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 9A29E42255C; Tue, 13 Oct 2026 14:46:12 +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=1791902773; cv=none; b=nBXgKuCp8VX+VHPIBE2PxLhWftuokO8UexGcToDdyEVf3FTRyXDH4bseOSYvEZ8k8zOEYRaDVbB8ImAeUZHW5KS8TkLls5rjepdBnxemfggyhy+hBEDwlQpascEsAYhkC07CG889iW3f2MMhYpx26OsmOOimNY8t/+kRPOgNKno= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791902773; c=relaxed/simple; bh=jjpBFTErSy0osbl4v/wsww4m1OSHMX3EC4VBJahpAfU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=hEL9daA8WxpaZEYbJOI+4HEKTRb3yqPXGAjfEr5oaEZ4lbCUwIX9lqTqSaA6EdG5IVxNUKzXdaSEFYFDvNRwp20zjUmw9UNfF3CnW3/VOEoWjWaAAJvzIId+rl44UWlyvs9h3E6aslsz7FSKImzJoi5HraYpv6j4pofA7TS2KqE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=R325y+e8; 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="R325y+e8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4B0B01F000FF; Tue, 13 Oct 2026 14:46:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791902772; bh=4F6Tw46VkpNnGRzthbkmTP21wgcYOdI2X0+e0mUpCFk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=R325y+e8Sm80qlgGpI6ejkQ0qXq2S1XHtB2tkNvLNBtXYPvujVNORQtOkp02R5DAm uvOzEBFct97UIjQUblh6DAlGRtaaw4zPKE+tTW1eU5A4ll4wuM3t4quqTJtl++2jbL x4AKLA7MIns2uDvWr493MCaD4Q4CSY7n2RvxVqBKWeBby4LSQNh3UT5i9FnwE/Kl3j KNWjrnDtzahxrqSrcBcLhQVBZgBSdLZNpS03271OAhLApWyapgdhc8GmTkp8AaVPQH mrmhjqkOn+R/15+ePbn0Bt9JdhPb6K7rbX3v7gQHh/CDDMIJRPvEaJ2nv77LyWTM9x eYIKbtEfCSX8w== Message-ID: <154d653f-c6ab-4357-b823-a345a8524470@kernel.org> Date: Tue, 13 Oct 2026 16:46:06 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] userfaultfd: fix UFFDIO_MOVE livelock on large folio with extra references To: "Cen Zhang (Microsoft)" , Andrew Morton , Mike Rapoport Cc: Peter Xu , Suren Baghdasaryan , Andrea Arcangeli , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, AutonomousCodeSecurity@microsoft.com, tgopinath@linux.microsoft.com References: <20261012201514.97015-1-cenzhang@linux.microsoft.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/12/26 22:15, Cen Zhang (Microsoft) wrote: > To move one page out of a large folio, move_pages_ptes() first has to > split the folio. __folio_split() refuses if the folio has any > reference beyond its page table mappings and the caller's own, and it > reports the refusal as -EAGAIN. move_pages() treats returned -EAGAIN > as a transient race and keeps retrying the same address, holding the VMA > read locks the whole time. This is incorrect since in some cases > userspace can hold such a reference indefinitely, such as by > registering the page as an io_uring buffer. > > An unprivileged user can trigger this with three threads: first holds > such a reference on a page of a THP, second calls UFFDIO_MOVE on another > page of the same THP and spins in the kernel forever holding the VMA > read locks, and third calls fork() or mprotect(), which takes mmap_lock > for writing and then waits forever for those read locks. From then on > every mmap_lock user of that mm hangs, including other processes > inspecting it through /proc//, until the process is SIGKILLed. > The hung task detector reports the blocked tasks (third one), and with > kernel.hung_task_panic=1 the machine panics: > > INFO: task wedge:95 blocked for more than 122 seconds. > task:wedge state:D stack:12528 pid:95 tgid:95 ppid:94 > Call Trace: > __schedule+0x596/0x1110 > schedule+0x36/0xf0 > __vma_start_exclude_readers+0x100/0x170 > __vma_start_write+0x51/0xd0 > __split_vma+0x208/0x3d0 > vma_modify+0x6a8/0x840 > vma_modify_flags+0xce/0x110 > mprotect_fixup+0xbe/0x480 > do_mprotect_pkey+0x2d9/0x580 > __x64_sys_mprotect+0x1a/0x30 > > Fix by returning -EBUSY instead of -EAGAIN in move_pages_ptes(), the > caller of split_folio(), when the split fails. Thus, move_pages() > does not retry -EBUSY and the ioctl drops its locks and returns to > userspace instead of spinning. > > Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI") > Reported-by: AutonomousCodeSecurity@microsoft.com > Assisted-by: LLM > Signed-off-by: Cen Zhang (Microsoft) > --- > mm/userfaultfd.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > index 74f04c323c50..500962be1162 100644 > --- a/mm/userfaultfd.c > +++ b/mm/userfaultfd.c > @@ -1663,8 +1663,12 @@ static long move_pages_ptes(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd > pte_unmap(dst_pte); > src_pte = dst_pte = NULL; > ret = split_folio(src_folio); > - if (ret) > + if (ret) { > + /* do not retry, extra refs may be held forever */ > + if (ret == -EAGAIN) > + ret = -EBUSY; > goto out; > + } > /* have to reacquire the folio after it got split */ > folio_unlock(src_folio); > folio_put(src_folio); Yeah, that makes sense to me. Acked-by: David Hildenbrand (Arm) -- Cheers, David