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Message-ID: <20170512225755.GU390@ZenIV.linux.org.uk>
Date: Fri, 12 May 2017 23:57:55 +0100
From: Al Viro <viro@...IV.linux.org.uk>
To: Rik van Riel <riel@...hat.com>
Cc: Kees Cook <keescook@...omium.org>,
Russell King - ARM Linux <linux@...linux.org.uk>,
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the arch/x86 maintainers <x86@...nel.org>,
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René Nyffenegger <mail@...enyffenegger.ch>,
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"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: Re: [PATCH v9 1/4] syscalls: Verify address
limit before returning to user-mode
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?
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