linux-kernel - Re: [PATCH 2/2] exec: increase BINPRM_BUF

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Date:   Tue, 19 Feb 2019 18:36:54 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ben Woodard <woodard@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] exec: increase BINPRM_BUF_SIZE to 256

On 02/19, Guenter Roeck wrote:
>
> On Tue, Feb 19, 2019 at 01:37:57PM +0100, Oleg Nesterov wrote:
> >
> > looks unrelated...
> >
>
> Indeed...
>
> The underlying problem is in the error handling code of ace_setup(),
> which calls put_disk() followed by blk_cleanup_queue(). put_disk()
> calls disk_release(), which calls blk_put_queue(), which in turn
> results in a call to blk_mq_hw_sysfs_release().
>
> Added debug code, with your patch reverted, shows:
>
>  ######### blk_mq_hw_sysfs_release hctx=cee4a800
>  ...
>  ######### blk_mq_run_hw_queue hctx=cee4a800
>
> blk_mq_hw_sysfs_release() calls kfree(htcx), so accessing it later is most
> definitely not a good idea.

Thanks!

> No idea why this only causes problems with your patch applied.

Well... blk_put_queue() may trigger kobject_uevent() which does call_usermodehelper.

So if one of the used-after-free datastructures was already re-allocated as
linux_binprm, then with my patch it can look "more corrupted"...

But honestly, I too have no idea.

Thanks Guenter.

Oleg.