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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=QMXrqqMs; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1D86F606A31A; Sun, 25 Oct 2026 10:56:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=QMXrqqMs Received: by smtp.kernel.org (Postfix) with ESMTPSA id CA25A1F01825; Sun, 25 Oct 2026 10:56:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792925765; bh=68QJafW62HgkjotUsbneO0EWoKRkIg6fDkzU/Qf+g1s=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list 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 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