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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=blPOGtHH; 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 91EE2606A31B; Sun, 25 Oct 2026 10:57:04 +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=blPOGtHH Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1336A1F01825; Sun, 25 Oct 2026 10:57:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792925824; bh=WxGHp2cRhL5SwPo0OwBRfhp0Jz+5ULz4/oEe6eLF81A=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=blPOGtHHu1tq3dB00Ujh+vtvVYU+cG9XGGANoibb6V930ieUlGbThJR+UuVR5cEbd VDxHHioU5Hje8/Bmd1zSATXRtXaqJ9kl4uv8M/A2r1CF7seZT7bVSEoy53/lqc5FJU hjOb1vaWSnLDDFQP7W6iS9d+m9w9BjQqTReqGEnGK0c++AFP05BLJJZ6qpqfHYt0we tDS/J1Oom1CD5WxjzTCiI+tVn5uu8mJ6/9kWgudDMDN9VJvTniA8vjcs5F0p2RdjqQ VnkTzliosOQ4wMTZFG7/8Lwjt8/PHd78lWpeF3K2r+jObJgsTzLeBkEVrVTjApt3vN QRD3NcP3ba2ZA== Message-ID: Date: Sun, 25 Oct 2026 11:56:16 +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: Ngoc Quy , akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org Cc: 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> 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 10/14/26 11:55, 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. > > Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs") > Cc: stable@vger.kernel.org > Assisted-by: Claude:claude-opus-5 > Signed-off-by: Ngoc Quy > --- > fs/proc/task_mmu.c | 9 +++++++-- > 1 file changed, 7 insertions(+), 2 deletions(-) > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > index 5c54aebe2118..8825b01f5e38 100644 > --- 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; > + > ret = pagemap_scan_get_args(&p.arg, uarg); > if (ret) > - return ret; > + goto out_mmput; > > p.masks_of_interest = p.arg.category_mask | p.arg.category_anyof_mask | > p.arg.return_mask; > ret = pagemap_scan_init_bounce_buffer(&p); > if (ret) > - return ret; > + goto out_mmput; > > for (walk_start = p.arg.start; walk_start < p.arg.end; > walk_start = p.arg.walk_end) { > @@ -3294,6 +3297,8 @@ static long do_pagemap_scan(struct mm_struct *mm, unsigned long uarg) > ret = -EFAULT; > > kfree(p.vec_buf); > +out_mmput: > + mmput(mm); > return ret; > } Acked-by: David Hildenbrand (Arm) -- Cheers, David