linux-kernel - Re: [PATCH] fs/exec: Test patch for syzkaller crash

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Message-ID: <202208121144.E9E5EE9E@keescook>
Date:   Fri, 12 Aug 2022 11:45:36 -0700
From:   Kees Cook <keescook@...omium.org>
To:     ira.weiny@...el.com
Cc:     syzbot+3250d9c8925ef29e975f@...kaller.appspotmail.com,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        ebiederm@...ssion.com, viro@...iv.linux.org.uk,
        sfr@...b.auug.org.au, syzkaller-bugs@...glegroups.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/exec: Test patch for syzkaller crash

On Thu, Aug 11, 2022 at 05:09:19PM -0700, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
> 
> Kees reported that it looked like the kmap_local_page() conversion in
> fs/exec.c was causing a crash with the syzkaller.[1]
> 
> At first glance it appeared this was due to the lack of pagefaults not
> being disabled as was done by kmap_atomic().
> 
> Unfortunately, after deeper investigation we don't see how this is a
> problem.  The crash does not appear to be happening in the
> memcpy_to_page() call.[2]
> 
> For testing, add back pagefault disable in copy_string_kernel() to see
> if it makes syzkaller happy.  If so more investigation will need to be
> done to understand exactly what is happening.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c6e8e36c6ae4b11bed5643317afb66b6c3cadba8
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/fs/exec.c?id=40d43a7507e1547dd45cb02af2e40d897c591870#n616
> 
> Cc: Kees Cook <keescook@...omium.org>
> Reported-by: syzbot+3250d9c8925ef29e975f@...kaller.appspotmail.com
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>

Thanks for spinning this. As noted in the other thread, I'm going to
wait and see if the crashes return. It looks like it may have been an
unrelated problem that got fix in -next (no crashes for a day now...)

But we'll have this in our back pocket if we need it. :)

-Kees

-- 
Kees Cook