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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 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 X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: E86D41C0003 X-Rspam-User: X-Stat-Signature: akjqcfccjzu8sperzkfdferek9jmh1jz X-HE-Tag: 1791902772-626226 X-HE-Meta: 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 hvIVIn2Z pKX4gmy5TeJXGTT2Y2cPtqUHDynDXMVwk28NwhXXxNWGl1goMTqIU9Qntxhv2wRYINaz+eOTaboIOPjE49zl9FmMQ0sCRRLXwbftlB5i6XrHEZvunQPpfBDCZzksm1/aWjtz+FP0MlS/AH6ys3lxk22nK8E5Lzg0OMUVXKjpghsozien7+uEkBPDMje64xiO2ZnOCyUWCFXXHlWl2TxYN5YCrzQco7VvzGn6vFVJKV+VBYaRKoEFHtufs8Xkwnucnc4hBoZysS+tS/s8CvDaMh4/PHFbPgwIKgmCuYPPq74ytVO/FIm8En82ExmPfHDoYfASo1cnFE9fz9K7Wti/cTonEAzsmlH1y7M//yTYpR/ybhHkxPlgvTwLZvlVCTOfuyancKXol+guqr/Q1UCVqhEV456X8SaXA/RufsffyO7m2qhI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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