locks: fix TOCTOU race when granting write lease

Thread A trying to acquire a write lease checks the value of i_readcount
and i_writecount in check_conflicting_open() to verify that its own fd
is the only fd referencing the file.

Thread B trying to open the file for read will call break_lease() in
do_dentry_open() before incrementing i_readcount, which leaves a small
window where thread A can acquire the write lease and then thread B
completes the open of the file for read without breaking the write lease
that was acquired by thread A.

Fix this race by incrementing i_readcount before checking for existing
leases, same as the case with i_writecount.

Use a helper put_file_access() to decrement i_readcount or i_writecount
in do_dentry_open() and __fput().

Fixes: 387e3746d01c ("locks: eliminate false positive conflicts for write lease")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 1 insertions, 6 deletions

diff --git a/fs/file_table.c b/fs/file_table.c
index 99c6796c9f28..dd88701e54a9 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -324,12 +324,7 @@ static void __fput(struct file *file)
 	}
 	fops_put(file->f_op);
 	put_pid(file->f_owner.pid);
-	if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
-		i_readcount_dec(inode);
-	if (mode & FMODE_WRITER) {
-		put_write_access(inode);
-		__mnt_drop_write(mnt);
-	}
+	put_file_access(file);
 	dput(dentry);
 	if (unlikely(mode & FMODE_NEED_UNMOUNT))
 		dissolve_on_fput(mnt);