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>
diff --git a/fs/internal.h b/fs/internal.h
index 87e96b9..31861e6 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -101,6 +101,16 @@ extern void chroot_fs_refs(const struct path *, const struct path *);
extern struct file *alloc_empty_file(int, const struct cred *);
extern struct file *alloc_empty_file_noaccount(int, const struct cred *);
+static inline void put_file_access(struct file *file)
+{
+ if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
+ i_readcount_dec(file->f_inode);
+ } else if (file->f_mode & FMODE_WRITER) {
+ put_write_access(file->f_inode);
+ __mnt_drop_write(file->f_path.mnt);
+ }
+}
+
/*
* super.c
*/
