From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 2A9DD53ECFB; Thu, 15 Oct 2026 16:07:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792080484; cv=none; b=H7jPtn1RbaxXLbgbYQFBr1gEoI2x4AbY5jrqSmhBAm0jZ/RwpOykPC8ZSEagsPDY00aA36cwt8Pxf3xEbFM/sIZa4mv719ZeUMOD7mUBPzanc6Tdou1hDvdQyOsMYqyzo3vjRYMhmqYAHSIaWr8WzfWGbVMFHbdrc0t9mp6Zbb4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792080484; c=relaxed/simple; bh=79SeBC4+XmuWTh61NJmw5Joul9l+RNS4U0+XJ/nMcHE=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=DL7ySaj+X9TpGeYF/qfBsOM62UIzjNkxvxs1V6+2p21lAKfZ7nWPTSPvywXsSjfZpvVDFbmZBg9CeagfFuPZF6eL34R0DNxstYJQzzRwI5YWDUods17HD7BB4Zh1MRxi/+IzkdIb268skj5njQzH0Yg0B10/fL2zEZmZDszLagc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=jszaSfE6; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="jszaSfE6" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E52151F000C4; Thu, 15 Oct 2026 16:07:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792080467; bh=tTPYoMFTTDcUOlKvocFSekvL0GWvlMdCBCB0mVspP+0=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=jszaSfE6JAU7Q5ThVr7+FWwpwsAeeGIgE3q35qW4nu2TEhjPyeR0W92lkSjFmaJ4c a+jFyOA5hVkGt9ZtlWTG4XYFJYvwrpyUM/Nn7f5ODZFagN+iiWe4lET1tgiBZBlvxh zFARwZn5eIqZPmLbUQsoerde49acYIs5NFaBQzTALwkqIwOuF8wXKJu8G41XZECrpd tyTnxQ6hxd+yap1SbXQYxqesFTEVO2uXiQvDx3SJlzrKT9xlduh4gZcyS0rvwjvIBI dhhPsfYYi2BM02toCPKkpqZkaKJTpfh0LxTilM2q3kQiaru3Lt87uHNuqog3xO2EL4 RJKVKsOpgYpeQ== From: "David Hildenbrand (Arm)" Date: Thu, 15 Oct 2026 18:07:10 +0200 Subject: [PATCH 04/10] mm/hugetlb: unmap the page where the other VMA maps it 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 Message-Id: <20261015-urgent_fixes-v1-4-42796911043b@kernel.org> References: <20261015-urgent_fixes-v1-0-42796911043b@kernel.org> In-Reply-To: <20261015-urgent_fixes-v1-0-42796911043b@kernel.org> To: Andrew Morton , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Pedro Falcato , Muchun Song , Oscar Salvador , Miaohe Lin , Naoya Horiguchi , Adam Litke , Mel Gorman , Rik van Riel , Peter Xu , "Kiryl Shutsemau (Meta)" , "Mike Rapoport (Microsoft)" , "Liam R. Howlett" , Josef Bacik Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, "David Hildenbrand (Arm)" , Chris Mason , stable@vger.kernel.org X-Mailer: b4 0.16.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=5795; i=david@kernel.org; h=from:subject:message-id; bh=9QOVq0s2+CpnNKu9MuupRapnouHfqvMDPJ5hA3vXoiA=; b=owGbwMvMwCF2/OVpCdfQ3ZWMp9WSGLIu/DKOuipbV7jY5bdE4XK5Dy1RP+e9+pOccpLZSvGJk kxGi6xQRykLgxgHg6yYIktQ8H2Dly/X1Va78THDzGFlAhnCwMUpABMJ2sHIsLyIP0Bxws2PJuXp P3J/rJQ6PYvl1jKmPXzhb3/zsVmHJzEyvK885ns54BzX4/eFndN5vsj1i6TPv8ohvdLFfPvdhX5 fWAE= X-Developer-Key: i=david@kernel.org; a=openpgp; fpr=1BD9CAAD735C4C3A460DFCCA4DDE10F700FF835A From: Josef Bacik If the owner of a MAP_PRIVATE hugetlb mapping fails to allocate a page for CoW, unmap_ref_private() unmaps the old page from all other private VMAs that have no reservation of their own. It finds them by the offset in the file, but then zaps each of them at the address of the faulting VMA. Nothing says that another VMA maps the offset at that address: a child can have moved its copy of the VMA with mremap(), and a second mapping of the file is wherever mmap() has put it. So __unmap_hugepage_range() walks the page tables of the other mm at an address outside of iter_vma, where i_mmap_rwsem of this file protects nothing and where anything can be mapped, and the page stays mapped where it really is. All of the following was seen on v7.3-rc6+ (g6c377d19d4a5) in KVM guests, as a user without any privilege. It takes a pool of 2 MiB pages with no free page that is not reserved, so that the allocation fails: the user can take or reserve what is free itself. - The child moves its VMA away. The page is never unmapped, the mapcount stays 2, and the owner goes round retry_avoidcopy in hugetlb_wp() until the child is gone or a page becomes free. SIGKILL to the owner does not end it. 5 of 5 runs. - The owner's mm has a second private mapping of the offset, made with MAP_NORESERVE. The owner zaps its own entry and reads zeroes where its data was. 5 of 5 runs. - The child moves its VMA away and maps a 1 GiB hugetlb page at the old address, so this one takes a pool with a 1 GiB page as well. huge_pte_offset() for the 2 MiB size steps through the PUD leaf, so the "PMD table" is the 1 GiB page itself and its "page table lock" is folio->private of that page, which holds 2. Kernels with lock debugging oops in __unmap_hugepage_range(), 7 of 7 runs. Others spin on that word for ever with preemption disabled, 5 of 5 runs: watchdog: BUG: soft lockup - CPU#0 stuck for 26s! [u_det3:130] RIP: 0010:queued_spin_lock_slowpath+0x8a/0x290 RAX: 0000000000000002 RBX: ffffea0001000028 Call Trace: __unmap_hugepage_range+0x154/0x6d0 unmap_hugepage_range+0xc1/0x140 hugetlb_wp+0x55c/0x700 hugetlb_fault+0x5f3/0x8f0 RCU stalls follow, and killing all tasks of the user changes nothing. Four such owners, whose children share a single 1 GiB page, took down the whole guest with its four CPUs, 2 of 2 runs: one reported soft lockups on all four, the other gave no sign of life any more. i_mmap_rwsem stays write locked. With a MAP_PRIVATE mapping of a hugetlbfs file that belongs to another user, and that the attacker can only read, the next mmap() of that file by its owner sleeps uninterruptibly in __mmap_region(), 4 of 4 runs. Compute the address from the offset for each VMA. The tree walk returns only VMAs that cover pgoff, and vm_start, vm_end and vm_pgoff of a hugetlb VMA are aligned to the huge page size, as pgoff is, so the whole range lies inside iter_vma. Its page tables can not go away there, since the VMA is unlinked from the file under i_mmap_rwsem first. With the right address the page is unmapped from all VMAs but the owner's, so the retry in hugetlb_wp() finds a mapcount of 1 and reuses the page. A child that has moved its copy of the VMA now loses the page, and gets SIGBUS when it faults on a missing page of that VMA. That is what the comment above the function has always promised to those who do not own the reservation, and what already happens to a child that has left the VMA in place. The address has been wrong from the start, and the second case needs only mmap() and fork(). mremap() of hugetlb VMAs, which the other cases use, came with commit 550a7d60bd5e ("mm, hugepages: add mremap() support for hugepage backed vma") in v5.16. Original fix generated by an LLM and sent in private by Josef. Fixes: 04f2cbe35699 ("hugetlb: guarantee that COW faults for a process that called mmap(MAP_PRIVATE) on hugetlbfs will succeed") Reported-by: Chris Mason Cc: stable@vger.kernel.org Signed-off-by: Josef Bacik [ david: switch to vma_filebacked_address, add VM_WARN_ON_ONCE, refine comments] Reviewed-by: Lorenzo Stoakes (ARM) Signed-off-by: David Hildenbrand (Arm) --- mm/hugetlb.c | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 86928d76ef12d..86fafd0782987 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5440,6 +5440,8 @@ static void unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma, struct address_space *mapping; pgoff_t pgoff; + VM_WARN_ON_ONCE(!folio_test_anon(folio)); + /* * vm_pgoff is in PAGE_SIZE units, hence the different calculation * from page cache lookup which is in HPAGE_SIZE units. @@ -5474,10 +5476,17 @@ static void unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma, * could insert a zeroed page instead of the data existing * from the time of fork. This would look like data corruption */ - if (!is_vma_resv_set(iter_vma, HPAGE_RESV_OWNER)) - unmap_hugepage_range(iter_vma, address, - address + huge_page_size(h), - folio, 0); + if (is_vma_resv_set(iter_vma, HPAGE_RESV_OWNER)) + continue; + + /* + * The address in other VMAs can differ. Even though our old + * folio is anon, we always operate on the underlying hugetlb + * file. + */ + address = vma_filebacked_address(iter_vma, pgoff, 1); + unmap_hugepage_range(iter_vma, address, + address + huge_page_size(h), folio, 0); } i_mmap_unlock_write(mapping); } -- 2.43.0