linux-kernel - Re: [PATCH] security: fix the key_permission LSM hook function type

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Date:   Mon, 15 Jun 2020 11:21:08 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>,
        James Morris <jamorris@...ux.microsoft.com>
Cc:     David Howells <dhowells@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        KP Singh <kpsingh@...gle.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Thomas Cedeno <thomascedeno@...gle.com>,
        Anders Roxell <anders.roxell@...aro.org>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] security: fix the key_permission LSM hook function type

On Mon, Jun 15, 2020 at 11:12:32AM -0700, Sami Tolvanen wrote:
> Commit 8c0637e950d6 ("keys: Make the KEY_NEED_* perms an enum rather than
> a mask") changed the type of the key_permission callback functions, but
> didn't change the type of the hook, which trips indirect call checking with
> Control-Flow Integrity (CFI). This change fixes the issue by changing the
> hook type to match the functions.
> 
> Fixes: 8c0637e950d6 ("keys: Make the KEY_NEED_* perms an enum rather than a mask")
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

Thanks for fixing this!

Acked-by: Kees Cook <keescook@...omium.org>

I wonder if there is any compiler option we can turn on to catch the
implicit enum/int casting that would help for these kinds of things
without absolutely exploding the build warnings.

I see -Wenum-conversion, but that seems to be between enums, not between
int.

I see this, but it's external:
https://noamlewis.wordpress.com/2017/10/05/type-safe-enums-in-c-using-a-clang-plugin/

-Kees

> ---
>  include/linux/lsm_hook_defs.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 6791813cd439..24f6683f1cfc 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -360,7 +360,7 @@ LSM_HOOK(int, 0, key_alloc, struct key *key, const struct cred *cred,
>  	 unsigned long flags)
>  LSM_HOOK(void, LSM_RET_VOID, key_free, struct key *key)
>  LSM_HOOK(int, 0, key_permission, key_ref_t key_ref, const struct cred *cred,
> -	 unsigned perm)
> +	 enum key_need_perm need_perm)
>  LSM_HOOK(int, 0, key_getsecurity, struct key *key, char **_buffer)
>  #endif /* CONFIG_KEYS */
>  
> 
> base-commit: b3a9e3b9622ae10064826dccb4f7a52bd88c7407
> -- 
> 2.27.0.290.gba653c62da-goog
> 

-- 
Kees Cook