From: Eric Biggers ebiggers@google.com
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 Signed-off-by: Eric Biggers ebiggers@google.com --- fs/verity/enable.c | 25 +++++++++++++------------ 1 file changed, 13 insertions(+), 12 deletions(-)
diff --git a/fs/verity/enable.c b/fs/verity/enable.c index e13db6507b38b..7a0e3a84d370b 100644 --- a/fs/verity/enable.c +++ b/fs/verity/enable.c @@ -8,7 +8,6 @@ #include "fsverity_private.h"
#include <linux/mount.h> -#include <linux/pagemap.h> #include <linux/sched/signal.h> #include <linux/uaccess.h>
@@ -367,25 +366,27 @@ 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);
base-commit: f959325e6ac3f499450088b8d9c626d1177be160
Reviewed-by: Victor Hsieh victorhsieh@google.com
On Tue, Mar 14, 2023 at 4:55 PM Eric Biggers ebiggers@kernel.org wrote:
From: Eric Biggers ebiggers@google.com
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 Signed-off-by: Eric Biggers ebiggers@google.com
fs/verity/enable.c | 25 +++++++++++++------------ 1 file changed, 13 insertions(+), 12 deletions(-)
diff --git a/fs/verity/enable.c b/fs/verity/enable.c index e13db6507b38b..7a0e3a84d370b 100644 --- a/fs/verity/enable.c +++ b/fs/verity/enable.c @@ -8,7 +8,6 @@ #include "fsverity_private.h"
#include <linux/mount.h> -#include <linux/pagemap.h> #include <linux/sched/signal.h> #include <linux/uaccess.h>
@@ -367,25 +366,27 @@ 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);
base-commit: f959325e6ac3f499450088b8d9c626d1177be160
2.39.2
linux-stable-mirror@lists.linaro.org
-
Eric Biggers -
Victor Hsieh