linux-kernel - Re: [PATCH] mm: Downgrade mmap_sem before locking or populating on mmap

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20121214072755.GR4939@ZenIV.linux.org.uk>
Date:	Fri, 14 Dec 2012 07:27:55 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Michel Lespinasse <walken@...gle.com>,
	Hugh Dickins <hughd@...gle.com>, J??rn Engel <joern@...fs.org>
Subject: Re: [PATCH] mm: Downgrade mmap_sem before locking or populating on
 mmap

On Thu, Dec 13, 2012 at 09:49:43PM -0800, Andy Lutomirski wrote:
> This is a serious cause of mmap_sem contention.  MAP_POPULATE
> and MCL_FUTURE, in particular, are disastrous in multithreaded programs.
> 
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
> 
> Sensible people use anonymous mappings.  I write kernel patches :)
> 
> I'm not entirely thrilled by the aesthetics of this patch.  The MAP_POPULATE case
> could also be improved by doing it without any lock at all.  This is still a big
> improvement, though.

Wait a minute.  get_user_pages() relies on ->mmap_sem being held.  Unless
I'm seriously misreading your patch it removes that protection.  And yes,
I'm aware of execve-related exception; it's in special circumstances -
bprm->mm is guaranteed to be not shared (and we need to rearchitect that
area anyway, but that's a separate story).
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/