[apparmor] [PATCH 03/16] AppArmor: Fix underflow in xindex calculation

[Kees Cook]

On Wed, Feb 22, 2012 at 09:10:28AM -0800, John Johansen wrote:
> If the xindex value stored in the accept tables is 0, the extraction of
> that value will result in an underflow (0 - 4).
> 
> In properly compiled policy this should not happen for file rules but
> it may be possible for other rule types in the future.
> 
> To exploit this underflow a user would have to be able to load a corrupt
> policy, which requires CAP_MAC_ADMIN, overwrite system policy in kernel
> memory or know of a compiler error resulting in the flaw being present
> for loaded policy (no such flaw is known at this time).
> 
> Signed-off-by: John Johansen <john.johansen at canonical.com>
> ---
>  security/apparmor/include/file.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/security/apparmor/include/file.h b/security/apparmor/include/file.h
> index ab8c6d8..f98fd47 100644
> --- a/security/apparmor/include/file.h
> +++ b/security/apparmor/include/file.h
> @@ -117,7 +117,7 @@ static inline u16 dfa_map_xindex(u16 mask)
>  		index |= AA_X_NAME;
>  	} else if (old_index == 3) {
>  		index |= AA_X_NAME | AA_X_CHILD;
> -	} else {
> +	} else if (old_index) {
>  		index |= AA_X_TABLE;
>  		index |= old_index - 4;
>  	}

What about the cases where old_index < 4, but != 0?

-- 
Kees Cook