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

[Jan Kara <jack@suse.cz>]

On Thu 04-06-15 09:18:49, Paul Moore wrote:
> On Thu, Jun 4, 2015 at 3:36 AM, Jan Kara <jack@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@redhat.com>
> >> > Signed-off-by: Jan Kara <jack@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?

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR