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 <[e-mail]>
Signed-off-by: Amir Goldstein <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/file_table.c

···
 	}
 	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);

fs/internal.h

···
 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
  */

fs/open.c

···
 		return 0;
 	}
 
-	if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
+	if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
+		i_readcount_inc(inode);
+	} else if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
 		error = get_write_access(inode);
 		if (unlikely(error))
 			goto cleanup_file;
···
 			goto cleanup_all;
 	}
 	f->f_mode |= FMODE_OPENED;
-	if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
-		i_readcount_inc(inode);
 	if ((f->f_mode & FMODE_READ) &&
 	     likely(f->f_op->read || f->f_op->read_iter))
 		f->f_mode |= FMODE_CAN_READ;
···
 	if (WARN_ON_ONCE(error > 0))
 		error = -EINVAL;
 	fops_put(f->f_op);
-	if (f->f_mode & FMODE_WRITER) {
-		put_write_access(inode);
-		__mnt_drop_write(f->f_path.mnt);
-	}
+	put_file_access(f);
 cleanup_file:
 	path_put(&f->f_path);
 	f->f_path.mnt = NULL;