linux-kernel - Re: general protection fault with prefetch

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]

Date:   Wed, 8 Aug 2018 15:16:46 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Paul Menzel <pmenzel+linux-mm@...gen.mpg.de>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Alex Deucher <alexander.deucher@....com>
Subject: Re: general protection fault with prefetch_freepointer

On 08/08/2018 01:54 PM, Vlastimil Babka wrote:
> Hmm I have looked at the splats in all the bugs you referenced and the
> Code part always has the de-obfuscation XORs. Then in comment 36 of
> [1] jian-hong says the problem disappeared, and in comment 40 posts
> a config that has CONFIG_SLAB_FREELIST_HARDENED disabled. Earlier
> posting of his config has it enabled and confirms the disassembly.
> Very suspicious, huh.

So I'm looking at 2482ddec670f ("mm: add SLUB free list pointer
obfuscation") from Kees, and one suspicious thing is:

before, prefetch_freepointer() was just:

	prefetch(object + s->offset);

after, it is

	if (object)
		prefetch(freelist_dereference(s, object + s->offset));

Where freelist_dereference() is either a simple dereference of address,
when FREELIST_HARDENED is disabled, or adds those XORs when enabled.
However, this had changed the prefetch intention! Previously it just
prefetched the address (object + s->offset), now it *dereferences it*,
optionally changes the value read with those XORs, and then prefetches
the result.

This unintentionally adds a non-prefetching read from (object +
s->offset), which may fault, and wasn't there before. It's safe from
NULL pointers, but not from bogus pointers, and faults that
get_freepointer_safe() prevents. Note that alone doesn't explain why
disabling SLAB_FREELIST_HARDENED would help, as the dereference is there
unconditionally. But IMHO it's a bug in the commit and minimally it
likely has some performance impact.