linux-kernel - Re: [PATCH 1/2] mmap.2: clarify MAP

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: <20150513144506.GD1227@akamai.com>
Date:	Wed, 13 May 2015 10:45:06 -0400
From:	Eric B Munson <emunson@...mai.com>
To:	Michal Hocko <miso@...p22.suse.cz>
Cc:	Michael Kerrisk <mtk.manpages@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>, linux-mm@...ck.org,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH 1/2] mmap.2: clarify MAP_LOCKED semantic

On Wed, 13 May 2015, Michal Hocko wrote:

> From: Michal Hocko <mhocko@...e.cz>
> 
> MAP_LOCKED had a subtly different semantic from mmap(2)+mlock(2) since
> it has been introduced.
> mlock(2) fails if the memory range cannot get populated to guarantee
> that no future major faults will happen on the range. mmap(MAP_LOCKED) on
> the other hand silently succeeds even if the range was populated only
> partially.
> 
> Fixing this subtle difference in the kernel is rather awkward because
> the memory population happens after mm locks have been dropped and so
> the cleanup before returning failure (munlock) could operate on something
> else than the originally mapped area.
> 
> E.g. speculative userspace page fault handler catching SEGV and doing
> mmap(fault_addr, MAP_FIXED|MAP_LOCKED) might discard portion of a racing
> mmap and lead to lost data. Although it is not clear whether such a
> usage would be valid, mmap page doesn't explicitly describe requirements
> for threaded applications so we cannot exclude this possibility.
> 
> This patch makes the semantic of MAP_LOCKED explicit and suggest using
> mmap + mlock as the only way to guarantee no later major page faults.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.cz>

Does the problem still happend when MAP_POPULATE | MAP_LOCKED is used
(AFAICT MAP_POPULATE will cause the mmap to fail if all the pages cannot
be made present).

Either way this is a good catch.

Acked-by: Eric B Munson <emunson@...mai.com>


Download attachment "signature.asc" of type "application/pgp-signature" (820 bytes)