if (!IS_ERR(cp)) {
-			memcpy(sbi->s_es->s_last_mounted, cp,
-			       sizeof(sbi->s_es->s_last_mounted));
+			strlcpy(sbi->s_es->s_last_mounted, cp,
+				sizeof(sbi->s_es->s_last_mounted));
 			ext4_mark_super_dirty(sb);
 		}
 	}