From: Vlastimil Babka <vbabka@suse.cz>
To: Paul Menzel <pmenzel+linux-mm@molgen.mpg.de>,
Christoph Lameter <cl@linux.com>,
Pekka Enberg <penberg@kernel.org>,
David Rientjes <rientjes@google.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Kees Cook <keescook@chromium.org>,
Alex Deucher <alexander.deucher@amd.com>
Subject: Re: general protection fault with prefetch_freepointer
Date: Wed, 8 Aug 2018 15:16:46 +0200 [thread overview]
Message-ID: <cc93080f-2d22-71fe-a1fb-d55d1fcc2441@suse.cz> (raw)
In-Reply-To: <4fcc1694-6c29-8c49-1183-fbfb832bf513@suse.cz>
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.
next prev parent reply other threads:[~2018-08-08 13:16 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-18 14:31 general protection fault with prefetch_freepointer Paul Menzel
2018-08-08 11:54 ` Vlastimil Babka
2018-08-08 13:16 ` Vlastimil Babka [this message]
2018-08-09 8:52 ` [PATCH] mm, slub: restore the original intention of prefetch_freepointer() Vlastimil Babka
2018-08-14 0:03 ` Kees Cook
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