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b=QMXrqqMso456ITXiNZr90kPAO74wrbYx+z54a3x+l9eEMuCfAezKeCbqq2Y+wX32C 0qz/HU8V/DjMllMVNsZ7dxS2kR7tjv05cgXujnvcOFDLsJgM6noFufS+AkA9cfsGBg 0C85Lpvl1/EXlcjibTd/iJTpsmVptJ+cf7juyho5braCwFt0YC5MlKZfE4ASGGgeSx JjecmP1+6N+QaI9Kppdd/CDp5h3GuqJBbtBE4rrInZxLZpDvcd3p/7+Xcl7UGbkQot QXQZQwBFz+bxBlFkiGuMQ5BrHn3TDOa7ZpsXKfvZyeHiTnvEKf1Uu/Ty6n4qEBXv+y 8Z1Dbxv9k8c7Q== Message-ID: <9aa49812-1132-458e-9bf5-6dba8b476b26@kernel.org> Date: Sun, 25 Oct 2026 11:55:59 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] fs/proc/task_mmu: pin mm_users in do_pagemap_scan() To: Andrew Morton , Ngoc Quy Cc: liam@infradead.org, ljs@kernel.org, Vlastimil Babka , Jann Horn , Pedro Falcato , Muhammad Usama Anjum , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, stable@vger.kernel.org References: <20261014095540.3261469-1-ngocquy0700@gmail.com> <20261019174659.42c3529d5e35563cc6e5244c@linux-foundation.org> 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-Queue-Id: 84DA91C0006 X-Rspam-User: X-Stat-Signature: x35cj71sdmyjgcz955pdu6naq44ygz1e X-Spam-Status: No, score=-12.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam03 X-HE-Tag: 1792925766-684227 X-HE-Meta: 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 sfX/kMbE g8SadmV86rXngYqMKAv7mePox3cnYA7AcEyPdDvLbZxNjfwUlVwDrA9sD/2uNHZlouiNIW3muJWF3Wor2qAxJJlyWoO8E4sskomjxxmU+LQSkICt80DHCIFuLRUySHRxnHngpoAluNBEil1YO77CX+Rip35XYRVvVY1BpBRUIODnNmcLOrdJImkQmDDJyz3JqYudSSmb5W0i2/B3Vxh01TO/1gEk9MLZtOY7dkCKaEnok/hSpKDzwQ8K160fLi7m1xRX8pVyTRrpOf3XL3qvcLlmUyXrzBHiMa9UDKpZwiQlNGsgkvEzI5yO+YLX2qePvqCPMRRL1nYUAp8M= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 10/20/26 02:46, Andrew Morton wrote: > On Wed, 14 Oct 2026 16:55:40 +0700 Ngoc Quy wrote: > >> The PAGEMAP_SCAN ioctl takes mmap_read_lock() on an mm obtained from >> /proc//pagemap without holding a reference to mm_users. >> >> proc_mem_open() deliberately grabs only mm_count, so a consumer that >> touches the address space has to call mmget_not_zero() first. Every >> other consumer in this file does -- m_start(), do_procmap_query(), >> show_smaps_rollup() and pagemap_read() -- but do_pagemap_scan() does >> not. >> >> Without that reference the target can reach exit_mmap() concurrently, >> and exit_mmap() calls unmap_vmas() while holding mmap_lock only for >> reading, which does not exclude the scanner. For ordinary VMAs this is >> harmless, as free_pgtables() and remove_vma() run afterwards under >> mmap_write_lock. A shared hugetlb VMA is different: __hugetlb_zap_end() >> frees the VMA's hugetlb_vma_lock inside the read-locked phase. >> >> walk_hugetlb_range() can therefore load vma->vm_private_data, have the >> lock freed underneath it, and then down_read() a freed rw_semaphore. >> The load happens before the freeing side clears the pointer, so this is >> a use-after-free and not a NULL dereference. Any unprivileged user can >> reach it with no huge pages configured, as hugetlb_reserve_pages() >> allocates the lock before the VM_NORESERVE early return. >> >> Pin mm_users across the scan, the way the other consumers do. mmput() >> from the exiting task can then no longer reach exit_mmap() while a walk >> is in progress. > > All looks good and checks out for me, thanks. > > Except... > >> --- a/fs/proc/task_mmu.c >> +++ b/fs/proc/task_mmu.c >> @@ -3231,15 +3231,18 @@ static long do_pagemap_scan(struct mm_struct *mm, unsigned long uarg) >> size_t n_ranges_out = 0; >> int ret; >> >> + if (!mm || !mmget_not_zero(mm)) >> + return -ESRCH; > > Why the new test for !mm? The current call chain guarantees this is a > cant-happen and the current do_pagemap_scan will cheerfully oops if > mm==NULL. I'd just keep it for consistency with all the other checks in this file? -- Cheers, David