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Fri, 08 May 2026 19:18:26 -0700 (PDT) Received: from google.com ([2a00:79e0:2031:6:96eb:c157:3755:4e22]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-c8267689d76sm3059171a12.11.2026.05.08.19.18.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 08 May 2026 19:18:26 -0700 (PDT) Date: Sat, 9 May 2026 11:18:22 +0900 From: Sergey Senozhatsky To: Richard Chang Cc: Minchan Kim , Sergey Senozhatsky , Jens Axboe , Andrew Morton , bgeffon@google.com, liumartin@google.com, linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2] zram: fix use-after-free in zram_writeback_endio Message-ID: References: <20260508084933.3730661-1-richardycc@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260508084933.3730661-1-richardycc@google.com> On (26/05/08 08:49), Richard Chang wrote: > A crash was observed in zram_writeback_endio due to a NULL pointer > dereference in wake_up. The root cause is a race condition between the > bio completion handler (zram_writeback_endio) and the writeback task. > > In zram_writeback_endio, wake_up() is called on &wb_ctl->done_wait after > releasing wb_ctl->done_lock. This creates a race window where the > writeback task can see num_inflight become 0, return, and free wb_ctl > before zram_writeback_endio calls wake_up(). > > CPU 0 (zram_writeback_endio) CPU 1 (writeback_store) > ============================ ============================ > zram_writeback_slots > zram_submit_wb_request > zram_submit_wb_request > wait_event(wb_ctl->done_wait) > spin_lock(&wb_ctl->done_lock); > list_add(&req->entry, &wb_ctl->done_reqs); > spin_unlock(&wb_ctl->done_lock); > wake_up(&wb_ctl->done_wait); > zram_complete_done_reqs > spin_lock(&wb_ctl->done_lock); > list_add(&req->entry, &wb_ctl->done_reqs); > spin_unlock(&wb_ctl->done_lock); > while (num_inflight) > 0) > spin_lock(&wb_ctl->done_lock); > list_del(&req->entry); > spin_unlock(&wb_ctl->done_lock); > // num_inflight becomes 0 > atomic_dec(num_inflight); > > // Leave zram_writeback_slots > // Free wb_ctl > release_wb_ctl(wb_ctl); > // UAF crash! > wake_up(&wb_ctl->done_wait); > > This patch fixes this race by using RCU. By protecting wb_ctl with > rcu_read_lock() in zram_writeback_endio and using kfree_rcu() to free > it, we ensure that wb_ctl remains valid during the execution of > zram_writeback_endio. > > Fixes: f405066a1f0d ("zram: introduce writeback bio batching") > Suggested-by: Sergey Senozhatsky > Suggested-by: Minchan Kim > Signed-off-by: Richard Chang Acked-by: Sergey Senozhatsky