[patch V2.1 11/20] x86/fpu: Dont use MCE safe fixups for writing FPU state to user space

From: Thomas Gleixner
Date: Tue Sep 07 2021 - 16:25:25 EST


Writes cannot raise #MC, so no point in pretending that the code can handle
in kernel #MC recovery.

Reported-by: Peter Ziljstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
V2: New patch
---
arch/x86/include/asm/fpu/internal.h | 48 ++++++++++++++++++++++++++++++------
1 file changed, 40 insertions(+), 8 deletions(-)

--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -92,7 +92,7 @@ extern void save_fpregs_to_fpstate(struc
* Returns 0 on success or the trap number when the operation raises an
* exception.
*/
-#define user_insn(insn, output, input...) \
+#define user_insn_mce_safe(insn, output, input...) \
({ \
int err; \
\
@@ -107,6 +107,25 @@ extern void save_fpregs_to_fpstate(struc
err; \
})

+#define user_insn(insn, output, input...) \
+({ \
+ int err; \
+ \
+ might_fault(); \
+ \
+ asm volatile(ASM_STAC "\n" \
+ "1: " #insn "\n" \
+ "2: " ASM_CLAC "\n" \
+ ".section .fixup,\"ax\"\n" \
+ "3: movl $-1,%[err]\n" \
+ " jmp 2b\n" \
+ ".previous\n" \
+ _ASM_EXTABLE(1b, 3b) \
+ : [err] "=a" (err), output \
+ : "0"(0), input); \
+ err; \
+})
+
#define kernel_insn_err(insn, output, input...) \
({ \
int err; \
@@ -161,9 +180,9 @@ static inline int fxrstor_safe(struct fx
static inline int fxrstor_from_user_sigframe(struct fxregs_state __user *fx)
{
if (IS_ENABLED(CONFIG_X86_32))
- return user_insn(fxrstor %[fx], "=m" (*fx), [fx] "m" (*fx));
+ return user_insn_mce_safe(fxrstor %[fx], "=m" (*fx), [fx] "m" (*fx));
else
- return user_insn(fxrstorq %[fx], "=m" (*fx), [fx] "m" (*fx));
+ return user_insn_mce_safe(fxrstorq %[fx], "=m" (*fx), [fx] "m" (*fx));
}

static inline void frstor(struct fregs_state *fx)
@@ -178,7 +197,7 @@ static inline int frstor_safe(struct fre

static inline int frstor_from_user_sigframe(struct fregs_state __user *fx)
{
- return user_insn(frstor %[fx], "=m" (*fx), [fx] "m" (*fx));
+ return user_insn_mce_safe(frstor %[fx], "=m" (*fx), [fx] "m" (*fx));
}

static inline void fxsave(struct fxregs_state *fx)
@@ -200,7 +219,7 @@ static inline void fxsave(struct fxregs_
* After this @err contains 0 on success or the trap number when the
* operation raises an exception.
*/
-#define XSTATE_OP(op, st, lmask, hmask, err) \
+#define XSTATE_OP_MCE_SAFE(op, st, lmask, hmask, err) \
asm volatile("1:" op "\n\t" \
"xor %[err], %[err]\n" \
"2:\n\t" \
@@ -209,6 +228,19 @@ static inline void fxsave(struct fxregs_
: "D" (st), "m" (*st), "a" (lmask), "d" (hmask) \
: "memory")

+#define XSTATE_OP(op, st, lmask, hmask, err) \
+ asm volatile("1:" op "\n\t" \
+ "xor %[err], %[err]\n" \
+ "2:\n\t" \
+ ".section .fixup,\"ax\"\n" \
+ "3: movl $-1,%[err]\n" \
+ " jmp 2b\n" \
+ ".previous\n" \
+ _ASM_EXTABLE(1b, 3b) \
+ : [err] "=a" (err) \
+ : "D" (st), "m" (*st), "a" (lmask), "d" (hmask) \
+ : "memory")
+
/*
* If XSAVES is enabled, it replaces XSAVEOPT because it supports a compact
* format and supervisor states in addition to modified optimization in
@@ -360,15 +392,15 @@ static inline int xrstor_from_user_sigfr
int err;

stac();
- XSTATE_OP(XRSTOR, xstate, lmask, hmask, err);
+ XSTATE_OP_MCE_SAFE(XRSTOR, xstate, lmask, hmask, err);
clac();

return err;
}

/*
- * Restore xstate from kernel space xsave area, return an error code instead of
- * an exception.
+ * Restore xstate from kernel space xsave area, return an error code when
+ * the operation raises an exception.
*/
static inline int os_xrstor_safe(struct xregs_state *xstate, u64 mask)
{