[PATCH] keys: Fix description size

[David Howells]

The maximum key description size is 4095.  Commit f771fde82051
inadvertantly reduced that to 255 and made sizes between 256 and 4095 work
weirdly, and any size whereby size & 255 == 0 would cause an assertion in
__key_link_begin() at the following line:

	BUG_ON(index_key->desc_len == 0);

This can be fixed by simply increasing the size of desc_len in struct
keyring_index_key to a u16.

Note the argument length test in keyutils only checked empty descriptions
and descriptions with a size around the limit (ie. 4095) and not for all
the values in between, so it missed this.  This has been addressed and

	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/keyutils.git/commit/?id=066bf56807c26cd3045a25f355b34c1d8a20a5aa

now exhaustively tests all possible lengths of type, description and
payload and then some.

The assertion failure looks something like:

 kernel BUG at security/keys/keyring.c:1245!
 ...
 RIP: 0010:__key_link_begin+0x88/0xa0
 ...
 Call Trace:
  key_create_or_update+0x211/0x4b0
  __x64_sys_add_key+0x101/0x200
  do_syscall_64+0x5b/0x1e0
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

It can be triggered by:

	keyctl add user "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" a @s

Fixes: f771fde82051 ("keys: Simplify key description management")
Reported-by: kernel test robot <rong.a.chen at intel.com>
Signed-off-by: David Howells <dhowells at redhat.com>
cc: Linus Torvalds <torvalds at linux-foundation.org>
---

 include/linux/key.h |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/key.h b/include/linux/key.h
index 91f391cd272e..50028338a4cc 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -94,11 +94,11 @@ struct keyring_index_key {
 	union {
 		struct {
 #ifdef __LITTLE_ENDIAN /* Put desc_len at the LSB of x */
-			u8	desc_len;
-			char	desc[sizeof(long) - 1];	/* First few chars of description */
+			u16	desc_len;
+			char	desc[sizeof(long) - 2];	/* First few chars of description */
 #else
-			char	desc[sizeof(long) - 1];	/* First few chars of description */
-			u8	desc_len;
+			char	desc[sizeof(long) - 2];	/* First few chars of description */
+			u16	desc_len;
 #endif
 		};
 		unsigned long x;