linux-kernel - Re: [PATCH] Pass correct length to strnlen

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Date:	Sat, 17 Sep 2011 10:03:21 +1000
From:	Ryan Mallon <rmallon@...il.com>
To:	halfdog <me@...fdog.net>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mark Salter <msalter@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Pass correct length to strnlen_user in fs/exec.c

On 17/09/11 02:36, halfdog wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Ryan Mallon wrote:
>
>>> ...
>> That's fine. I originally went looking after a discussion with
>> Mark about the weird strnlen_user semantics  and this usage looked
>> incorrect to me because it wasn't obviously checking >=
>> MAX_ARG_STRLEN.
>>
>> The rework I think makes it a bit more clear and passes the correct
>> max length to strnlen_user. Its a bit odd to pass MAX_ARG_STRLEN
>> and then check if it is longer than bprm->len, and I guess assumes
>> that bprm->len is less than MAX_ARG_STRLEN.
>>
>> Feel free to drop the patch if you think it is just churn.
> As you are already busy with fs/exec.c and strnlen_user, could you
> please check, if the timerace (argv/envp corruption) using
> strnlen_user could also be fixed within the same patch (if bug still
> in kernel). See
>
> https://bugzilla.kernel.org/show_bug.cgi?id=39222

With kernel.org currently being down I cannot see that bug report
unfortunately, so I don't know what the bug you are referring to is.

> The race occurs when one thread issues exec while another one with
> shared memory pages is flipping char/0-bytes of the args or envptrs,
> causing random split (number greater maxargs - no problem, silently
> droped)/fusion (greater MAX_ARG_STRLEN - might harm dumb programs) or
> removal of all args including program name on top of new stack when
> calling via binfmt handlers (might have influenced old ldlinux/libc
> pre 2009?? when handling suid binaries)

It doesn't sound like my patch would have fixed that. The only issue
that my patch would have corrected is the case where bprm->len is
greater than MAX_ARG_STRLEN. MAX_ARG_STRLEN is pretty big though (32 *
PAGE_SIZE), so I don't see that being an issue very often.

~Ryan

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