Re: XSAVE / RDPKRU on Intel 11th Gen Core CPUs

[Brian Geffon <bgeffon@xxxxxxxxxx>]

Hi Andy,

On Tue, Nov 9, 2021 at 9:58 AM Andy Lutomirski <luto@xxxxxxxxxx> 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@xxxxxxxxxx>
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@xxxxxxxxxx>
---
 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