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[184.145.104.227]) by smtp.gmail.com with ESMTPSA id a22sm1020526qtp.80.2021.04.28.17.08.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 Apr 2021 17:08:46 -0700 (PDT) Date: Wed, 28 Apr 2021 20:08:45 -0400 From: Peter Xu To: Axel Rasmussen Cc: Andrea Arcangeli , Andrew Morton , Hugh Dickins , Lokesh Gidra , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] userfaultfd: release page in error path to avoid BUG_ON Message-ID: <20210429000845.GA8339@xz-x1> References: <20210428180109.293606-1-axelrasmussen@google.com> <20210428230858.348400-1-axelrasmussen@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20210428230858.348400-1-axelrasmussen@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 28, 2021 at 04:08:58PM -0700, Axel Rasmussen wrote: > Consider the following sequence of events: > > 1. Userspace issues a UFFD ioctl, which ends up calling into > shmem_mfill_atomic_pte(). We successfully account the blocks, we > shmem_alloc_page(), but then the copy_from_user() fails. We return > -ENOENT. We don't release the page we allocated. > 2. Our caller detects this error code, tries the copy_from_user() after > dropping the mmap_lock, and retries, calling back into > shmem_mfill_atomic_pte(). > 3. Meanwhile, let's say another process filled up the tmpfs being used. > 4. So shmem_mfill_atomic_pte() fails to account blocks this time, and > immediately returns - without releasing the page. > > This triggers a BUG_ON in our caller, which asserts that the page > should always be consumed, unless -ENOENT is returned. > > To fix this, detect if we have such a "dangling" page when accounting > fails, and if so, release it before returning. > > Fixes: cb658a453b93 ("userfaultfd: shmem: avoid leaking blocks and used blocks in UFFDIO_COPY") > Reported-by: Hugh Dickins > Signed-off-by: Axel Rasmussen Reviewed-by: Peter Xu Thanks, -- Peter Xu