Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

Al Viro
2017-05-12T22:57:55Z

On Fri, May 12, 2017 at 05:47:55PM -0400, Rik van Riel wrote:

> > Seriously, look at these beasts.  Overwriting ->addr_limit is nowhere
> > near
> > the top threat.  If attacker can overwrite thread_info, you have
> > lost.
>
> That is why THREAD_INFO_IN_TASK exists. It moves
> the struct thread_info to a location away from the
> stack, which means a stack overflow will not overwrite
> the thread_info.

... in which case such attacks on ->addr_limit also become a non-issue.

AFAICS, we are mixing several unrelated issues here:
* amount of places where set_fs() is called. Sure, reducing it
is a good idea and we want to move to primitives like kernel_write() et.al.
Fewer users => lower odds of screwing it up.
* making sure that remaining callers are properly paired. Ditto.
* switching to ->read_iter()/->write_iter() where it makes sense.
Again, no problem with that.
* providing sane environment for places like perf/oprofile. Again,
a good idea, and set_fs(USER_DS) is only a part of what's needed there.
* switching _everything_ to ->read_iter()/->write_iter(). Flat-out
insane and AFAICS nobody is signing up for that.
* getting rid of set_fs() entirely. I'm afraid that it's not feasible
without the previous one and frankly, I don't see much point.
* sanity-checking on return to userland. Maybe useful, maybe not.
* taking thread_info out of the way of stack overflows. Reasonable,
but has very little to do with the rest of that.
* protecting against Lovecraftian horrors slithering in from the outer
space only to commit unspeakable acts against ->addr_limit and ignoring much
tastier targets next to it, but then what do you expect from degenerate
spawn of Great Old Ones - sanity?