linux-kernel - Re: NULL pointer dereference in coredump code

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Date:   Tue, 19 May 2020 09:16:52 +0200
From:   Thibaut Sautereau <thibaut.sautereau@...p-os.org>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: NULL pointer dereference in coredump code

On Mon, Mar 30, 2020 at 10:31:59AM +0200, Thibaut Sautereau wrote:
> I hit a kernel NULL pointer dereference caused by the following call chain:
> 
> do_coredump()
>   file_start_write(cprm.file) # cprm.file is NULL
>     file_inode(file) # NULL ptr deref
> 
> The `ispipe` path is followed in do_coredump(), and:
>     # cat /proc/sys/kernel/core_pattern
>     |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h
> 
> It seems that cprm.file can be NULL after the call to the usermode
> helper, especially when setting CONFIG_STATIC_USERMODEHELPER=y and
> CONFIG_STATIC_USERMODEHELPER_PATH="", which is the case for me.
> 
> One may say it's a strange combination of configuration options but I
> think it should not crash the kernel anyway. As I don't know much about
> coredumps in general and this code, I don't know what's the best way to
> fix this issue in a clean and comprehensive way.
> 
> I attached the patch I used to temporarily work around this issue, if
> that can clarify anything.
> 
> Thanks,

For the record, this had previously been reported [1] and was eventually
fixed by 3740d93e3790 ("coredump: fix crash when umh is disabled").

[1] https://bugzilla.kernel.org/show_bug.cgi?id=199795

-- 
Thibaut Sautereau
CLIP OS developer