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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id q23sm1188988qtl.25.2021.04.28.16.57.10 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Wed, 28 Apr 2021 16:57:11 -0700 (PDT) Date: Wed, 28 Apr 2021 16:56:59 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Axel Rasmussen cc: Andrea Arcangeli , Andrew Morton , Hugh Dickins , Peter Xu , 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 In-Reply-To: <20210428230858.348400-1-axelrasmussen@google.com> Message-ID: References: <20210428180109.293606-1-axelrasmussen@google.com> <20210428230858.348400-1-axelrasmussen@google.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 28 Apr 2021, 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 Acked-by: Hugh Dickins Thanks! > --- > mm/shmem.c | 12 +++++++++++- > 1 file changed, 11 insertions(+), 1 deletion(-) > > diff --git a/mm/shmem.c b/mm/shmem.c > index 26c76b13ad23..8def03d3f32a 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -2375,8 +2375,18 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm, > pgoff_t offset, max_off; > > ret = -ENOMEM; > - if (!shmem_inode_acct_block(inode, 1)) > + if (!shmem_inode_acct_block(inode, 1)) { > + /* > + * We may have got a page, returned -ENOENT triggering a retry, > + * and now we find ourselves with -ENOMEM. Release the page, to > + * avoid a BUG_ON in our caller. > + */ > + if (unlikely(*pagep)) { > + put_page(*pagep); > + *pagep = NULL; > + } > goto out; > + } > > if (!*pagep) { > page = shmem_alloc_page(gfp, info, pgoff); > -- > 2.31.1.498.g6c1eba8ee3d-goog