linux-kernel - [PATCH 05/11] x86/fpu: set PKRU state for kernel threads

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Date:   Thu,  4 Oct 2018 16:05:41 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 05/11] x86/fpu: set PKRU state for kernel threads

The PKRU value is not set for kernel threads because they do not have
the ->initialized value set. As a result the kernel thread has a random
PKRU value set which it inherits from the previous task.
It has been suggested by Paolo Bonzini to set it for kernel threads, too
because it might be a fix.
I *think* this is not required because the kernel threads don't copy
data to/from userland and don't have access to any userspace mm in
general.
However there is this use_mm(). If we gain a mm by use_mm() we don't
have a matching PKRU value because those are per thread. It has been
suggested to use 0 as the PKRU value but this would bypass PKRU.

Set the initial (default) PKRU value for kernel threads.

Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 arch/x86/include/asm/fpu/internal.h | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 956d967ca824a..4ecaf4d22954e 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -14,6 +14,7 @@
 #include <linux/compat.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
+#include <linux/pkeys.h>
 
 #include <asm/user.h>
 #include <asm/fpu/api.h>
@@ -573,20 +574,23 @@ static inline void switch_fpu_finish(struct fpu *new_fpu, int cpu)
 	bool load_fpu;
 
 	load_fpu = static_cpu_has(X86_FEATURE_FPU) && new_fpu->initialized;
-	if (!load_fpu)
-		return;
-
-	__fpregs_load_activate(new_fpu, cpu);
-
 #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
 	if (static_cpu_has(X86_FEATURE_OSPKE)) {
 		struct pkru_state *pk;
 
-		pk = __raw_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
-		if (pk->pkru != __read_pkru())
-			__write_pkru(pk->pkru);
+		if (!load_fpu) {
+			pkru_set_init_value();
+		} else {
+			pk = __raw_xsave_addr(&new_fpu->state.xsave,
+					      XFEATURE_PKRU);
+			if (pk->pkru != __read_pkru())
+				__write_pkru(pk->pkru);
+		}
 	}
 #endif
+	if (!load_fpu)
+		return;
+	__fpregs_load_activate(new_fpu, cpu);
 }
 
 /*
-- 
2.19.0