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Tue, 13 Aug 2024 13:25:40 -0700 (PDT) Received: from localhost ([2a00:79e0:9d:4:a608:a4cb:f4c2:6573]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-429dea45445sm943535e9.6.2024.08.13.13.25.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 13 Aug 2024 13:25:40 -0700 (PDT) From: Jann Horn Subject: [PATCH v2 0/2] userfaultfd: fix races around pmd_trans_huge() check Date: Tue, 13 Aug 2024 22:25:20 +0200 Message-Id: <20240813-uffd-thp-flip-fix-v2-0-5efa61078a41@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-B4-Tracking: v=1; b=H4sIADDBu2YC/32NQQrCMBBFr1Jm7UgmVLSuvId0YZNJMlCbktSil Nzd2AO4+fA+/Pc3yJyEM1ybDRKvkiVOFfShARMek2cUWxm00q26kMaXcxaXMKMbpYa8USvXkSM 60dBB3c2Ja707733lIHmJ6bNfrPRr/9lWQoWtM2SHszHWqpuP0Y98NPEJfSnlC91gx0CzAAAA To: Andrew Morton , Pavel Emelianov , Andrea Arcangeli , Hugh Dickins Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, David Hildenbrand , Qi Zheng , Jann Horn , stable@vger.kernel.org X-Mailer: b4 0.15-dev X-Developer-Signature: v=1; a=ed25519-sha256; t=1723580736; l=2085; i=jannh@google.com; s=20240730; h=from:subject:message-id; bh=uPZbHC8CGUlf/x8Q+x+KQ3vLMFUd1MWESPzfxfGXEFE=; b=T2zzW+NCfodiI+EDaSAYC8DlWgR0OKZs3VhkAe8fiH7o38AHgADYuunZE4nL6DePHkDrsi+Nq V7e6FCKZvtHDSpQhBGqViYvRbDaHTR1ErIRFdP5Ddtww9JAY/XVgJpQ X-Developer-Key: i=jannh@google.com; a=ed25519; pk=AljNtGOzXeF6khBXDJVVvwSEkVDGnnZZYqfWhP1V+C8= 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 --- 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