file reference counts can get corrupted

Callers of zfs_file_get and zfs_file_put can corrupt the reference
counts for the file structure resulting in a panic or a soft lockup.
When zfs send/recv runs, it will add a reference count to the
open file, and begin to send or recv the stream. If the file descriptor
is closed, then when dmu_recv_stream() or dmu_send() return we will
call zfs_file_put to remove the reference we placed on the file
structure. Unfortunately, because zfs_file_put() uses the file
descriptor to lookup the file structure, it may end up finding that
the file descriptor table no longer contains the file struct, thus
leaking the file structure. Or it might end up finding a file
descriptor for a different file and blindly updating its reference
counts. Other failure modes probably exists.

This change reworks the zfs_file_[get|put] interface to not rely
on the file descriptor but instead pass the zfs_file_t pointer around.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Co-authored-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: George Wilson <gwilson@delphix.com>
External-issue: DLPX-76119
Closes #12299

module/os/freebsd/zfs/zfs_file_os.c

@@ -241,28 +241,21 @@ zfs_file_fsync(zfs_file_t *fp, int flags)
 	return (zfs_vop_fsync(fp->f_vnode));
 }
 
-int
-zfs_file_get(int fd, zfs_file_t **fpp)
+zfs_file_t *
+zfs_file_get(int fd)
 {
 	struct file *fp;
 
 	if (fget(curthread, fd, &cap_no_rights, &fp))
-		return (SET_ERROR(EBADF));
+		return (NULL);
 
-	*fpp = fp;
-	return (0);
+	return (fp);
 }
 
 void
-zfs_file_put(int fd)
+zfs_file_put(zfs_file_t *fp)
 {
-	struct file *fp;
-
-	/* No CAP_ rights required, as we're only releasing. */
-	if (fget(curthread, fd, &cap_no_rights, &fp) == 0) {
-		fdrop(fp, curthread);
-		fdrop(fp, curthread);
-	}
+	fdrop(fp, curthread);
 }
 
 loff_t

module/os/linux/zfs/zfs_file_os.c

@@ -407,36 +407,22 @@ zfs_file_unlink(const char *path)
  * Get reference to file pointer
  *
  * fd - input file descriptor
- * fpp - pointer to file pointer
  *
- * Returns 0 on success EBADF on failure.
+ * Returns pointer to file struct or NULL
  */
-int
-zfs_file_get(int fd, zfs_file_t **fpp)
+zfs_file_t *
+zfs_file_get(int fd)
 {
-	zfs_file_t *fp;
-
-	fp = fget(fd);
-	if (fp == NULL)
-		return (EBADF);
-
-	*fpp = fp;
-
-	return (0);
+	return (fget(fd));
 }
 
 /*
  * Drop reference to file pointer
  *
- * fd - input file descriptor
+ * fp - input file struct pointer
  */
 void
-zfs_file_put(int fd)
+zfs_file_put(zfs_file_t *fp)
 {
-	struct file *fp;
-
-	if ((fp = fget(fd)) != NULL) {
-		fput(fp);
-		fput(fp);
-	}
+	fput(fp);
 }