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

[Jarkko Sakkinen]

On Fri Sep 13, 2024 at 10:09 AM EEST, Chen Ridong wrote:
> We meet the same issue with the LINK, which reads memory out of bounds:

Nit: don't use "we" anywhere".

Tbh, I really don't understand the sentence above. I don't what
"the same issue with the LINK" really is.

> 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.

"The issue cannot be easily reproduced but by analyzing the code
it can be broken into following steps:"

> 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@xxxxxxxxxx>
> ---
>  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))


BR, Jarkko