[v1,4.20.y,stable] CIFS: fix deadlock in cached root handling

Prevent deadlock between open_shroot() and
cifs_mark_open_files_invalid() by releasing the lock before entering
SMB2_open, taking it again after and checking if we still need to use
the result.

Link: https://lore.kernel.org/linux-cifs/684ed01c-cbca-2716-bc28-b0a59a0f8521@prodrive-technologies.com/T/#u
Fixes: 3d4ef9a15343 ("smb3: fix redundant opens on root")
Signed-off-by: Aurelien Aptel <aaptel@suse.com>
---

This patch applies on top of the 4.20 stable tree.

Please review this carefuly, I have not tested it.

 XXX: do we need to call SMB2_close() when the work was already done
      concurrently?  if yes we need to do it outside the critical
      section otherwise we hit the same issue again

 fs/cifs/smb2ops.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

сб, 6 июл. 2019 г. в 12:17, Aurelien Aptel <aaptel@suse.com>:
>
> Prevent deadlock between open_shroot() and
> cifs_mark_open_files_invalid() by releasing the lock before entering
> SMB2_open, taking it again after and checking if we still need to use
> the result.
>
> Link: https://lore.kernel.org/linux-cifs/684ed01c-cbca-2716-bc28-b0a59a0f8521@prodrive-technologies.com/T/#u
> Fixes: 3d4ef9a15343 ("smb3: fix redundant opens on root")
> Signed-off-by: Aurelien Aptel <aaptel@suse.com>
> ---
>
> This patch applies on top of the 4.20 stable tree.
>
> Please review this carefuly, I have not tested it.
>
>  XXX: do we need to call SMB2_close() when the work was already done
>       concurrently?  if yes we need to do it outside the critical
>       section otherwise we hit the same issue again

Yes, we do need to close a handle we just opened. Otherwise it will
leak a handle on the server.

>
>  fs/cifs/smb2ops.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
> index aa71e620f3cd..55991e43d74f 100644
> --- a/fs/cifs/smb2ops.c
> +++ b/fs/cifs/smb2ops.c
> @@ -609,7 +609,27 @@ int open_shroot(unsigned int xid, struct cifs_tcon *tcon, struct cifs_fid *pfid)
>         oparams.fid = pfid;
>         oparams.reconnect = false;
>
> +       /*
> +        * We do not hold the lock for the open because in case
> +        * SMB2_open needs to reconnect, it will end up calling
> +        * cifs_mark_open_files_invalid() which takes the lock again
> +        * thus causing a deadlock
> +        */
> +       mutex_unlock(&tcon->crfid.fid_mutex);
>         rc = SMB2_open(xid, &oparams, &srch_path, &oplock, NULL, NULL, NULL);
> +       mutex_lock(&tcon->crfid.fid_mutex);
> +
> +       /*
> +        * Now we need to check again as the cached root might have
> +        * been successfully re-opened from a concurrent process
> +        */
> +
> +       if (tcon->crfid.is_valid) {
> +               /* work was already done */
> +               rc = 0;
> +               goto out;
> +       }
> +
>         if (rc == 0) {
>                 memcpy(tcon->crfid.fid, pfid, sizeof(struct cifs_fid));
>                 tcon->crfid.tcon = tcon;
> @@ -617,6 +637,8 @@ int open_shroot(unsigned int xid, struct cifs_tcon *tcon, struct cifs_fid *pfid)
>                 kref_init(&tcon->crfid.refcount);
>                 kref_get(&tcon->crfid.refcount);
>         }
> +
> +out:
>         mutex_unlock(&tcon->crfid.fid_mutex);
>         return rc;
>  }
> --
> 2.16.4
>


--
Best regards,
Pavel Shilovsky