linux-kernel - Re: [PATCH 2/2] exec: Avoid pathological argc, envc, and bprm->p values

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Date: Fri, 21 Jun 2024 06:21:15 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Kees Cook <kees@...nel.org>
Cc: Eric Biederman <ebiederm@...ssion.com>,
 Justin Stitt <justinstitt@...gle.com>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 2/2] exec: Avoid pathological argc, envc, and bprm->p
 values

On 6/21/24 00:00, Kees Cook wrote:
> On Thu, Jun 20, 2024 at 05:19:55PM -0700, Guenter Roeck wrote:
>> Hi,
>>
>> On Sun, May 19, 2024 at 07:16:12PM -0700, Kees Cook wrote:
>>> Make sure nothing goes wrong with the string counters or the bprm's
>>> belief about the stack pointer. Add checks and matching self-tests.
>>>
>>> For 32-bit validation, this was run under 32-bit UML:
>>> $ tools/testing/kunit/kunit.py run --make_options SUBARCH=i386 exec
>>>
>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>
>> With this patch in linux-next, the qemu m68k:mcf5208evb emulation
>> fails to boot. The error is:
> 
> Eeek. Thanks for the report! I've dropped this patch from my for-next
> tree.
> 
>> Run /init as init process
>> Failed to execute /init (error -7)
> 
> -7 is E2BIG, so it's certainly one of the 3 new added checks. I must
> have made a mistake in my reasoning about how bprm->p is initialized;
> the other two checks seems extremely unlikely to be tripped.
> 
> I will try to get qemu set up and take a close look at what's happening.
> While I'm doing that, if it's easy for you, can you try it with just
> this removed (i.e. the other 2 new -E2BIG cases still in place):
> 
> 	/* Avoid a pathological bprm->p. */
> 	if (bprm->p < limit)
> 		return -E2BIG;

I added a printk:

argc: 1 envc: 2 p: 262140 limit: 2097152
                 ^^^^^^^^^^^^^^^^^^^^^^^^
Removing the check above does indeed fix the problem.

Guenter