Re: TLB flushes on fixmap changes

[Masami Hiramatsu]

On Sun, 26 Aug 2018 20:26:09 -0700
Nadav Amit <nadav.amit@xxxxxxxxx> wrote:

> at 8:03 PM, Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> 
> > On Sun, 26 Aug 2018 11:09:58 +0200
> > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > 
> >> On Sat, Aug 25, 2018 at 09:21:22PM -0700, Andy Lutomirski wrote:
> >>> 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();.
> >> 
> >> I don't think anybody ever cared about performance here. Only
> >> correctness. That whole text_poke_bp() thing is entirely tricky.
> > 
> > Agreed. Self modification is a special event.
> > 
> >> FWIW, before text_poke_bp(), text_poke() would only be used from
> >> stop_machine, so all the other CPUs would be stuck busy-waiting with
> >> IRQs disabled. These days, yeah, that's lots more dodgy, but yes
> >> text_mutex should be serializing all that.
> > 
> > I'm still not sure that speculative page-table walk can be done
> > over the mutex. Also, if the fixmap area is for aliasing
> > pages (which always mapped to memory), what kind of
> > security issue can happen?
> 
> The PTE is accessible from other cores, so just as we assume for L1TF that
> the every addressable memory might be cached in L1, we should assume and
> PTE might be cached in the TLB when it is present.

Ok, so other cores can accidentally cache the PTE in TLB, (and no way
to shoot down explicitly?)

> Although the mapping is for an alias, there are a couple of issues here.
> First, this alias mapping is writable, so it might an attacker to change the
> kernel code (following another initial attack).

Combined with some buffer overflow, correct? If the attacker already can
write a kernel data directly, he is in the kernel mode.

> Second, the alias mapping is
> never explicitly flushed. We may assume that once the original mapping is
> removed/changed, a full TLB flush would take place, but there is no
> guarantee it actually takes place.

Hmm, would this means a full TLB flush will not flush alias mapping?
(or, the full TLB flush just doesn't work?)

> > Anyway, from the viewpoint of kprobes, either per-cpu fixmap or
> > changing CR3 sounds good to me. I think we don't even need per-cpu,
> > it can call a thread/function on a dedicated core (like the first
> > boot processor) and wait :) This may prevent leakage of pte change
> > to other cores.
> 
> I implemented per-cpu fixmap, but I think that it makes more sense to take
> peterz approach and set an entry in the PGD level. Per-CPU fixmap either
> requires to pre-populate various levels in the page-table hierarchy, or
> conditionally synchronize whenever module memory is allocated, since they
> can share the same PGD, PUD & PMD. While usually the synchronization is not
> needed, the possibility that synchronization is needed complicates locking.
> 

Could you point which PeterZ approach you said? I guess it will be
make a clone of PGD and use it for local page mapping (as new mm).
If so, yes it sounds perfectly fine to me.

> Anyhow, having fixed addresses for the fixmap can be used to circumvent
> KASLR.

I think text_poke doesn't mind using random address :)

> I don’t think a dedicated core is needed. Anyhow there is a lock
> (text_mutex), so use_mm() can be used after acquiring the mutex.

Hmm, use_mm() said;

/*
 * use_mm
 *      Makes the calling kernel thread take on the specified
 *      mm context.
 *      (Note: this routine is intended to be called only
 *      from a kernel thread context)
 */

So maybe we need a dedicated kernel thread for safeness?

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>