On Fri, May 12, 2017 at 12:08 PM, Linus Torvalds
[off-list ref] wrote:

On Fri, May 12, 2017 at 12:01 PM, Kees Cook [off-list ref] wrote:

quoted

Yeah, the risk for "corrupted addr_limit" is mainly a concern for
archs with addr_limit on the kernel stack. If I'm reading things
correctly, that means, from the archs I've been paying closer
attention to, it's an issue for arm, mips, and powerpc:

I don't understand why people are looking at addr_limit as some kind
of special thing.

If somebody is smashing the stack and corrupting thread info data, the
game is over. addr_limit is the *least* of your problems, and it's not
even all that likely that it will be increasing (it's much more likely
that it would be overwritten with a smaller value).

Quite frankly, this kind of idiotic discussion just makes me question
the whole idea of the patch.

Any "security" that is this specific is not real security, it's just
masturbatory garbage.

It may be worth checking that people use "set_fs()" properly. But stop
this idiotic crap. It just makes the kernel security people look like
the crazies.

There are enough incompetent crazy security people, don't go there.
The kinds of things it is worth protecting against are the big class
of generic issues, not the kind of "oh, but imagine if a cosmic ray
flips this particular word in memory" kind of crap that ignores all
the other words of memory.

Seriously, Kees. You are just making security people look bad. Stop it.

I'm clearly not explaining things well enough. I shouldn't say
"corruption", I should say "malicious manipulation". The methodology
of attacks against the stack are quite different from the other kinds
of attacks like use-after-free, heap overflow, etc. Being able to
exhaust the kernel stack (either due to deep recursion or unbounded
alloca()) means attackers can control a write to addr_limit, and then
leverage that into an actual arbitrary write via subsequent calls to
copy_to_user() pointed at kernel memory. This isn't theoretical, this
is how those attacks are performed. It may sound crazy, but it's real.

With thread_info off the stack, the whole problem goes away. It's
wonderful that this has happened for x86, arm64, and s390. There are
always going to be new methods of attack for everything, but while we
slowly address the design weakness (set_fs()), we can fix the low
hanging fruit too.

-Kees

-- 
Kees Cook
Pixel Security