Messages in this thread | | | Date | Tue, 10 May 2022 15:08:56 -0700 | From | Kees Cook <> | Subject | Re: [PATCH V10 14/44] mm/pkeys: Introduce pks_set_readwrite() |
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On Tue, May 10, 2022 at 02:33:03PM -0700, Ira Weiny wrote: > On Mon, May 09, 2022 at 02:38:38PM -0700, Kees Cook wrote: > > [...] > > Better yet would be: > > > > preempt_disable(); > > rdmsrl(MSR_IA32_PKRS, pkrs); > > pkrs = pkey_update_pkval(pkrs, pkey, protection); > > pks_write_pkrs(pkrs); > > current->thread.pkrs = pkrs; > > preempt_enable(); > > > > Then cross-thread attacks cannot corrupt the _other_ PKS keys (i.e. > > write the desired changes to target's current->thread.kprs and trigger > > an update to a different pkey, resulting in flushing the attacker's > > changes to that CPU's pkey state. > > Unfortunately I don't think this entirely prevents an attack through the > thread.pkrs value. thread.pkrs has to be used to set the MSR when a thread is > scheduled. Therefore the rdmsrl above will by definition pick up the > thread.pkrs but from an earlier time.
Ooh, good point, yeah.
> I'm not opposed to doing this as I think it does reduce the time window of such > an attack but I wanted to mention it. Especially since I specifically avoided > ever reading the MSR to improve performance. > > I'm going to run some tests. Perhaps the MSR read is not that big of a deal > and I can convince myself that the performance diff is negligible.
Yeah, given "loaded at scheduling" point, there's not much benefit in read/write pair. I think my first suggestion about only writing to thread.pkrs after the write, etc, still stands. I'll ponder this a bit more.
> > While adding these, can you please also add pks_set_nowrite()? This > > will be needed for protecting writes to memory that should be otherwise > > readable. > > I have a patch to add pks_set_readonly() but I was advised to not send it > because this series does not include a use case for it. (PMEM does not need > it.) > > Dave, Dan? Are you ok adding that back? > > Kees would you prefer pks_set_nowrite() as a name?
I think nowrite is the better name (in the sense that "read-only" can sometimes imply non-executable).
> > > > With these changes it should be possible to protect the kernel's page > > table entries from "stray" writes. :) > > Yes, Rick has done some great work in that area.
Oh! I would _love_ to see this series. I was trying to scope the work yesterday but gave up after I couldn't figure out the qemu PKS trick. :)
-- Kees Cook
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