fanotify: Allow copying of file handle to userspace

When file handle is embedded inside fanotify_event and usercopy checks
are enabled, we get a warning like:

Bad or missing usercopy whitelist? Kernel memory exposure attempt detected
from SLAB object 'fanotify_event' (offset 40, size 8)!
WARNING: CPU: 1 PID: 7649 at mm/usercopy.c:78 usercopy_warn+0xeb/0x110
mm/usercopy.c:78

Annotate handling in fanotify_event properly to mark copying it to
userspace is fine.

Reported-by: [e-mail]
Fixes: a8b13aa20afb ("fanotify: enable FAN_REPORT_FID init flag")
Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: Amir Goldstein <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

fs/notify/fanotify/fanotify_user.c

···
 {
 	struct fanotify_event_info_fid info = { };
 	struct file_handle handle = { };
+	unsigned char bounce[FANOTIFY_INLINE_FH_LEN], *fh;
 	size_t fh_len = event->fh_len;
 	size_t len = fanotify_event_info_len(event);
 
···
 
 	buf += sizeof(handle);
 	len -= sizeof(handle);
-	if (copy_to_user(buf, fanotify_event_fh(event), fh_len))
+	/*
+	 * For an inline fh, copy through stack to exclude the copy from
+	 * usercopy hardening protections.
+	 */
+	fh = fanotify_event_fh(event);
+	if (fh_len <= FANOTIFY_INLINE_FH_LEN) {
+		memcpy(bounce, fh, fh_len);
+		fh = bounce;
+	}
+	if (copy_to_user(buf, fh, fh_len))
 		return -EFAULT;
 
 	/* Pad with 0's */