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

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Date:	Fri, 16 Sep 2011 16:36:38 +0000
From:	halfdog <me@...fdog.net>
To:	Ryan Mallon <rmallon@...il.com>
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

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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

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)

Kind Regards,
Roman

- -- 
http://www.halfdog.net/
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