From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 1EE773803DB for ; Mon, 20 Apr 2026 17:29:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776706175; cv=none; b=R0b7w+FP+EcXswrS5W7312O010dExFIaAH04yFEHmlx1xllXE1qJ+j2CXb0DJakp6RxsdGL4b+S79eFJXeZ4KcMfaflotI6soztVFueIJomie/JmkvsXKdLuoLjQEgZo32SgKl2EKWoMq5JcHXuQHKJXmsqBHVR/i1kg11nNPx4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776706175; c=relaxed/simple; bh=t0CE49eVKMLTx91QRp0lyV3rGXkJh80xJLleLIAMJro=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=ob8UcwLmV+CDaqb+VOwKlC2UpWe9RTKX5rtIHlU0Nbi+WnMGre1c1r13vt7MsM46rYLyp3ds7hq05IW6TehGfkYTTMioWleQzzCuFEhOoPxVCF5eojxYtSqV1aW7UqhDdxDqDgj7XIjstAIEJfDPe1i9z0KzsgFKlqT64sfs+0k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=pgRJ/lTm; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="pgRJ/lTm" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 19DD3C19425; Mon, 20 Apr 2026 17:29:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776706174; bh=t0CE49eVKMLTx91QRp0lyV3rGXkJh80xJLleLIAMJro=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=pgRJ/lTmLBV5gmbteonTkwHBnfUx0s7HUzoNUMS51gfS5GJJTnFpb4+rH7JUfJ/F4 hatc0p5Rs7nzpfOOGhXsRhTN/dd5Z2LsyNacusGK6vTC/Bm40DSPDmKlFJq6/r9d8i KjJKKVNkssOqbl4+Ai9u2mLyoSC1D/B/01n7XqswoGB4vIu1rUzVe9bihw6dyrUliU x1BnvlcAWbWHimn59eHyVfnuaHAit+p1aYGgzM5gajdaAp6TJI9y/pzX22OHK8srdI 73+9NLV6QDHZ7WX6ldOunPMQqpoux+gF+kIe/2/nqflFbPDWaN45S8UNjotYOYJ/wg TqZlK6NdccTyA== From: Sasha Levin To: stable@vger.kernel.org Cc: Breno Leitao , Dennis Zhou , Shakeel Butt , David Hildenbrand , Jens Axboe , Johannes Weiner , Josef Bacik , JP Kobryn , Liam Howlett , "Lorenzo Stoakes (Oracle)" , Martin KaFai Lau , Michal Hocko , Mike Rapoport , Suren Baghdasaryan , Tejun Heo , Andrew Morton , Sasha Levin Subject: [PATCH 5.15.y] mm: blk-cgroup: fix use-after-free in cgwb_release_workfn() Date: Mon, 20 Apr 2026 13:29:31 -0400 Message-ID: <20260420172931.1421876-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <2026042027-thinly-unrigged-91da@gregkh> References: <2026042027-thinly-unrigged-91da@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Breno Leitao [ Upstream commit 8f5857be99f1ed1fa80991c72449541f634626ee ] cgwb_release_workfn() calls css_put(wb->blkcg_css) and then later accesses wb->blkcg_css again via blkcg_unpin_online(). If css_put() drops the last reference, the blkcg can be freed asynchronously (css_free_rwork_fn -> blkcg_css_free -> kfree) before blkcg_unpin_online() dereferences the pointer to access blkcg->online_pin, resulting in a use-after-free: BUG: KASAN: slab-use-after-free in blkcg_unpin_online (./include/linux/instrumented.h:112 ./include/linux/atomic/atomic-instrumented.h:400 ./include/linux/refcount.h:389 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 block/blk-cgroup.c:1367) Write of size 4 at addr ff11000117aa6160 by task kworker/71:1/531 Workqueue: cgwb_release cgwb_release_workfn Call Trace: blkcg_unpin_online (./include/linux/instrumented.h:112 ./include/linux/atomic/atomic-instrumented.h:400 ./include/linux/refcount.h:389 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 block/blk-cgroup.c:1367) cgwb_release_workfn (mm/backing-dev.c:629) process_scheduled_works (kernel/workqueue.c:3278 kernel/workqueue.c:3385) Freed by task 1016: kfree (./include/linux/kasan.h:235 mm/slub.c:2689 mm/slub.c:6246 mm/slub.c:6561) css_free_rwork_fn (kernel/cgroup/cgroup.c:5542) process_scheduled_works (kernel/workqueue.c:3302 kernel/workqueue.c:3385) ** Stack based on commit 66672af7a095 ("Add linux-next specific files for 20260410") I am seeing this crash sporadically in Meta fleet across multiple kernel versions. A full reproducer is available at: https://github.com/leitao/debug/blob/main/reproducers/repro_blkcg_uaf.sh (The race window is narrow. To make it easily reproducible, inject a msleep(100) between css_put() and blkcg_unpin_online() in cgwb_release_workfn(). With that delay and a KASAN-enabled kernel, the reproducer triggers the splat reliably in less than a second.) Fix this by moving blkcg_unpin_online() before css_put(), so the cgwb's CSS reference keeps the blkcg alive while blkcg_unpin_online() accesses it. Link: https://lore.kernel.org/20260413-blkcg-v1-1-35b72622d16c@debian.org Fixes: 59b57717fff8 ("blkcg: delay blkg destruction until after writeback has finished") Signed-off-by: Breno Leitao Reviewed-by: Dennis Zhou Reviewed-by: Shakeel Butt Cc: David Hildenbrand Cc: Jens Axboe Cc: Johannes Weiner Cc: Josef Bacik Cc: JP Kobryn Cc: Liam Howlett Cc: Lorenzo Stoakes (Oracle) Cc: Martin KaFai Lau Cc: Michal Hocko Cc: Mike Rapoport Cc: Suren Baghdasaryan Cc: Tejun Heo Cc: Signed-off-by: Andrew Morton Signed-off-by: Sasha Levin --- mm/backing-dev.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/mm/backing-dev.c b/mm/backing-dev.c index afdd132768455..0a00c9efafd84 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -404,12 +404,13 @@ static void cgwb_release_workfn(struct work_struct *work) wb_shutdown(wb); css_put(wb->memcg_css); - css_put(wb->blkcg_css); - mutex_unlock(&wb->bdi->cgwb_release_mutex); /* triggers blkg destruction if no online users left */ blkcg_unpin_online(blkcg); + css_put(wb->blkcg_css); + mutex_unlock(&wb->bdi->cgwb_release_mutex); + fprop_local_destroy_percpu(&wb->memcg_completions); spin_lock_irq(&cgwb_lock); -- 2.53.0