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Tue, 3 Nov 2026 12:13:00 +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 MgVXMsvP6WqucAAAD6G6ig (envelope-from ); Tue, 03 Nov 2026 12:13:00 +0000 Date: Tue, 3 Nov 2026 12:12:58 +0000 From: Pedro Falcato To: "Jose Fernandez (Anthropic)" Cc: "Matthew Wilcox (Oracle)" , Jan Kara , Andrew Morton , David Hildenbrand , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Krystian Kaniewski , stable@vger.kernel.org Subject: Re: [PATCH v2] mm/filemap: stop fault-around when the walk steps back Message-ID: References: <20261031-jf-mm-filemap-map-pages-lower-bound-v2-1-21029744e589@linux.dev> 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: <20261031-jf-mm-filemap-map-pages-lower-bound-v2-1-21029744e589@linux.dev> X-Rspamd-Queue-Id: 078272203B X-Spam-Level: X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Action: no action X-Spam-Flag: NO X-Spamd-Result: default: False [-13.51 / 50.00]; SUSE_INTERNAL_BYPASS(-9.00)[]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; RCVD_VIA_SMTP_AUTH(0.00)[]; MIME_TRACE(0.00)[0:+]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; MISSING_XM_UA(0.00)[]; ARC_NA(0.00)[]; RCVD_TLS_ALL(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[infradead.org,suse.cz,linux-foundation.org,kernel.org,vger.kernel.org,kvack.org,gmail.com]; RCPT_COUNT_SEVEN(0.00)[10]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,suse.de:dkim]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.de:+] X-Spam-Score: -13.51 On Sat, Oct 31, 2026 at 04:33:59AM +0000, Jose Fernandez (Anthropic) wrote: > filemap_map_pages() can dereference a PTE pointer outside the page table > it locked. KASAN has reported this as a use-after-free read [1]. The > same path can also write out of bounds. When the stray entry reads as > none, the fault-around loop maps a page there with set_pte_range(). > > Its fault-around loop walks the page cache and moves vmf->pte by the > change in index. A lower index moves it back. xas_find() can step back > to a lower index when a multi-index entry is split during the walk. > > [1] fixes xas_find(), but the loop still relies on the walk never > stepping back. Stop the fault-around when the walk steps back. ->fault() > maps the faulting page if the loop did not. I'm not sure I understand. xas_find() was fixed. Is there any other case where this loop can step backwards? If not, I don't think this should be Cc: stable, and should probably be a WARN_ON(). As it's a hardening patch and not an actual bugfix. -- Pedro