linux-kernel - [PATCH] x86/pkeys: Manually set X86_FEATURE

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Date:   Wed, 26 Feb 2020 15:16:15 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jacob Keller <jacob.e.keller@...el.com>
Subject: [PATCH] x86/pkeys: Manually set X86_FEATURE_OSPKE to preserve existing changes

Explicitly set X86_FEATURE_OSPKE via set_cpu_cap() instead of calling
get_cpu_cap() to pull the feature bit from CPUID after enabling CR4.PKE.
Invoking get_cpu_cap() effectively wipes out any {set,clear}_cpu_cap()
changes that were made between this_cpu->c_init() and setup_pku(), as
all non-synthetic feature words are reinitialized from the CPU's CPUID
values.

Blasting away capability updates manifests most visibility when running
on a VMX capable CPU, but with VMX disabled by BIOS.  To indicate that
VMX is disabled, init_ia32_feat_ctl() clears X86_FEATURE_VMX, using
clear_cpu_cap() instead of setup_clear_cpu_cap() so that KVM can report
which CPU is misconfigured (KVM needs to probe every CPU anyways).
Restoring X86_FEATURE_VMX from CPUID causes KVM to think VMX is enabled,
ultimately leading to an unexpected #GP when KVM attempts to do VMXON.

Arguably, init_ia32_feat_ctl() should use setup_clear_cpu_cap() and let
KVM figure out a different way to report the misconfigured CPU, but VMX
is not the only feature bit that is affected, i.e. there is precedent
that tweaking feature bits via {set,clear}_cpu_cap() after ->c_init()
is expected to work.  Most notably, x86_init_rdrand()'s clearing of
X86_FEATURE_RDRAND when RDRAND malfunctions is also overwritten.

Fixes: 0697694564c8 ("x86/mm/pkeys: Actually enable Memory Protection Keys in the CPU")
Cc: stable@...r.kernel.org
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Reported-by: Jacob Keller <jacob.e.keller@...el.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---

I considered adding a WARN_ON check of cpuid_ecx(7) to ensure OSPKE is set
and bail if it's not, but that seemed like useless paranoia.

I don't love that using clear_cpu_cap() is a bit fragile, especially where
init_ia32_feat_ctl() is concerned, but the existing RDRAND usage, which
predates setup_pku(), makes a pretty strong argument that setup_pku() is
in the wrong.

The other potential issue is late microcode updates, which refreshes the
feature bits from CPUID, but at least in that case the kernel will print a
warning if something changed.

 arch/x86/kernel/cpu/common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 52c9bfbbdb2a..4cdb123ff66a 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -445,7 +445,7 @@ static __always_inline void setup_pku(struct cpuinfo_x86 *c)
 	 * cpuid bit to be set.  We need to ensure that we
 	 * update that bit in this CPU's "cpu_info".
 	 */
-	get_cpu_cap(c);
+	set_cpu_cap(c, X86_FEATURE_OSPKE);
 }

 #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
-- 
2.24.1