linux-kernel - Re: [PATCH] xfs: do not call xfs_buf_hash

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Message-ID: <20170124183437.GJ4780@birch.djwong.org>
Date:   Tue, 24 Jan 2017 10:34:37 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     "Bill O'Donnell" <billodo@...hat.com>
Cc:     Colin Ian King <colin.king@...onical.com>,
        Eric Sandeen <sandeen@...deen.net>, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xfs: do not call xfs_buf_hash_destroy on a NULL pag

On Tue, Jan 24, 2017 at 09:04:57AM -0600, Bill O'Donnell wrote:
> On Fri, Jan 20, 2017 at 11:04:42PM +0000, Colin Ian King wrote:
> > On 20/01/17 20:47, Darrick J. Wong wrote:
> > > On Fri, Jan 20, 2017 at 01:26:12PM -0600, Eric Sandeen wrote:
> > >> On 1/20/17 8:26 AM, Colin King wrote:
> > >>> From: Colin Ian King <colin.king@...onical.com>
> > >>>
> > >>> If pag cannot be allocated, the current error exit path will trip
> > >>> a null pointer deference error when calling xfs_buf_hash_destroy
> > >>> with a null pag.  Fix this by adding a new error exit lable and
> > >>> jumping to this, avoiding the hash destroy and unnecessary kmem_free
> > >>> on pag.
> > >>>
> > >>> Fixes CoverityScan CID#1397628 ("Dereference after null check")
> > >>>
> > >>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> > >>
> > >> Hm, I think this leaves the code with issues.
> > >>
> > >>> ---
> > >>>  fs/xfs/xfs_mount.c | 3 ++-
> > >>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > >>>
> > >>> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> > >>> index 9b9540d..4e66cd19 100644
> > >>> --- a/fs/xfs/xfs_mount.c
> > >>> +++ b/fs/xfs/xfs_mount.c
> > >>> @@ -207,7 +207,7 @@ xfs_initialize_perag(
> > >>>  
> > >>>  		pag = kmem_zalloc(sizeof(*pag), KM_MAYFAIL);
> > >>>  		if (!pag)
> > >>> -			goto out_unwind;
> > >>> +			goto out_unwind_pags;
> > >>
> > >> So let's say we got to index == 3 at the top of the loop, and
> > >> this fails.
> > >>
> > >> We succeeded in initializing 0, 1, and 2, but 3 failed.
> > >>
> > >> So we go to out_unwind_pags with index == 3...
> > >>
> > >>>  		pag->pag_agno = index;
> > >>>  		pag->pag_mount = mp;
> > >>>  		spin_lock_init(&pag->pag_ici_lock);
> > >>> @@ -242,6 +242,7 @@ xfs_initialize_perag(
> > >>>  out_unwind:
> > >>>  	xfs_buf_hash_destroy(pag);
> > >>>  	kmem_free(pag);
> > >>> +out_unwind_pags:
> > >>
> > >> ... where index == 3, and:
> > >>
> > >>>  	for (; index > first_initialised; index--) {
> > >>>  		pag = radix_tree_delete(&mp->m_perag_tree, index);
> > >>
> > >> this should fail, because it never got inserted, and...
> > >>
> > >>>  		xfs_buf_hash_destroy(pag);
> > >>
> > >> this still tries to destroy a NULL pag, no?
> > >>
> > >> There also seems to be an existing issue w/the code where ag 0 is
> > >> never torn down in the error case, because first_initialized doesn't
> > >> stay set to 0:
> > >>
> > >>                 if (!first_initialised)
> > >>                         first_initialised = index;
> > >>
> > >> And we don't even tear down ag 1, because:
> > >>
> > >>>  	for (; index > first_initialised; index--) {
> > >>>  		pag = radix_tree_delete(&mp->m_perag_tree, index);
> > >>
> > >> when the loop reaches the first initialized AG, it stops.
> > >>
> > >> So we seem to always leak at least 2 if we managed to get far enough
> > >> to initialize them.
> > > 
> > > Ugh, yeah, the the whole error exit from that function is fubar...
> > > Anyone want to clean this up?
> > 
> > I may step back on this if somebody else wants to fix this up properly.
> 
> I'll take it.

Acknowledged. :)

--D

> -Bill
> 
> 
> > 
> > > 
> > > --D
> > > 
> > >>
> > >> -Eric
> > >>
> > >>>
> > >> --
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> > >> the body of a message to majordomo@...r.kernel.org
> > >> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > 
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