From nobody Sat Feb 7 20:44:12 2026 Received: from out-174.mta0.migadu.com (out-174.mta0.migadu.com [91.218.175.174]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 81FB8143C61 for ; Tue, 21 Oct 2025 01:05:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.174 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761008704; cv=none; b=kSsZNmGYUotv64Yn7FbcVEiwBicc0x/5DR5LabhWsASekUl/J6RGWSVt8B5/LWkP/AQnvoyKr432L9lPhhFtMwuEjz7ojvW7vB78cGpIOHeInijllcfFryGLHrLV/tInOfXk4LFrdubi+FNVWMstgvoC4ZUQWeOkLhVnCc6Y7Ao= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761008704; c=relaxed/simple; bh=6aN2ovqyybXzCMwblB9oyJ2EfGZvvf/XwIjAojm4NZs=; h=From:To:Cc:Subject:Date:Message-Id:MIME-Version; b=m/0xxYPYIt3uz67ftCPANsK3ZPmPbZcBj+ILfJMCnm1nMBsFXSu0vGSBw66C/GTbEyQPL2CaaCtK3hkIo9YJUm9pU1TWC74s6T3LZFZ+4U4hfEJWxay2DBwmcEcyNtWzX0lFIlAU5MAfXJ7TIEr0kTG8LcIsH7cmCxCFxeRGUK8= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=TqylpQ8j; arc=none smtp.client-ip=91.218.175.174 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="TqylpQ8j" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1761008689; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=/uhNugh/EUyo0G9PtnWbQz5kQB6FYcMFt9ZcmcW3jq8=; b=TqylpQ8jaM+tbH6RZRwPSgDqSyExtn/o1YOdg6CC3a+gfj0xGkpuyk1O6LrMagcfAc88iq erQ6AWTHO0yiIyOSjRHExx+Oz54RKFO4QVqX/mKymo0e/bl2mj0+vR7PKMlldkNwXxTDoK PaRg2JM/VgIA1BzjEE7euqijCyQ0Pac= From: Hao Ge To: Vlastimil Babka , 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 Subject: [PATCH v3] slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts Date: Tue, 21 Oct 2025 09:03:53 +0800 Message-Id: <20251021010353.1187193-1-hao.ge@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset="utf-8" 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 uncondi= tionally") Cc: Suggested-by: Suren Baghdasaryan Signed-off-by: Hao Ge Reviewed-by: Harry Yoo Reviewed-by: Suren Baghdasaryan --- v3: According to Suren's suggestion, simplify the commit message and the co= de comments. Thanks for Suren. v2: Incorporate handling for the scenario where, if mark_failed_objexts_all= oc wins the race, the other process (that previously succeeded in allocation) will lose t= he 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) =20 static inline void mark_failed_objexts_alloc(struct slab *slab) { - slab->obj_exts =3D OBJEXTS_ALLOC_FAIL; + cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL); } =20 static inline void handle_failed_objexts_alloc(unsigned long obj_exts, @@ -2136,6 +2136,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kme= m_cache *s, #ifdef CONFIG_MEMCG new_exts |=3D MEMCG_DATA_OBJEXTS; #endif +retry: old_exts =3D 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 kme= m_cache *s, * be simply assigned. */ slab->obj_exts =3D new_exts; - } else if ((old_exts & ~OBJEXTS_FLAGS_MASK) || - cmpxchg(&slab->obj_exts, old_exts, new_exts) !=3D 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 kme= m_cache *s, else kfree(vec); return 0; + } else if (cmpxchg(&slab->obj_exts, old_exts, new_exts) !=3D old_exts) { + /* Retry if a racing thread changed slab->obj_exts from under us. */ + goto retry; } =20 if (allow_spin) --=20 2.25.1