[PATCH] vfs: protect updates to file->f_sb_err with f_lock

[Jeff Layton <jlayton@kernel.org>]

When I added the ability for syncfs to report writeback errors, I
neglected to adequately protect file->f_sb_err. We could have racing
updates to that value if two tasks are issuing syncfs() on the same
fd at the same time.

Fix this by protecting the f_sb_err field with the file->f_lock.

Cc: stable@vger.kernel.org # v5.8+
Fixes: 735e4ae5ba28 ("vfs: track per-sb writeback errors and report them to syncfs")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/sync.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/sync.c b/fs/sync.c
index 1373a610dc78..3be26ff72431 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -162,7 +162,7 @@ SYSCALL_DEFINE1(syncfs, int, fd)
 {
 	struct fd f = fdget(fd);
 	struct super_block *sb;
-	int ret, ret2;
+	int ret, ret2 = 0;
 
 	if (!f.file)
 		return -EBADF;
@@ -172,7 +172,12 @@ SYSCALL_DEFINE1(syncfs, int, fd)
 	ret = sync_filesystem(sb);
 	up_read(&sb->s_umount);
 
-	ret2 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
+	if (errseq_check(&sb->s_wb_err, f.file->f_sb_err)) {
+		/* Something changed, must use slow path */
+		spin_lock(&f.file->f_lock);
+		ret2 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
+		spin_unlock(&f.file->f_lock);
+	}
 
 	fdput(f);
 	return ret ? ret : ret2;
-- 
2.28.0