Re: [PATCH v2 1/2] auditsc: audit_krule mask accesses need bounds checking

Andy Lutomirski
2014-05-29T02:27:33Z

On Wed, May 28, 2014 at 7:23 PM, Eric Paris <eparis@redhat.com> wrote:
> On Wed, 2014-05-28 at 18:44 -0700, Andy Lutomirski wrote:
>> Fixes an easy DoS and possible information disclosure.
>>
>> This does nothing about the broken state of x32 auditing.
>>
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
>> ---
>> kernel/auditsc.c | 27 ++++++++++++++++++---------
>> 1 file changed, 18 insertions(+), 9 deletions(-)
>>
>> diff --git a/kernel/auditsc.c b/kernel/auditsc.c
>> index f251a5e..7ccd9db 100644
>> --- a/kernel/auditsc.c
>> +++ b/kernel/auditsc.c
>> @@ -728,6 +728,22 @@ static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
>> return AUDIT_BUILD_CONTEXT;
>> }
>>
>> +static bool audit_in_mask(const struct audit_krule *rule, unsigned long val)
>> +{
>> + int word, bit;
>> +
>> + if (val > 0xffffffff)
>> + return false;
>
> Why is this necessary?

To avoid an integer overflow. Admittedly, this particular overflow
won't cause a crash, but it will cause incorrect results.

>
>> +
>> + word = AUDIT_WORD(val);
>> + if (word >= AUDIT_BITMASK_SIZE)
>> + return false;
>
> Since this covers it and it extensible...
>
>> +
>> + bit = AUDIT_BIT(val);
>> +
>> + return rule->mask[word] & bit;
>
> Returning an int as a bool creates worse code than just returning the
> int. (although in this case if the compiler chooses to inline it might
> be smart enough not to actually convert this int to a bool and make
> worse assembly...) I'd suggest the function here return an int. bools
> usually make the assembly worse...

I'm ambivalent. The right assembly would use flags on x86, not an int
or a bool, and I don't know what the compiler will do.

--Andy