Re: [PATCH] fs/binfmt_elf: Add padding NULL when argc == 0

[Eric W. Biederman]

Kees Cook <keescook@xxxxxxxxxxxx> writes:

> On Wed, Jan 26, 2022 at 08:08:14PM +0000, Matthew Wilcox wrote:
>> On Wed, Jan 26, 2022 at 11:58:39AM -0800, Kees Cook wrote:
>> > We can't mutate argc; it'll turn at least some userspace into an
>> > infinite loop:
>> > https://sources.debian.org/src/valgrind/1:3.18.1-1/none/tests/execve.c/?hl=22#L22
>> 
>> How does that become an infinite loop?  We obviously wouldn't mutate
>> argc in the caller, just the callee.
>
> Oh, sorry, I misread. It's using /bin/true, not argv[0] (another bit of
> code I found was using argv[0]). Yeah, {"", NULL} could work.
>
>> Also, there's a version of this where we only mutate argc if we're
>> executing a setuid program, which would remove the privilege
>> escalation part of things.
>
> True; though I'd like to keep the logic as non-specialized as possible.
> I don't like making stuff conditional on privilege boundaries if we can
> make it always happen.

Which I think means turning the argc == 0 case into { "", NULL }.
I think we can always do that, and it is already valid in userspace.

The only case I can imagine breaking would be an explicitly testing
for argc == 0 and behaving completely differently if that is passed
to the program.

Eric