linux-kernel - [PATCH] x86/special_insn: reverse __force

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Message-Id: <20200901161857.566142-1-namit@vmware.com>
Date:   Tue,  1 Sep 2020 09:18:57 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Cc:     Nadav Amit <namit@...are.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>, stable@...r.kernel.org
Subject: [PATCH] x86/special_insn: reverse __force_order logic

From: Nadav Amit <namit@...are.com>

The __force_order logic seems to be inverted. __force_order is
supposedly used to manipulate the compiler to use its memory
dependencies analysis to enforce orders between CR writes and reads.
Therefore, the memory should behave as a "CR": when the CR is read, the
memory should be "read" by the inline assembly, and __force_order should
be an output. When the CR is written, the memory should be "written".

This change should allow to remove the "volatile" qualifier from CR
reads at a later patch.

While at it, remove the extra new-line from the inline assembly, as it
only confuses GCC when it estimates the cost of the inline assembly.

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Kees Cook <keescook@...omium.org>
Cc: stable@...r.kernel.org
Signed-off-by: Nadav Amit <namit@...are.com>

---

Unless I misunderstand the logic, __force_order should also be used by
rdpkru() and wrpkru() which do not have dependency on __force_order. I
also did not understand why native_write_cr0() has R/W dependency on
__force_order, and why native_write_cr4() no longer has any dependency
on __force_order.
---
 arch/x86/include/asm/special_insns.h | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 5999b0b3dd4a..dff5e5b01a3c 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -24,32 +24,32 @@ void native_write_cr0(unsigned long val);
 static inline unsigned long native_read_cr0(void)
 {
 	unsigned long val;
-	asm volatile("mov %%cr0,%0\n\t" : "=r" (val), "=m" (__force_order));
+	asm volatile("mov %%cr0,%0" : "=r" (val) : "m" (__force_order));
 	return val;
 }
 
 static __always_inline unsigned long native_read_cr2(void)
 {
 	unsigned long val;
-	asm volatile("mov %%cr2,%0\n\t" : "=r" (val), "=m" (__force_order));
+	asm volatile("mov %%cr2,%0" : "=r" (val) : "m" (__force_order));
 	return val;
 }
 
 static __always_inline void native_write_cr2(unsigned long val)
 {
-	asm volatile("mov %0,%%cr2": : "r" (val), "m" (__force_order));
+	asm volatile("mov %1,%%cr2" : "=m" (__force_order) : "r" (val));
 }
 
 static inline unsigned long __native_read_cr3(void)
 {
 	unsigned long val;
-	asm volatile("mov %%cr3,%0\n\t" : "=r" (val), "=m" (__force_order));
+	asm volatile("mov %%cr3,%0" : "=r" (val) : "m" (__force_order));
 	return val;
 }
 
 static inline void native_write_cr3(unsigned long val)
 {
-	asm volatile("mov %0,%%cr3": : "r" (val), "m" (__force_order));
+	asm volatile("mov %1,%%cr3" : "=m" (__force_order) : "r" (val));
 }
 
 static inline unsigned long native_read_cr4(void)
@@ -64,10 +64,10 @@ static inline unsigned long native_read_cr4(void)
 	asm volatile("1: mov %%cr4, %0\n"
 		     "2:\n"
 		     _ASM_EXTABLE(1b, 2b)
-		     : "=r" (val), "=m" (__force_order) : "0" (0));
+		     : "=r" (val) : "m" (__force_order), "0" (0));
 #else
 	/* CR4 always exists on x86_64. */
-	asm volatile("mov %%cr4,%0\n\t" : "=r" (val), "=m" (__force_order));
+	asm volatile("mov %%cr4,%0" : "=r" (val) : "m" (__force_order));
 #endif
 	return val;
 }
-- 
2.25.1