Re: TLB flushes on fixmap changes

[Nadav Amit]

at 9:43 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> On Sat, Aug 25, 2018 at 9:21 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> On Sat, Aug 25, 2018 at 7:23 PM, Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>>> On Fri, 24 Aug 2018 21:23:26 -0700
>>> Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>> Couldn't text_poke() use kmap_atomic()?  Or, even better, just change CR3?
>>> 
>>> No, since kmap_atomic() is only for x86_32 and highmem support kernel.
>>> In x86-64, it seems that returns just a page address. That is not
>>> good for text_poke, since it needs to make a writable alias for RO
>>> code page. Hmm, maybe, can we mimic copy_oldmem_page(), it uses ioremap_cache?
>> 
>> I just re-read text_poke().  It's, um, horrible.  Not only is the
>> implementation overcomplicated and probably buggy, but it's SLOOOOOW.
>> It's totally the wrong API -- poking one instruction at a time
>> basically can't be efficient on x86.  The API should either poke lots
>> of instructions at once or should be text_poke_begin(); ...;
>> text_poke_end();.
>> 
>> Anyway, the attached patch seems to boot.  Linus, Kees, etc: is this
>> too scary of an approach?  With the patch applied, text_poke() is a
>> fantastic exploit target.  On the other hand, even without the patch
>> applied, text_poke() is every bit as juicy.
> 
> I tried to convince Ingo to use this method for doing "write rarely"
> and he soundly rejected it. :) I've always liked this because AFAICT,
> it's local to the CPU. I had proposed it in
> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/write-rarely&id=9ab0cb2618ebbc51f830ceaa06b7d2182fe1a52d
> 
> With that, text_poke() mostly becomes:
> 
> rare_write_begin()
> memcpy(addr, opcode, len);
> rare_write_end()
> 
> As for juiciness, if an attacker already has execution control, they
> can do more interesting things than text_poke(). But regardless, yes,
> it's always made me uncomfortable. :)

I think that the key to harden the security of text_poke() against its use
as a gadget in a ROP/JOP attack is to add a check/assertion for the old
(expected) value, such as:

rare_write_begin()
if (*addr == prev_opcode)
	memcpy(addr, opcode, len);
rare_write_end()