From patchwork Tue Mar 28 17:32:26 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Eric Biggers X-Patchwork-Id: 13191419 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 5851EAD30 for ; Tue, 28 Mar 2023 17:33:07 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id ED243C4339B; Tue, 28 Mar 2023 17:33:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1680024787; bh=4HwFt+u19oZDx6r8WlYLiMQmmYR06P41KH3Z/3WL0os=; h=From:To:Cc:Subject:Date:From; b=XxmXS8dRj4wc1QIFZd7x8YJF+4Ztz8IDuCGCRME9D0HoN+HBialYG+dZK5hfQ2Asp PZAfCnbEDMfuvV1azTYi+oYheRxD38LZ9shwDEzQlkXjJEh1QxBfHwjr/XRZas2G29 6bERYq3ofM9Z5VvPOYIDzfqqq1A6kw3/gJZ7sorhfblEnq8vKOs0HCGjucnThYlea4 Enx3GTudAxbxh2KMzWTdhnGCKnNNsz22XtdF9z8YbT6Nttdjpi33YWiAr4wUu3ij+e N+WAFYYpZYEowIzbjZD5j4OKKg7B/zr0KnlpU4CopyvFAVPX5xT7Kn2sQjXe6m7tHH /nOkiT2O0LTBQ== From: Eric Biggers To: stable@vger.kernel.org Cc: fsverity@lists.linux.dev, Victor Hsieh Subject: [PATCH 6.1,5.15] fsverity: don't drop pagecache at end of FS_IOC_ENABLE_VERITY Date: Tue, 28 Mar 2023 10:32:26 -0700 Message-Id: <20230328173226.65244-1-ebiggers@kernel.org> X-Mailer: git-send-email 2.40.0 Precedence: bulk X-Mailing-List: fsverity@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Eric Biggers commit a075bacde257f755bea0e53400c9f1cdd1b8e8e6 upstream. [Please apply to 6.1-stable and 5.15-stable.] The full pagecache drop at the end of FS_IOC_ENABLE_VERITY is causing performance problems and is hindering adoption of fsverity. It was intended to solve a race condition where unverified pages might be left in the pagecache. But actually it doesn't solve it fully. Since the incomplete solution for this race condition has too much performance impact for it to be worth it, let's remove it for now. Fixes: 3fda4c617e84 ("fs-verity: implement FS_IOC_ENABLE_VERITY ioctl") Cc: stable@vger.kernel.org Reviewed-by: Victor Hsieh Link: https://lore.kernel.org/r/20230314235332.50270-1-ebiggers@kernel.org Signed-off-by: Eric Biggers --- fs/verity/enable.c | 24 +++++++++++++----------- 1 file changed, 13 insertions(+), 11 deletions(-) diff --git a/fs/verity/enable.c b/fs/verity/enable.c index df6b499bf6a14..400c264bf8930 100644 --- a/fs/verity/enable.c +++ b/fs/verity/enable.c @@ -390,25 +390,27 @@ int fsverity_ioctl_enable(struct file *filp, const void __user *uarg) goto out_drop_write; err = enable_verity(filp, &arg); - if (err) - goto out_allow_write_access; /* - * Some pages of the file may have been evicted from pagecache after - * being used in the Merkle tree construction, then read into pagecache - * again by another process reading from the file concurrently. Since - * these pages didn't undergo verification against the file digest which - * fs-verity now claims to be enforcing, we have to wipe the pagecache - * to ensure that all future reads are verified. + * We no longer drop the inode's pagecache after enabling verity. This + * used to be done to try to avoid a race condition where pages could be + * evicted after being used in the Merkle tree construction, then + * re-instantiated by a concurrent read. Such pages are unverified, and + * the backing storage could have filled them with different content, so + * they shouldn't be used to fulfill reads once verity is enabled. + * + * But, dropping the pagecache has a big performance impact, and it + * doesn't fully solve the race condition anyway. So for those reasons, + * and also because this race condition isn't very important relatively + * speaking (especially for small-ish files, where the chance of a page + * being used, evicted, *and* re-instantiated all while enabling verity + * is quite small), we no longer drop the inode's pagecache. */ - filemap_write_and_wait(inode->i_mapping); - invalidate_inode_pages2(inode->i_mapping); /* * allow_write_access() is needed to pair with deny_write_access(). * Regardless, the filesystem won't allow writing to verity files. */ -out_allow_write_access: allow_write_access(filp); out_drop_write: mnt_drop_write_file(filp);