From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-214.mta0.migadu.com [91.218.175.214]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 80E3223395D for ; Sun, 18 Oct 2026 06:43:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.214 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792305796; cv=none; b=oxTqF4ESJTsjXPpCTkpgxmOsCNkXwZm6WM6Fr2IDDP0+AEp2qqi0PHPkp5jisKrbNcjgi0pY3wloL8ytThRonV+9bGAf0jmWtcWQ/ZuqlNaLUq3RIHR2v0JAoj4YF0dJ5yny6y6T6YZFby3Iml/dphaorrmXCFNB1ZNiWQLKYc8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792305796; c=relaxed/simple; bh=Uw0YIVaZtfH6o8cYYNicP3BPPfnY5BFJtNnGIlhTWb4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=RwoHGEKmlsqA+SijuQ7bSknaR3dUON68jWu9MaljiK8oH4NNy49qnAix3d2K/yVb3+BzbbxmHk2NII7QSFASDCUzUPrqWKXNCnc4AGN33OgFI4QRZ/4RDAyr9IeoYm4y7ok4NqqkYnd63Pta32YsfpWkZvQX1sJ36v2HsCpvzW4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=wzQtTrpD; arc=none smtp.client-ip=91.218.175.214 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="wzQtTrpD" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=Uw0YIVaZtfH6o8cYYNicP3BPPfnY5BFJtNnGIlhTWb4=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1792305791; v=1; x=1792910591; b=wzQtTrpDmaMrrqBaJyyLJamCuw7QHX2tgQT3VnVCIYmoDpc8KhryI/vrM+dr4djjC7+qDXLu 4QPBmwW5RFy2jMn+4Z9tGAN9OlHmRPTzGY/1HmxfkQvHzSo+vCADmFthl0XkHPjEkWK8cprJG6W IfPw3eZuxm5jhw8R6ApeP3sU= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 0bd3a34f4381111f; Sun, 18 Oct 2026 06:43:10 +0000 X-Mizu-Trace-ID: 0bd3a34f4381111f X-Migadu-Flow: FLOW_OUT From: Lance Yang To: david@kernel.org Cc: akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, muchun.song@linux.dev, osalvador@suse.de, linmiaohe@huawei.com, nao.horiguchi@gmail.com, agl@us.ibm.com, mel@csn.ul.ie, riel@surriel.com, peterx@redhat.com, kas@kernel.org, rppt@kernel.org, josef@toxicpanda.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, mason@kernel.org, stable@vger.kernel.org Subject: Re: [PATCH 04/10] mm/hugetlb: unmap the page where the other VMA maps it Date: Sun, 18 Oct 2026 14:43:04 +0800 Message-ID: <20261018064304.17357-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20261015-urgent_fixes-v1-4-42796911043b@kernel.org> References: <20261015-urgent_fixes-v1-4-42796911043b@kernel.org> 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: 8bit On Thu, Oct 15, 2026 at 06:07:10PM +0200, David Hildenbrand (Arm) wrote: >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) >--- Good catch1 Reviewed-by: Lance Yang