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Mon, 26 Oct 2026 12:49:32 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id a5SyJ1xM32rFTAAAD6G6ig (envelope-from ); Mon, 26 Oct 2026 12:49:32 +0000 Date: Mon, 26 Oct 2026 12:49:30 +0000 From: Pedro Falcato To: "Lorenzo Stoakes (ARM)" Cc: Andrew Morton , David Hildenbrand , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , Davidlohr Bueso , SJ Park , Kees Cook , Colin Cross , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH 1/4] mm: do not unintentionally overwrite atomic VMA flags Message-ID: References: <20261023-fix-vma-atomic-v1-0-8ff4fae1fc85@kernel.org> <20261023-fix-vma-atomic-v1-1-8ff4fae1fc85@kernel.org> 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 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261023-fix-vma-atomic-v1-1-8ff4fae1fc85@kernel.org> X-Spamd-Result: default: False [-13.30 / 50.00]; SUSE_INTERNAL_BYPASS(-9.00)[]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; RCPT_COUNT_TWELVE(0.00)[16]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; RCVD_TLS_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; MISSING_XM_UA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo,suse.de:email] X-Spam-Score: -13.30 X-Spam-Level: X-Spam-Flag: NO On Fri, Oct 23, 2026 at 02:47:27PM +0100, Lorenzo Stoakes (ARM) wrote: > madvise, mlock and mprotect take a copy of a VMA's flags holding only the > VMA read lock, later writing it back under the write lock (in > madvise_update_vma(), mlock_fixup(), mlock_vma_pages_range() and > mprotect_fixup()). > > This was done on the assumption that VMA flags cannot change with a read > lock held. > > However this changed in commit 2ab7f1bbafc9 ("mm/madvise: allow guard page > install/remove under VMA lock"), which allowed VM_ATOMIC_SET_ALLOWED > flags (currently only VMA_MAYBE_GUARD_BIT) to be written concurrently under > a VMA read lock. > > The actual write itself is safe as this is done under the VMA write lock > at which point no further concurrent writes can occur, however at this > point the stale flags mean that these callers can unintentionally overwrite > atomic flags incorrectly. > > The consequences of this are significant - a missing VMA_MAYBE_GUARD_BIT > could result in page tables not being copied on fork (the vma_needs_copy() > check can return false) or being overwritten on file-backed THP > collapse (the file_backed_vma_is_retractable() can return true). > > Resolve the issue by always taking care to retain existing atomic flags > when overwriting VMA flags, replacing the inconsistently named > vma_flags_reset_once() with vma_overwrite_flags() to do so, and always > invoking it when flags are overwritten like this. > > Reads are a problem too - madvise, mlock and mprotect all compare VMA flags > without a VMA write lock held via vma_flags_same_pair(). > > This has more serious impact, as concurrent changes to atomic VMA flags can > cause two sets of VMA flags to be wrongly considered different. I really wish we had a new field for atomic flags :/ > > For mprotect and madvise this causes a useless split, but mlock is impacted > more seriously as it can result in mm->locked_vm underflow. > > Fix this by updating vma_flags_same_pair() to disregard > VM_ATOMIC_SET_ALLOWED VMAs - VMA equality is the same as VMA mergeability, > and atomic VMA flags are disregarded when comparing VMA flags for merge - > so this is justified semantically. > > Additionally, remove the redundant vma_flags_same_mask() and the unused > vma_flags_same() as part of the change, and replace VM_ATOMIC_SET_ALLOWED > with VMA_ATOMIC_SET_ALLOWED, using the new VMA flag API. > > Finally, update userland VMA tests to reflect the changes. > > Suggested-by: David Hildenbrand (Arm) > Fixes: 2ab7f1bbafc9 ("mm/madvise: allow guard page install/remove under VMA lock") > Cc: > Signed-off-by: Lorenzo Stoakes (ARM) Anyway, LGTM Reviewed-by: Pedro Falcato -- Pedro