linux-kernel - [PATCH AUTOSEL 4.14 10/21] x86/fpu: Return proper error codes from user access functions

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Message-Id: <20210710235212.3222375-10-sashal@kernel.org>
Date:   Sat, 10 Jul 2021 19:52:01 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...e.de>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 4.14 10/21] x86/fpu: Return proper error codes from user access functions

From: Thomas Gleixner <tglx@...utronix.de>

[ Upstream commit aee8c67a4faa40a8df4e79316dbfc92d123989c1 ]

When *RSTOR from user memory raises an exception, there is no way to
differentiate them. That's bad because it forces the slow path even when
the failure was not a fault. If the operation raised eg. #GP then going
through the slow path is pointless.

Use _ASM_EXTABLE_FAULT() which stores the trap number and let the exception
fixup return the negated trap number as error.

This allows to separate the fast path and let it handle faults directly and
avoid the slow path for all other exceptions.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Borislav Petkov <bp@...e.de>
Link: https://lkml.kernel.org/r/20210623121457.601480369@linutronix.de
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/include/asm/fpu/internal.h | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index fa2c93cb42a2..b8c935033d21 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -103,6 +103,7 @@ static inline void fpstate_init_fxstate(struct fxregs_state *fx)
 }
 extern void fpstate_sanitize_xstate(struct fpu *fpu);
 
+/* Returns 0 or the negated trap number, which results in -EFAULT for #PF */
 #define user_insn(insn, output, input...)				\
 ({									\
 	int err;							\
@@ -110,14 +111,14 @@ extern void fpstate_sanitize_xstate(struct fpu *fpu);
 	might_fault();							\
 									\
 	asm volatile(ASM_STAC "\n"					\
-		     "1:" #insn "\n\t"					\
+		     "1: " #insn "\n"					\
 		     "2: " ASM_CLAC "\n"				\
 		     ".section .fixup,\"ax\"\n"				\
-		     "3:  movl $-1,%[err]\n"				\
+		     "3:  negl %%eax\n"					\
 		     "    jmp  2b\n"					\
 		     ".previous\n"					\
-		     _ASM_EXTABLE(1b, 3b)				\
-		     : [err] "=r" (err), output				\
+		     _ASM_EXTABLE_FAULT(1b, 3b)				\
+		     : [err] "=a" (err), output				\
 		     : "0"(0), input);					\
 	err;								\
 })
@@ -221,16 +222,20 @@ static inline void copy_fxregs_to_kernel(struct fpu *fpu)
 #define XRSTOR		".byte " REX_PREFIX "0x0f,0xae,0x2f"
 #define XRSTORS		".byte " REX_PREFIX "0x0f,0xc7,0x1f"
 
+/*
+ * After this @err contains 0 on success or the negated trap number when
+ * the operation raises an exception. For faults this results in -EFAULT.
+ */
 #define XSTATE_OP(op, st, lmask, hmask, err)				\
 	asm volatile("1:" op "\n\t"					\
 		     "xor %[err], %[err]\n"				\
 		     "2:\n\t"						\
 		     ".pushsection .fixup,\"ax\"\n\t"			\
-		     "3: movl $-2,%[err]\n\t"				\
+		     "3: negl %%eax\n\t"				\
 		     "jmp 2b\n\t"					\
 		     ".popsection\n\t"					\
-		     _ASM_EXTABLE(1b, 3b)				\
-		     : [err] "=r" (err)					\
+		     _ASM_EXTABLE_FAULT(1b, 3b)				\
+		     : [err] "=a" (err)					\
 		     : "D" (st), "m" (*st), "a" (lmask), "d" (hmask)	\
 		     : "memory")
 
-- 
2.30.2