From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Dave Hansen <dave.hansen@intel.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
"Andy Lutomirski" <luto@kernel.org>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Radim Krčmář" <rkrcmar@redhat.com>,
kvm@vger.kernel.org, "Jason A. Donenfeld" <Jason@zx2c4.com>,
"Rik van Riel" <riel@surriel.com>,
"Dave Hansen" <dave.hansen@linux.intel.com>
Subject: Re: [PATCH 14/22] x86/fpu: Eager switch PKRU state
Date: Mon, 11 Mar 2019 12:06:05 +0100 [thread overview]
Message-ID: <20190311110605.2xxjx2ym3n63lpr2@linutronix.de> (raw)
In-Reply-To: <b0f3ffa8-faec-73b2-f6f2-ea37ece3a3b1@intel.com>
On 2019-03-08 11:01:25 [-0800], Dave Hansen wrote:
> On 3/8/19 10:08 AM, Sebastian Andrzej Siewior wrote:
> > On 2019-02-25 10:16:24 [-0800], Dave Hansen wrote:
> >>> + if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
> >>> + return;
> >>> +
> >>> + if (current->mm) {
> >>> + pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
> >>> + WARN_ON_ONCE(!pk);
…
> Nothing will break, but the warning will trigger, which isn't nice.
the warning should trigger if something goes south, I was not expecting
it to happen.
> > My understanding is that the in-kernel XSAVE will always save everything
> > so we should never "lose" the XFEATURE_PKRU no matter what user space
> > does.
> >
> > So as test case you want
> > xsave (-1 & ~XFEATURE_PKRU)
> > xrestore (-1 & ~XFEATURE_PKRU)
> >
> > in userland and then a context switch to see if the warning above
> > triggers?
>
> I think you need an XRSTOR with RFBM=-1 (or at least with the PKRU bit
> set) and the PKRU bit in the XFEATURES field in the XSAVE buffer set to 0.
let me check that, write a test case in userland and I come back with
the results. I can remove that warning but I wasn't expecting it to
trigger so let me verify that first.
> >>> + if (pk)
> >>> + pkru_val = pk->pkru;
> >>> + }> + __write_pkru(pkru_val);
> >>> }
> >>
> >> A comment above __write_pkru() would be nice to say that it only
> >> actually does the slow instruction on changes to the value.
> >
> > Could we please not do this? It is a comment above one of the callers
> > function and we have two or three. And we have that comment already
> > within __write_pkru().
>
> I looked at this code and thought "writing PKRU is slow", and "this
> writes PKRU unconditionally", and "the __ version of the function
> shoudn't have much logic in it".
>
> I got 2/3 wrong. To me that means this site needs a 1-line comment.
> Feel free to move one of the other comments to here if you think it's
> over-commented, but this site needs one.
right because things changed as part of patch series.
You wanted to have in __write_pkru() the same semantic like in
__read_pkru() which is currently the case because __write_pkru() has the
check. It would be great if we could rename it to something else and
avoid the comment. (Because if this user gets a comment then other
should, too and I think this is an overkill).
> > Last time we talked about this we agreed (or this was my impression) that
> > 0 should be written so that the kernel thread should always be able to
> > write to user space in case it borrowed its mm (otherwise it has none
> > and it would fail anyway).
>
> We can't write to userspace when borrowing an mm. If the kernel borrows
> an mm, we might as well be on the init_mm which has no userspace mappings.
If a kernel thread borrows a mm from a user task via use_mm() then it
_can_ write to that task's user land memory from a kthread.
> > We didn't want to leave PKRU alone because the outcome (whether or not
> > the write by the kernel thread succeeds) should not depend on the last
> > running task (and be random) but deterministic.
>
> Right, so let's make it deterministically restrictive: either
> init_pkru_value, or -1 since kernel threads shouldn't be touching
> userspace in the first place.
I'm fine either way, just tell me what you want. Just consider the
use_mm() part above I wrote. (I remember you/luto suggest to have an API
for something like that so that the PKRU value can be
Sebastian
next prev parent reply other threads:[~2019-03-11 11:06 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-21 11:49 [PATCH v7] x86: load FPU registers on return to userland Sebastian Andrzej Siewior
2019-02-21 11:49 ` [PATCH 01/22] x86/fpu: Remove fpu->initialized usage in __fpu__restore_sig() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 02/22] x86/fpu: Remove fpu__restore() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 03/22] x86/fpu: Remove preempt_disable() in fpu__clear() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 04/22] x86/fpu: Always init the `state' " Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in copy_fpstate_to_sigframe() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 06/22] x86/fpu: Don't save fxregs for ia32 frames " Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 07/22] x86/fpu: Remove fpu->initialized Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 08/22] x86/fpu: Remove user_fpu_begin() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 09/22] x86/fpu: Add (__)make_fpregs_active helpers Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 10/22] x86/fpu: Make __raw_xsave_addr() use feature number instead of mask Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 11/22] x86/fpu: Make get_xsave_field_ptr() and get_xsave_addr() " Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 12/22] x86/fpu: Only write PKRU if it is different from current Sebastian Andrzej Siewior
2019-02-25 18:08 ` Dave Hansen
2019-03-08 17:24 ` Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 13/22] x86/pkeys: Don't check if PKRU is zero before writting it Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 14/22] x86/fpu: Eager switch PKRU state Sebastian Andrzej Siewior
2019-02-25 18:16 ` Dave Hansen
2019-03-08 18:08 ` Sebastian Andrzej Siewior
2019-03-08 19:01 ` Dave Hansen
2019-03-11 11:06 ` Sebastian Andrzej Siewior [this message]
2019-03-11 14:30 ` Sebastian Andrzej Siewior
[not found] ` <cd1a34e6-f122-33e3-864f-e23c1833a6c0@intel.com>
2019-03-21 17:10 ` Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 15/22] x86/entry: Add TIF_NEED_FPU_LOAD Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 16/22] x86/fpu: Always store the registers in copy_fpstate_to_sigframe() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 17/22] x86/fpu: Prepare copy_fpstate_to_sigframe() for TIF_NEED_FPU_LOAD Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 18/22] x86/fpu: Update xstate's PKRU value on write_pkru() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 19/22] x86/fpu: Inline copy_user_to_fpregs_zeroing() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 20/22] x86/fpu: Let __fpu__restore_sig() restore the !32bit+fxsr frame from kernel memory Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 21/22] x86/fpu: Merge the two code paths in __fpu__restore_sig() Sebastian Andrzej Siewior
2019-02-21 11:50 ` [PATCH 22/22] x86/fpu: Defer FPU state load until return to userspace Sebastian Andrzej Siewior
-- strict thread matches above, loose matches on Subject: below --
2019-01-09 11:47 [PATCH v6] x86: load FPU registers on return to userland Sebastian Andrzej Siewior
2019-01-09 11:47 ` [PATCH 14/22] x86/fpu: Eager switch PKRU state Sebastian Andrzej Siewior
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