From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EB7A52C9C for ; Tue, 7 Feb 2023 13:06:38 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 51137C433EF; Tue, 7 Feb 2023 13:06:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1675775198; bh=i2h5QWv1pKWSJYkxbpOEQbmV6AZdrWL5NEdW9AwPDLg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=oHFo388V8uOAfZE60S8RSyjwf4IPK6L2szSGeX1b49bS8uKlaDrnV87aDNbbhC1aW pv6k9OK95MqtxAQ+aaTGsCrFxesLbAcW1k5gC+xC3DOrcKfLd0FP6hQ1KywLl7LMib HF9epF+oG4y6MVkzA5wkO16MZU2Py8QTwKUZfMS4= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Jann Horn , Zach OKeefe , "Kirill A. Shutemov" , Yang Shi , David Hildenbrand , Andrew Morton Subject: [PATCH 6.1 169/208] mm/khugepaged: fix ->anon_vma race Date: Tue, 7 Feb 2023 13:57:03 +0100 Message-Id: <20230207125642.069846036@linuxfoundation.org> X-Mailer: git-send-email 2.39.1 In-Reply-To: <20230207125634.292109991@linuxfoundation.org> References: <20230207125634.292109991@linuxfoundation.org> User-Agent: quilt/0.67 Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: Jann Horn commit 023f47a8250c6bdb4aebe744db4bf7f73414028b upstream. If an ->anon_vma is attached to the VMA, collapse_and_free_pmd() requires it to be locked. Page table traversal is allowed under any one of the mmap lock, the anon_vma lock (if the VMA is associated with an anon_vma), and the mapping lock (if the VMA is associated with a mapping); and so to be able to remove page tables, we must hold all three of them. retract_page_tables() bails out if an ->anon_vma is attached, but does this check before holding the mmap lock (as the comment above the check explains). If we racily merged an existing ->anon_vma (shared with a child process) from a neighboring VMA, subsequent rmap traversals on pages belonging to the child will be able to see the page tables that we are concurrently removing while assuming that nothing else can access them. Repeat the ->anon_vma check once we hold the mmap lock to ensure that there really is no concurrent page table access. Hitting this bug causes a lockdep warning in collapse_and_free_pmd(), in the line "lockdep_assert_held_write(&vma->anon_vma->root->rwsem)". It can also lead to use-after-free access. Link: https://lore.kernel.org/linux-mm/CAG48ez3434wZBKFFbdx4M9j6eUwSUVPd4dxhzW_k_POneSDF+A@mail.gmail.com/ Link: https://lkml.kernel.org/r/20230111133351.807024-1-jannh@google.com Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages") Signed-off-by: Jann Horn Reported-by: Zach O'Keefe Acked-by: Kirill A. Shutemov Reviewed-by: Yang Shi Cc: David Hildenbrand Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman --- mm/khugepaged.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1649,7 +1649,7 @@ static int retract_page_tables(struct ad * has higher cost too. It would also probably require locking * the anon_vma. */ - if (vma->anon_vma) { + if (READ_ONCE(vma->anon_vma)) { result = SCAN_PAGE_ANON; goto next; } @@ -1678,6 +1678,18 @@ static int retract_page_tables(struct ad if ((cc->is_khugepaged || is_target) && mmap_write_trylock(mm)) { /* + * Re-check whether we have an ->anon_vma, because + * collapse_and_free_pmd() requires that either no + * ->anon_vma exists or the anon_vma is locked. + * We already checked ->anon_vma above, but that check + * is racy because ->anon_vma can be populated under the + * mmap lock in read mode. + */ + if (vma->anon_vma) { + result = SCAN_PAGE_ANON; + goto unlock_next; + } + /* * When a vma is registered with uffd-wp, we can't * recycle the pmd pgtable because there can be pte * markers installed. Skip it only, so the rest mm/vma