[RFC PATCH 1/3] lsm: cleanup the size counters in security_getselfattr()

[Casey Schaufler]

On 10/24/2023 6:43 PM, Paul Moore wrote:
> On Tue, Oct 24, 2023 at 6:23 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
>> On 10/24/2023 2:35 PM, Paul Moore wrote:
>>> Zero out all of the size counters in the -E2BIG case (buffer too
>>> small) to help make the current code a bit more robust in the face of
>>> future code changes.
>> I don't see how this change would have the described effect.
>> What it looks like it would do is change the return from -E2BIG
>> to 0, which would not have the desired result.
> When @toobig is true, which it will be when one of the individual LSMs
> return -E2BIG, the return value of security_getselfattr() is fixed to
> -E2BIG (check the if-statements at the end of the function).  Setting
> @rc to zero as in this patch simply preserves some sanity in the
> @count variable as we are no longer subtracting the E2BIG errno from
> the @count value.  Granted, in the @toobig case, @count doesn't do
> anything meaningful, but I believe this does harden the code against
> future changes.
>
> Look at the discussion between Mickaël and I in the v15 04/11 patch
> for more background.
>
> https://lore.kernel.org/linux-security-module/20230912205658.3432-5-casey@schaufler-ca.com

OK. My bad for not looking beyond the patch.

Acked-by: Casey Schaufler <casey at schaufler-ca.com>

>
>>> Signed-off-by: Paul Moore <paul at paul-moore.com>
>>> ---
>>>  security/security.c | 3 ++-
>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/security/security.c b/security/security.c
>>> index 988483fcf153..9c63acded4ee 100644
>>> --- a/security/security.c
>>> +++ b/security/security.c
>>> @@ -3951,8 +3951,9 @@ int security_getselfattr(unsigned int attr, struct lsm_ctx __user *uctx,
>>>                       continue;
>>>               }
>>>               if (rc == -E2BIG) {
>>> -                     toobig = true;
>>> +                     rc = 0;
>>>                       left = 0;
>>> +                     toobig = true;
>>>               } else if (rc < 0)
>>>                       return rc;
>>>               else