[PATCH] audit: Fix check of return value of strnlen_user()

[Paul Moore]

On Thu, Jun 4, 2015 at 5:32 PM, Jan Kara <jack at suse.cz> wrote:
> On Thu 04-06-15 09:18:49, Paul Moore wrote:
>> On Thu, Jun 4, 2015 at 3:36 AM, Jan Kara <jack at suse.cz> wrote:
>> > On Wed 03-06-15 14:56:18, Paul Moore wrote:
>> >> On Tuesday, June 02, 2015 05:08:29 PM Jan Kara wrote:
>> >> > strnlen_user() returns 0 when it hits fault, not -1. Fix the test in
>> >> > audit_log_single_execve_arg(). Luckily this shouldn't ever happen unless
>> >> > there's a kernel bug so it's mostly a cosmetic fix.
>> >> >
>> >> > CC: Paul Moore <pmoore at redhat.com>
>> >> > Signed-off-by: Jan Kara <jack at suse.cz>
>> >> > ---
>> >> >  kernel/auditsc.c | 2 +-
>> >> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >> >
>> >> > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
>> >> > index 9fb9d1cb83ce..bb947ceeee4d 100644
>> >> > --- a/kernel/auditsc.c
>> >> > +++ b/kernel/auditsc.c
>> >> > @@ -1023,7 +1023,7 @@ static int audit_log_single_execve_arg(struct
>> >> > audit_context *context, * for strings that are too long, we should not have
>> >> > created
>> >> >      * any.
>> >> >      */
>> >> > -   if (unlikely((len == -1) || len > MAX_ARG_STRLEN - 1)) {
>> >> > +   if (unlikely((len == 0) || len > MAX_ARG_STRLEN - 1)) {
>> >>
>> >> While we're at it, should we make it just "len > MAX_ARG_STRLEN" as well?
>> >> Reading the comments in include/uapi/linux/binfmts.h as well as
>> >> valid_arg_len() that seems to be the correct logic.
>> >
>> >   Umm, but audit_log_single_execve_arg() does decrement 1 from
>> > strnlen_user() result before doing the comparison. So the current test
>> > seems to match the one in valid_arg_len() exactly...
>>
>> For reference (taken from fs/exec.c in Linus' tree just now):
>>
>>   static bool valid_arg_len(struct linux_binprm *bprm, long len)
>>   {
>>           return len <= MAX_ARG_STRLEN;
>>   }
>>
>> The valid_arg_len() returns true when the length is less than or equal
>> to MAX_ARG_STRLEN, implying that lengths greater than MAX_ARG_STRLEN
>> are invalid.  The existing test in audit_log_single_execve_arg()
>> treats lengths greater than (MAX_ARG_STRLEN-1) as invalid.
>   But the 'len' passed to valid_arg_len() is the return value of
> strnlen_user() and that one returns lenght *including* the terminating
> '\0'. So in fact valid_arg_len() tests whether the argument length is <=
> (MAX_ARG_STRLEN-1). Hmm?

I must have spaced on the fact that audit subtracts the NULL from the
output of strnlen_user() when it calls it earlier in the function.

-- 
paul moore
www.paul-moore.com