fsverity: don't drop pagecache at end of FS_IOC_ENABLE_VERITY

[ Upstream commit a075bacde2 ]

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: 3fda4c617e ("fs-verity: implement FS_IOC_ENABLE_VERITY ioctl")
Cc: stable@vger.kernel.org
Reviewed-by: Victor Hsieh <victorhsieh@google.com>
Link: https://lore.kernel.org/r/20230314235332.50270-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/verity/enable.c

@@ -391,25 +391,27 @@ 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
+	 * We no longer drop the inode's pagecache after enabling verity.  This
-	 * being used in the Merkle tree construction, then read into pagecache
+	 * used to be done to try to avoid a race condition where pages could be
-	 * again by another process reading from the file concurrently.  Since
+	 * evicted after being used in the Merkle tree construction, then
-	 * these pages didn't undergo verification against the file digest which
+	 * re-instantiated by a concurrent read.  Such pages are unverified, and
-	 * fs-verity now claims to be enforcing, we have to wipe the pagecache
+	 * the backing storage could have filled them with different content, so
-	 * to ensure that all future reads are verified.
+	 * 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);