[PATCH] security/keys: fix slab-out-of-bounds in key_task_permission

[Chen Ridong]

We meet the same issue with the LINK, which reads memory out of bounds:
BUG: KASAN: slab-out-of-bounds in __kuid_val include/linux/uidgid.h:36
BUG: KASAN: slab-out-of-bounds in uid_eq include/linux/uidgid.h:63 [inline]
BUG: KASAN: slab-out-of-bounds in key_task_permission+0x394/0x410
security/keys/permission.c:54
Read of size 4 at addr ffff88813c3ab618 by task stress-ng/4362

CPU: 2 PID: 4362 Comm: stress-ng Not tainted 5.10.0-14930-gafbffd6c3ede #15
Call Trace:
 __dump_stack lib/dump_stack.c:82 [inline]
 dump_stack+0x107/0x167 lib/dump_stack.c:123
 print_address_description.constprop.0+0x19/0x170 mm/kasan/report.c:400
 __kasan_report.cold+0x6c/0x84 mm/kasan/report.c:560
 kasan_report+0x3a/0x50 mm/kasan/report.c:585
 __kuid_val include/linux/uidgid.h:36 [inline]
 uid_eq include/linux/uidgid.h:63 [inline]
 key_task_permission+0x394/0x410 security/keys/permission.c:54
 search_nested_keyrings+0x90e/0xe90 security/keys/keyring.c:793
 keyring_search_rcu+0x1b6/0x310 security/keys/keyring.c:922
 search_cred_keyrings_rcu+0x111/0x2e0 security/keys/process_keys.c:459
 search_process_keyrings_rcu+0x1d/0x310 security/keys/process_keys.c:544
 lookup_user_key+0x782/0x12e0 security/keys/process_keys.c:762
 keyctl_invalidate_key+0x20/0x190 security/keys/keyctl.c:434
 __do_sys_keyctl security/keys/keyctl.c:1978 [inline]
 __se_sys_keyctl+0x1de/0x5b0 security/keys/keyctl.c:1880
 do_syscall_64+0x30/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x67/0xd1

However, we can't reproduce this issue.
After our analysis, it can make this issue by following steps.
1.As syzkaller reported, the memory is allocated for struct
  assoc_array_shortcut in the assoc_array_insert_into_terminal_node
  functions.
2.In the search_nested_keyrings, when we go through the slots in a node,
  (bellow tag ascend_to_node), and the slot ptr is meta and
  node->back_pointer != NULL, we will proceed to  descend_to_node.
  However, there is an exception. If node is the root, and one of the
  slots points to a shortcut, it will be treated as a keyring.
3.Whether the ptr is keyring decided by keyring_ptr_is_keyring function.
  However, KEYRING_PTR_SUBTYPE is 0x2UL, the same as
  ASSOC_ARRAY_PTR_SUBTYPE_MASK,
4.As mentioned above, If a slot of the root is a shortcut, it may be
  mistakenly be transferred to a key*, leading to an read out-of-bounds
  read.

To fix this issue, one should jump to descend_to_node if the pointer is a
shortcut.

Link: https://syzkaller.appspot.com/bug?id=68a5e206c2a8e08d317eb83f05610c0484ad10b9
Fixes: b2a4df200d57 ("KEYS: Expand the capacity of a keyring")
Signed-off-by: Chen Ridong <chenridong at huawei.com>
---
 security/keys/keyring.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index 4448758f643a..7958486ac834 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -772,7 +772,9 @@ static bool search_nested_keyrings(struct key *keyring,
 	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
 		ptr = READ_ONCE(node->slots[slot]);
 
-		if (assoc_array_ptr_is_meta(ptr) && node->back_pointer)
+		if ((assoc_array_ptr_is_meta(ptr) && node->back_pointer) ||
+		    (assoc_array_ptr_is_meta(ptr) &&
+		     assoc_array_ptr_is_shortcut(ptr)))
 			goto descend_to_node;
 
 		if (!keyring_ptr_is_keyring(ptr))
-- 
2.34.1