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Date:   Sat, 12 Sep 2020 08:47:13 +0200
From:   Salvatore Bonaccorso <carnil@debian.org>
To:     Bob Peterson <rpeterso@redhat.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Sasha Levin <sashal@kernel.org>,
        Daniel Craig <Daniel.Craig@csiro.au>,
        Nicolas Courtel <courtel@cena.fr>
Subject: Re: [PATCH 4.19 142/206] gfs2: fix use-after-free on transaction ail
 lists
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Hi Bob, hi Greg,

On Fri, Sep 11, 2020 at 08:49:14AM -0400, Bob Peterson wrote:
> ----- Original Message -----
> > On Fri, Sep 11, 2020 at 08:08:35AM -0400, Bob Peterson wrote:
> > > ----- Original Message -----
> > > > On Thu, Sep 10, 2020 at 09:43:19PM +0200, Salvatore Bonaccorso wrote:
> > > > > Hi,
> > > > > 
> > > > > On Tue, Jun 23, 2020 at 09:57:50PM +0200, Greg Kroah-Hartman wrote:
> > > > > > From: Bob Peterson <rpeterso@redhat.com>
> > > > > > 
> > > > > > [ Upstream commit 83d060ca8d90fa1e3feac227f995c013100862d3 ]
> > > > > > 
> > > > > > Before this patch, transactions could be merged into the system
> > > > > > transaction by function gfs2_merge_trans(), but the transaction ail
> > > > > > lists were never merged. Because the ail flushing mechanism can run
> > > > > > separately, bd elements can be attached to the transaction's buffer
> > > > > > list during the transaction (trans_add_meta, etc) but quickly moved
> > > > > > to its ail lists. Later, in function gfs2_trans_end, the transaction
> > > > > > can be freed (by gfs2_trans_end) while it still has bd elements
> > > > > > queued to its ail lists, which can cause it to either lose track of
> > > > > > the bd elements altogether (memory leak) or worse, reference the bd
> > > > > > elements after the parent transaction has been freed.
> > > > > > 
> > > > > > Although I've not seen any serious consequences, the problem becomes
> > > > > > apparent with the previous patch's addition of:
> > > > > > 
> > > > > > 	gfs2_assert_warn(sdp, list_empty(&tr->tr_ail1_list));
> > > > > > 
> > > > > > to function gfs2_trans_free().
> > > > > > 
> > > > > > This patch adds logic into gfs2_merge_trans() to move the merged
> > > > > > transaction's ail lists to the sdp transaction. This prevents the
> > > > > > use-after-free. To do this properly, we need to hold the ail lock,
> > > > > > so we pass sdp into the function instead of the transaction itself.
> > > > > > 
> > > > > > Signed-off-by: Bob Peterson <rpeterso@redhat.com>
> > > > > > Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
> > > > > > Signed-off-by: Sasha Levin <sashal@kernel.org>
> > > (snip)
> > > > > 
> > > > > In Debian two user confirmed issues on writing on a GFS2 partition
> > > > > with this commit applied. The initial Debian report is at
> > > > > https://bugs.debian.org/968567 and Daniel Craig reported it into
> > > > > Bugzilla at https://bugzilla.kernel.org/show_bug.cgi?id=209217 .
> > > > > 
> > > > > Writing to a gfs2 filesystem fails and results in a soft lookup of the
> > > > > machine for kernels with that commit applied. I cannot reporduce the
> > > > > issue myself due not having a respective setup available, but Daniel
> > > > > described a minimal serieos of steps to reproduce the issue.
> > > > > 
> > > > > This might affect as well other stable series where this commit was
> > > > > applied, as there was a similar report for someone running 5.4.58 in
> > > > > https://www.redhat.com/archives/linux-cluster/2020-August/msg00000.html
> > > > 
> > > > Can you report this to the gfs2 developers?
> > > > 
> > > > thanks,
> > > > 
> > > > greg k-h
> > > 
> > > Hi Greg,
> > > 
> > > No need. The patch came from the gfs2 developers. I think he just wants
> > > it added to a stable release.
> > 
> > What commit needs to be added to a stable release?
> > 
> > confused,
> > 
> > greg k-h
> 
> Sorry Greg,
> 
> It's pretty early here and the caffeine hadn't quite hit my system.
> The problem is most likely that 4.19.132 is missing this upstream patch:
> 
> cbcc89b630447ec7836aa2b9242d9bb1725f5a61
> 
> I'm not sure how or why 83d060ca8d90fa1e3feac227f995c013100862d3 got
> put into stable without a stable CC but cbcc89b6304 is definitely
> required.
> 
> I'd like to suggest Salvatore try cherry-picking this patch to see if
> it fixes the problem, and if so, perhaps Greg can add it to stable.

I can confirm (Daniel was able to test): Applying cbcc89b63044 ("gfs2:
initialize transaction tr_ailX_lists earlier") fixes the issue. So
would be great if you can pick that up for stable for those series
which had 83d060ca8d90 ("gfs2: fix use-after-free on transaction ail
lists") as well.

Regards,
Salvatore