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Message-Id: <20240813-uffd-thp-flip-fix-v2-0-5efa61078a41@google.com>
Date: Tue, 13 Aug 2024 22:25:20 +0200
From: Jann Horn <jannh@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
Pavel Emelianov <xemul@...tuozzo.com>,
Andrea Arcangeli <aarcange@...hat.com>, Hugh Dickins <hughd@...gle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
David Hildenbrand <david@...hat.com>, Qi Zheng <zhengqi.arch@...edance.com>,
Jann Horn <jannh@...gle.com>, stable@...r.kernel.org
Subject: [PATCH v2 0/2] userfaultfd: fix races around pmd_trans_huge()
check
The pmd_trans_huge() code in mfill_atomic() is wrong in three different
ways depending on kernel version:
1. The pmd_trans_huge() check is racy and can lead to a BUG_ON() (if you hit
the right two race windows) - I've tested this in a kernel build with
some extra mdelay() calls. See the commit message for a description
of the race scenario.
On older kernels (before 6.5), I think the same bug can even
theoretically lead to accessing transhuge page contents as a page table
if you hit the right 5 narrow race windows (I haven't tested this case).
2. As pointed out by Qi Zheng, pmd_trans_huge() is not sufficient for
detecting PMDs that don't point to page tables.
On older kernels (before 6.5), you'd just have to win a single fairly
wide race to hit this.
I've tested this on 6.1 stable by racing migration (with a mdelay()
patched into try_to_migrate()) against UFFDIO_ZEROPAGE - on my x86
VM, that causes a kernel oops in ptlock_ptr().
3. On newer kernels (>=6.5), for shmem mappings, khugepaged is allowed
to yank page tables out from under us (though I haven't tested that),
so I think the BUG_ON() checks in mfill_atomic() are just wrong.
I decided to write two separate fixes for these (one fix for bugs 1+2,
one fix for bug 3), so that the first fix can be backported to kernels
affected by bugs 1+2.
Signed-off-by: Jann Horn <jannh@...gle.com>
---
Changes in v2:
- in patch 1/2:
- change title
- get rid of redundant early pmd_trans_huge() check
- also check for swap PMDs and devmap PMDs (Qi Zheng)
- Link to v1: https://lore.kernel.org/r/20240812-uffd-thp-flip-fix-v1-0-4fc1db7ccdd0@google.com
---
Jann Horn (2):
userfaultfd: Fix checks for huge PMDs
userfaultfd: Don't BUG_ON() if khugepaged yanks our page table
mm/userfaultfd.c | 29 ++++++++++++++++-------------
1 file changed, 16 insertions(+), 13 deletions(-)
---
base-commit: d4560686726f7a357922f300fc81f5964be8df04
change-id: 20240812-uffd-thp-flip-fix-20f91f1151b9
--
Jann Horn <jannh@...gle.com>
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