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bh=UKa23txq5VZUcJ3KpkIgUf0mRuHZ2KAGGjulu7Eo99s=; b=UY6yhdWykkfCRRy6ZK+k+gNZnRHDbGZoS0fdr/1JW+GyCqSE81lGzy8Qag0A9z1RALUzXW 6Z7z4QJlNfYtq8Aw== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id 4B696139D2; Tue, 21 Oct 2025 07:04:35 +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 owgTEoMw92h/OwAAD6G6ig (envelope-from ); Tue, 21 Oct 2025 07:04:35 +0000 Message-ID: <1e1376fd-1d38-4dde-918a-d4e937d4feac@suse.cz> Date: Tue, 21 Oct 2025 09:04:35 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts To: Hao Ge , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Suren Baghdasaryan Cc: Shakeel Butt , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hao Ge , stable@vger.kernel.org References: <20251021010353.1187193-1-hao.ge@linux.dev> Content-Language: en-US From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 669F6211E9 X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Spamd-Result: default: False [-4.51 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.cz:s=susede2_rsa,suse.cz: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]; RCPT_COUNT_TWELVE(0.00)[12]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.cz:mid,suse.cz:dkim,kylinos.cn:email]; DKIM_TRACE(0.00)[suse.cz:+]; TO_DN_SOME(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCVD_TLS_ALL(0.00)[]; DNSWL_BLOCKED(0.00)[2a07:de40:b281:106:10:150:64:167:received,2a07:de40:b281:104:10:150:64:97:from]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from] X-Rspamd-Action: no action X-Spam-Flag: NO X-Spam-Score: -4.51 X-Spam-Level: On 10/21/25 03:03, Hao Ge wrote: > From: Hao Ge > > If two competing threads enter alloc_slab_obj_exts() and one of them > fails to allocate the object extension vector, it might override the > valid slab->obj_exts allocated by the other thread with > OBJEXTS_ALLOC_FAIL. This will cause the thread that lost this race and > expects a valid pointer to dereference a NULL pointer later on. > > Update slab->obj_exts atomically using cmpxchg() to avoid > slab->obj_exts overrides by racing threads. > > Thanks for Vlastimil and Suren's help with debugging. > > Fixes: f7381b911640 ("slab: mark slab->obj_exts allocation failures unconditionally") > Cc: > Suggested-by: Suren Baghdasaryan > Signed-off-by: Hao Ge Added to slab/for-next-fixes, thanks! > --- > v3: According to Suren's suggestion, simplify the commit message and the code comments. > Thanks for Suren. > > v2: Incorporate handling for the scenario where, if mark_failed_objexts_alloc wins the race, > the other process (that previously succeeded in allocation) will lose the race, based on Suren's suggestion. > Add Suggested-by: Suren Baghdasaryan > --- > mm/slub.c | 9 ++++++--- > 1 file changed, 6 insertions(+), 3 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index 2e4340c75be2..d4403341c9df 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2054,7 +2054,7 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) > > static inline void mark_failed_objexts_alloc(struct slab *slab) > { > - slab->obj_exts = OBJEXTS_ALLOC_FAIL; > + cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL); > } > > static inline void handle_failed_objexts_alloc(unsigned long obj_exts, > @@ -2136,6 +2136,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > #ifdef CONFIG_MEMCG > new_exts |= MEMCG_DATA_OBJEXTS; > #endif > +retry: > old_exts = READ_ONCE(slab->obj_exts); > handle_failed_objexts_alloc(old_exts, vec, objects); > if (new_slab) { > @@ -2145,8 +2146,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > * be simply assigned. > */ > slab->obj_exts = new_exts; > - } else if ((old_exts & ~OBJEXTS_FLAGS_MASK) || > - cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) { > + } else if (old_exts & ~OBJEXTS_FLAGS_MASK) { > /* > * If the slab is already in use, somebody can allocate and > * assign slabobj_exts in parallel. In this case the existing > @@ -2158,6 +2158,9 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > else > kfree(vec); > return 0; > + } else if (cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) { > + /* Retry if a racing thread changed slab->obj_exts from under us. */ > + goto retry; > } > > if (allow_spin)