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SubjectRe: XSAVE / RDPKRU on Intel 11th Gen Core CPUs
Hi Andy,

On Tue, Nov 9, 2021 at 9:58 AM Andy Lutomirski <luto@kernel.org> wrote:
> Here's an excerpt from an old email that I, perhaps unwisely, sent to Dave but not to a public list:
>
> static inline void write_pkru(u32 pkru)
> {
> struct pkru_state *pk;
>
> if (!boot_cpu_has(X86_FEATURE_OSPKE))
> return;
>
> pk = get_xsave_addr(&current->thread.fpu.state.xsave,
> XFEATURE_PKRU);
>
> /*
> * The PKRU value in xstate needs to be in sync with the value
> that is
> * written to the CPU. The FPU restore on return to userland would
> * otherwise load the previous value again.
> */
> fpregs_lock();
> if (pk)
> pk->pkru = pkru;
>
> ^^^
> else we just write to the PKRU register but leave XINUSE[PKRU] clear on
> return to usermode? That seems... unwise.
>
> __write_pkru(pkru);
> fpregs_unlock();
> }
>
> I bet you're hitting exactly this bug. The fix ended up being a whole series of patches, but the gist of it is that the write_pkru() slow path needs to set the xfeature bit in the xsave buffer and then do the write. It should be possible to make a little patch to do just this in a couple lines of code.

I think you've got the right idea, the following patch does seem to
fix the problem on this CPU, this is based on 5.13. It seems the
changes to asm/pgtable.h were not enough, I also had to modify
fpu/internal.h to get it working properly.

From e5e184d68ac6ca93c3cd2cc88d61af3260d1c014 Mon Sep 17 00:00:00 2001
From: Brian Geffon <bgeffon@google.com>
Date: Tue, 9 Nov 2021 17:08:25 +0000
Subject: [PATCH] x86/fpu: Set XFEATURE_PKRU when writing to pkru

On kernels prior to 5.14 the write_pkru path could
end up writing to the pkru register without updating
the corresponding state.

Signed-off-by: Brian Geffon <bgeffon@google.com>
---
arch/x86/include/asm/fpu/internal.h | 22 ++++++++++------------
arch/x86/include/asm/pgtable.h | 5 +++--
2 files changed, 13 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h
b/arch/x86/include/asm/fpu/internal.h
index 16bf4d4a8159..ed2ce7d1afeb 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -564,18 +564,16 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
* PKRU state is switched eagerly because it needs to be valid before we
* return to userland e.g. for a copy_to_user() operation.
*/
- if (!(current->flags & PF_KTHREAD)) {
- /*
- * If the PKRU bit in xsave.header.xfeatures is not set,
- * then the PKRU component was in init state, which means
- * XRSTOR will set PKRU to 0. If the bit is not set then
- * get_xsave_addr() will return NULL because the PKRU value
- * in memory is not valid. This means pkru_val has to be
- * set to 0 and not to init_pkru_value.
- */
- pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
- pkru_val = pk ? pk->pkru : 0;
- }
+ /*
+ * If the PKRU bit in xsave.header.xfeatures is not set,
+ * then the PKRU component was in init state, which means
+ * XRSTOR will set PKRU to 0. If the bit is not set then
+ * get_xsave_addr() will return NULL because the PKRU value
+ * in memory is not valid. This means pkru_val has to be
+ * set to 0 and not to init_pkru_value.
+ */
+ pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
+ pkru_val = pk ? pk->pkru : 0;
__write_pkru(pkru_val);
}

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index b1099f2d9800..d00fc2df4cfe 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -137,18 +137,19 @@ static inline u32 read_pkru(void)
static inline void write_pkru(u32 pkru)
{
struct pkru_state *pk;
+ struct fpu *fpu = &current->thread.fpu;

if (!boot_cpu_has(X86_FEATURE_OSPKE))
return;

- pk = get_xsave_addr(&current->thread.fpu.state.xsave, XFEATURE_PKRU);
-
/*
* The PKRU value in xstate needs to be in sync with the value that is
* written to the CPU. The FPU restore on return to userland would
* otherwise load the previous value again.
*/
+ fpu->state.xsave.header.xfeatures |= XFEATURE_MASK_PKRU;
fpregs_lock();
+ pk = get_xsave_addr(&fpu->state.xsave, XFEATURE_PKRU);
if (pk)
pk->pkru = pkru;
__write_pkru(pkru);
--
2.34.0.rc0.344.g81b53c2807-goog
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