[PATCH][RFC][resend] potential NULL pointer deref in XFS on failed mount

[PATCH][RFC][resend] potential NULL pointer deref in XFS on failed mount

[Jesper Juhl]

(got no reply on this when I originally send it on 20061031, so resending
 now that a bit of time has passed.  The patch still applies cleanly to
 Linus' git tree as of today.)


The Coverity checker spotted a potential problem in XFS.

The problem is that if, in xfs_mount(), this code triggers:

	...
	if (!mp->m_logdev_targp)
		goto error0;
	...

Then we'll end up calling xfs_unmountfs_close() with a NULL 
'mp->m_logdev_targp'. 
This in turn will result in a call to xfs_free_buftarg() with its 'btp' 
argument == NULL. xfs_free_buftarg() dereferences 'btp' leading to
a NULL pointer dereference and crash.

I think this can happen, since the fatal call to xfs_free_buftarg() 
happens when 'm_logdev_targp != m_ddev_targp' and due to a check of
'm_ddev_targp' against NULL in xfs_mount() (and subsequent return if it is 
NULL) the two will never both be NULL when we hit the error0 label from 
the two lines cited above.

Comments welcome (please keep me on Cc: on replies).

Here's a proposed patch to fix this by testing 'btp' against NULL in 
xfs_free_buftarg().


Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
---

 fs/xfs/linux-2.6/xfs_buf.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
index db5f5a3..6ef1860 100644
--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c
@@ -1450,6 +1450,9 @@ xfs_free_buftarg(
 	xfs_buftarg_t		*btp,
 	int			external)
 {
+	if (unlikely(!btp))
+		return;
+
 	xfs_flush_buftarg(btp, 1);
 	if (external)
 		xfs_blkdev_put(btp->bt_bdev);

Re: [PATCH][RFC][resend] potential NULL pointer deref in XFS on failed mount

[Shailendra Tripathi]

Hey Jesper,
                Rather,  it can be done as below. Nothing to say that 
your code wouldn't work. Just that catch it early, so that potential 
function call overhead to call xfs_free_buftarg can be avoided.

void
xfs_unmountfs_close(xfs_mount_t *mp, struct cred *cr)
{
       if (mp->m_logdev_targp && (mp->m_logdev_targp != mp->m_ddev_targp))
                xfs_free_buftarg(mp->m_logdev_targp, 1);
        if (mp->m_rtdev_targp)
                xfs_free_buftarg(mp->m_rtdev_targp, 1);
        xfs_free_buftarg(mp->m_ddev_targp, 0);
}


Jesper Juhl wrote:
> (got no reply on this when I originally send it on 20061031, so resending
>  now that a bit of time has passed.  The patch still applies cleanly to
>  Linus' git tree as of today.)
>
>
> The Coverity checker spotted a potential problem in XFS.
>
> The problem is that if, in xfs_mount(), this code triggers:
>
> 	...
> 	if (!mp->m_logdev_targp)
> 		goto error0;
> 	...
>
> Then we'll end up calling xfs_unmountfs_close() with a NULL 
> 'mp->m_logdev_targp'. 
> This in turn will result in a call to xfs_free_buftarg() with its 'btp' 
> argument == NULL. xfs_free_buftarg() dereferences 'btp' leading to
> a NULL pointer dereference and crash.
>
> I think this can happen, since the fatal call to xfs_free_buftarg() 
> happens when 'm_logdev_targp != m_ddev_targp' and due to a check of
> 'm_ddev_targp' against NULL in xfs_mount() (and subsequent return if it is 
> NULL) the two will never both be NULL when we hit the error0 label from 
> the two lines cited above.
>
> Comments welcome (please keep me on Cc: on replies).
>
> Here's a proposed patch to fix this by testing 'btp' against NULL in 
> xfs_free_buftarg().
>
>
> Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
> ---
>
>  fs/xfs/linux-2.6/xfs_buf.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
>
> diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
> index db5f5a3..6ef1860 100644
> --- a/fs/xfs/linux-2.6/xfs_buf.c
> +++ b/fs/xfs/linux-2.6/xfs_buf.c
> @@ -1450,6 +1450,9 @@ xfs_free_buftarg(
>  	xfs_buftarg_t		*btp,
>  	int			external)
>  {
> +	if (unlikely(!btp))
> +		return;
> +
>  	xfs_flush_buftarg(btp, 1);
>  	if (external)
>  		xfs_blkdev_put(btp->bt_bdev);
>
>
>
>   

Re: [PATCH][RFC][resend] potential NULL pointer deref in XFS on failed mount

[Shailendra Tripathi]

Jesper Juhl wrote:

> The reason I want to fix it in the freeing function is that many other
> functions in the kernel that free resources are safe to call with NULL
> pointers and this would make xfs_free_buftarg() follow that
> convention.  This would perhaps also allow for some cleanups in other
> places that call the function since then there's no longer a need for
> explicit NULL checks any more (haven't checked if there's anything to
> gain there though).
> I don't think the function call overhead matters much since this is in
> a case of a failed mount, so it should happen very rarely.
>
I agree with you.  However, cleanup functions should(/must?) check for  
NULL etc and
in this case it is already doing so for other cases. So, perhaps not 
required. Just a different viewpoint.
Your choice.

>> void
>> xfs_unmountfs_close(xfs_mount_t *mp, struct cred *cr)
>> {
>>        if (mp->m_logdev_targp && (mp->m_logdev_targp != 
>> mp->m_ddev_targp))
>>                 xfs_free_buftarg(mp->m_logdev_targp, 1);
>>         if (mp->m_rtdev_targp)
>>                 xfs_free_buftarg(mp->m_rtdev_targp, 1);
>>         xfs_free_buftarg(mp->m_ddev_targp, 0);
>> }
>>
>
>

Re: [PATCH][RFC][resend] potential NULL pointer deref in XFS on failed mount

[Jesper Juhl]

[-- Attachment #1: Type: text/plain, Size: 4472 bytes --]

On 16/11/06, Jesper Juhl <jesper.juhl@gmail.com> wrote:
> On 16/11/06, David Chinner <dgc@sgi.com> wrote:
> > On Thu, Nov 16, 2006 at 10:18:26PM +0100, Jesper Juhl wrote:
> > > (got no reply on this when I originally send it on 20061031, so resending
> > >  now that a bit of time has passed.  The patch still applies cleanly to
> > >  Linus' git tree as of today.)
> > >
> > >
> > > The Coverity checker spotted a potential problem in XFS.
> > >
> > > The problem is that if, in xfs_mount(), this code triggers:
> > >
> > >       ...
> > >       if (!mp->m_logdev_targp)
> > >               goto error0;
> > >       ...
> > >
> > > Then we'll end up calling xfs_unmountfs_close() with a NULL
> > > 'mp->m_logdev_targp'.
> > > This in turn will result in a call to xfs_free_buftarg() with its 'btp'
> > > argument == NULL. xfs_free_buftarg() dereferences 'btp' leading to
> > > a NULL pointer dereference and crash.
> >
> > Interesting that coverity found that, but failed to find the other
> > leaks in that function from exactly the same code and error
> > case.....
> >
> > > I think this can happen, since the fatal call to xfs_free_buftarg()
> > > happens when 'm_logdev_targp != m_ddev_targp' and due to a check of
> > > 'm_ddev_targp' against NULL in xfs_mount() (and subsequent return if it is
> > > NULL) the two will never both be NULL when we hit the error0 label from
> > > the two lines cited above.
> > >
> > > Comments welcome (please keep me on Cc: on replies).
> > >
> > > Here's a proposed patch to fix this by testing 'btp' against NULL in
> > > xfs_free_buftarg().
> >
> > Not the right fix - we should only be trying to free valid
> > buftargs, which means xfs_unmountfs_close() is the correct
> > place to fix this....
> >
>
> Ok.
>
> > e.g:
> >
> > -       if (mp->m_logdev_targp != mp->m_ddev_targp)
> > +       if (mp->m_logdev_targp && (mp->m_logdev_targp != mp->m_ddev_targp))
> >
> > As to the afore-mentioned leaks, if we fail to allocate a realtime
> > buftarg, then we will leak a reference to both the rtdev and logdev,
> > and if we fail to allocate an external log buftarg we'll leak a
> > reference to the logdev. i.e., we fail to do one or both of:
> >
> >         xfs_blkdev_put(logdev);
> >         xfs_blkdev_put(rtdev);
> >
> > To remove the bdev references we may have gained earlier. Normally,
> > these references are released by xfs_free_buftarg(), but because we
> > failed to allocate the buftarg, we can't drop the references via
> > that method....
> >
>

How about something like the attached patch ?

(sorry about the attachment, but I can't inline the patch from my
current location - a whitespace damaged version is below though for
easy review)



Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
---

 fs/xfs/xfs_mount.c  |    2 +-
 fs/xfs/xfs_vfsops.c |   10 ++++++++--
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 9dfae18..3497128 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -1135,7 +1135,7 @@ #endif
 void
 xfs_unmountfs_close(xfs_mount_t *mp, struct cred *cr)
 {
-       if (mp->m_logdev_targp != mp->m_ddev_targp)
+       if (mp->m_logdev_targp && mp->m_logdev_targp != mp->m_ddev_targp)
                xfs_free_buftarg(mp->m_logdev_targp, 1);
        if (mp->m_rtdev_targp)
                xfs_free_buftarg(mp->m_rtdev_targp, 1);
diff --git a/fs/xfs/xfs_vfsops.c b/fs/xfs/xfs_vfsops.c
index 62336a4..6d7e8f1 100644
--- a/fs/xfs/xfs_vfsops.c
+++ b/fs/xfs/xfs_vfsops.c
@@ -473,13 +473,19 @@ xfs_mount(
        }
        if (rtdev) {
                mp->m_rtdev_targp = xfs_alloc_buftarg(rtdev, 1);
-               if (!mp->m_rtdev_targp)
+               if (!mp->m_rtdev_targp) {
+                       xfs_blkdev_put(logdev);
+                       xfs_blkdev_put(rtdev);
                        goto error0;
+               }
        }
        mp->m_logdev_targp = (logdev && logdev != ddev) ?
                                xfs_alloc_buftarg(logdev, 1) : mp->m_ddev_targp;
-       if (!mp->m_logdev_targp)
+       if (!mp->m_logdev_targp) {
+               xfs_blkdev_put(logdev);
+               xfs_blkdev_put(rtdev);
                goto error0;
+       }

        /*
         * Setup flags based on mount(2) options and then the superblock


-- 
Jesper Juhl <jesper.juhl@gmail.com>
Don't top-post  http://www.catb.org/~esr/jargon/html/T/top-post.html
Plain text mails only, please      http://www.expita.com/nomime.html

[-- Attachment #2: patch.txt --]
[-- Type: text/plain, Size: 1317 bytes --]


Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
---

 fs/xfs/xfs_mount.c  |    2 +-
 fs/xfs/xfs_vfsops.c |   10 ++++++++--
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 9dfae18..3497128 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -1135,7 +1135,7 @@ #endif
 void
 xfs_unmountfs_close(xfs_mount_t *mp, struct cred *cr)
 {
-	if (mp->m_logdev_targp != mp->m_ddev_targp)
+	if (mp->m_logdev_targp && mp->m_logdev_targp != mp->m_ddev_targp)
 		xfs_free_buftarg(mp->m_logdev_targp, 1);
 	if (mp->m_rtdev_targp)
 		xfs_free_buftarg(mp->m_rtdev_targp, 1);
diff --git a/fs/xfs/xfs_vfsops.c b/fs/xfs/xfs_vfsops.c
index 62336a4..6d7e8f1 100644
--- a/fs/xfs/xfs_vfsops.c
+++ b/fs/xfs/xfs_vfsops.c
@@ -473,13 +473,19 @@ xfs_mount(
 	}
 	if (rtdev) {
 		mp->m_rtdev_targp = xfs_alloc_buftarg(rtdev, 1);
-		if (!mp->m_rtdev_targp)
+		if (!mp->m_rtdev_targp) {
+			xfs_blkdev_put(logdev);
+			xfs_blkdev_put(rtdev);
 			goto error0;
+		}
 	}
 	mp->m_logdev_targp = (logdev && logdev != ddev) ?
 				xfs_alloc_buftarg(logdev, 1) : mp->m_ddev_targp;
-	if (!mp->m_logdev_targp)
+	if (!mp->m_logdev_targp) {
+		xfs_blkdev_put(logdev);
+		xfs_blkdev_put(rtdev);
 		goto error0;
+	}
 
 	/*
 	 * Setup flags based on mount(2) options and then the superblock