linux-kernel - Re: mm: compaction: buffer overflow in isolate_migratepages

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: <CAPAsAGwk7kF6XtJNz6Y41zn0SHHzEt1Nwi_wC0gWgt0fpdp-ZQ@mail.gmail.com>
Date:	Thu, 14 Aug 2014 22:07:40 +0400
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Rafael Aquini <aquini@...hat.com>,
	Konstantin Khlebnikov <koct9i@...il.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Dave Jones <davej@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrey Ryabinin <a.ryabinin@...sung.com>
Subject: Re: mm: compaction: buffer overflow in isolate_migratepages_range

2014-08-14 19:13 GMT+04:00 Rafael Aquini <aquini@...hat.com>:
>> Yeah, it happens because I failed to anticipate a race window opening where
>> balloon_page_movable() can stumble across an anon page being released --
>> somewhere in the midway of __page_cache_release() & free_pages_prepare()
>> down on the put_page() codepath -- while isolate_migratepages_range() performs
>> its loop in the (lru) unlocked case.
>>
>
> Giving it a second thought, I see my first analisys (above) isn't accurate,
> as if we had raced against a page being released at the point I mentioned,
> balloon_page_movable() would have bailed out while performing its
> page_flags_cleared() checkpoint.
>
> But I now can see from where this occurrence is coming from, actually.
>
> The real race window for this issue opens when balloon_page_movable()
> checkpoint @ isolate_migratepages_range() stumbles across a (new)
> page under migration at:
>
> static int move_to_new_page(struct page *newpage, struct page *page, ...
> {
>    ...
>    newpage->mapping = page->mapping;
>
>
> At this point, *newpage points to a fresh page coming out from the allocator
> (just as any other possible ballooned page), but it gets its ->mapping
> pointer set, which can create the conditions to the access (for mapping flag
> checking purposes only) KASAN is complaining about, if *page happens to
> be pointing to an anon page.
>
>
>> Although harmless, IMO, as we only go for the isolation step if we hold the
>> lru lock (and the check is re-done under lock safety) this is an
>> annoying thing we have to get rid of to not defeat the purpose of having
>> the kasan in place.
>>
>
> It still a harmless condition as before, but considering what goes above
> I'm now convinced & confident the patch proposed by Andrey is the real fix
> for such occurrences.
>

I don't think that it's harmless, because we could cross page boundary here and
try to read from a memory hole.
And this code has more potential problems like use after free. Since
we don't hold locks properly here,
page->mapping could point to freed struct address_space.

We discussed this with Konstantin and he suggested a better solution for this.
If I understood him correctly the main idea was to store bit
identifying ballon page
in struct page (special value in _mapcount), so we won't need to check
mapping->flags.


-- 
Best regards,
Andrey Ryabinin
--
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/