ceph: fix oops due to invalid pointer for kfree() in parse_longname()

This fixes a kernel oops when reading ceph snapshot directories (.snap),
for example by simply running `ls /mnt/my_ceph/.snap`.

The variable str is guarded by __free(kfree), but advanced by one for
skipping the initial '_' in snapshot names. Thus, kfree() is called
with an invalid pointer. This patch removes the need for advancing the
pointer so kfree() is called with correct memory pointer.

Steps to reproduce:

1. Create snapshots on a cephfs volume (I've 63 snaps in my testcase)

2. Add cephfs mount to fstab
$ echo "samba-fileserver@.files=/volumes/datapool/stuff/3461082b-ecc9-4e82-8549-3fd2590d3fb6 /mnt/test/stuff ceph acl,noatime,_netdev 0 0" >> /etc/fstab

3. Reboot the system
$ systemctl reboot

4. Check if it's really mounted
$ mount | grep stuff

5. List snapshots (expected 63 snapshots on my system)
$ ls /mnt/test/stuff/.snap

Now ls hangs forever and the kernel log shows the oops.

Cc: [e-mail]
Fixes: 101841c38346 ("[ceph] parse_longname(): strrchr() expects NUL-terminated string")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220807
Suggested-by: Helge Deller <[e-mail]>
Signed-off-by: Daniel Vogelbacher <[e-mail]>
Reviewed-by: Viacheslav Dubeyko <[e-mail]>
Signed-off-by: Ilya Dryomov <[e-mail]>

fs/ceph/crypto.c

···
 	struct ceph_vino vino = { .snap = CEPH_NOSNAP };
 	char *name_end, *inode_number;
 	int ret = -EIO;
-	/* NUL-terminate */
-	char *str __free(kfree) = kmemdup_nul(name, *name_len, GFP_KERNEL);
+	/* Snapshot name must start with an underscore */
+	if (*name_len <= 0 || name[0] != '_')
+		return ERR_PTR(-EIO);
+	/* Skip initial '_' and NUL-terminate */
+	char *str __free(kfree) = kmemdup_nul(name + 1, *name_len - 1, GFP_KERNEL);
 	if (!str)
 		return ERR_PTR(-ENOMEM);
-	/* Skip initial '_' */
-	str++;
 	name_end = strrchr(str, '_');
 	if (!name_end) {
 		doutc(cl, "failed to parse long snapshot name: %s\n", str);