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Date: Wed, 9 Sep 2015 17:27:16 +0200 From: Vlastimil Babka <vbabka@...e.cz> To: "Kirill A. Shutemov" <kirill@...temov.name>, Sasha Levin <sasha.levin@...cle.com>, Rik van Riel <riel@...hat.com>, Andrew Morton <akpm@...ux-foundation.org> Cc: Andrey Konovalov <andreyknvl@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: Re: Multiple potential races on vma->vm_flags On 09/07/2015 01:40 PM, Kirill A. Shutemov wrote: > On Sun, Sep 06, 2015 at 03:21:05PM -0400, Sasha Levin wrote: >> ================================================================== >> ThreadSanitizer: data-race in munlock_vma_pages_range >> >> Write of size 8 by thread T378 (K2633, CPU3): >> [<ffffffff81212579>] munlock_vma_pages_range+0x59/0x3e0 mm/mlock.c:425 >> [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549 >> [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589 >> [< inlined >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651 >> [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643 >> [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71 >> arch/x86/entry/entry_64.S:186 > > ... > >> Previous read of size 8 by thread T398 (K2623, CPU2): >> [<ffffffff8121d198>] try_to_unmap_one+0x78/0x4f0 mm/rmap.c:1208 >> [< inlined >] rmap_walk+0x147/0x450 rmap_walk_file mm/rmap.c:1540 >> [<ffffffff8121e7b7>] rmap_walk+0x147/0x450 mm/rmap.c:1559 >> [<ffffffff8121ef72>] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423 >> [<ffffffff81211bb0>] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129 >> [<ffffffff81212066>] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331 >> [<ffffffff812128a0>] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476 >> [<ffffffff81212ac9>] mlock_fixup+0x1c9/0x280 mm/mlock.c:549 >> [<ffffffff81212ccc>] do_mlock+0x14c/0x180 mm/mlock.c:589 >> [< inlined >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651 >> [<ffffffff812130b4>] SyS_munlock+0x74/0xb0 mm/mlock.c:643 >> [<ffffffff81eb352e>] entry_SYSCALL_64_fastpath+0x12/0x71 >> arch/x86/entry/entry_64.S:186 > > Okay, the detected race is mlock/munlock vs. rmap. > > On rmap side we check vma->vm_flags in few places without taking > vma->vm_mm->mmap_sem. The vma cannot be freed since we hold i_mmap_rwsem > or anon_vma_lock, but nothing prevent vma->vm_flags from changing under > us. > > In this particular case, speculative check in beginning of > try_to_unmap_one() is fine, since we re-check it under mmap_sem later in > the function. > > False-negative is fine too here, since we will mlock the page in > __mm_populate() on mlock side after mlock_fixup(). > > BUT. > > We *must* have all speculative vm_flags accesses wrapped READ_ONCE() to > avoid all compiler trickery, like duplication vm_flags access with > inconsistent results. Doesn't taking a semaphore, as in try_to_unmap_one(), already imply a compiler barrier forcing vm_flags to be re-read? > I looked only on VM_LOCKED checks, but there are few other flags checked > in rmap. All of them must be handled carefully. At least READ_ONCE() is > required. > > Other solution would be to introduce per-vma spinlock to protect > vma->vm_flags and probably other vma fields and offload this duty > from mmap_sem. > But that's much bigger project. Sounds like an overkill, unless we find something more serious than this. -- 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/
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