[PATCH 2/2] x86/paravirt: add extra clobbers with ZERO_CALL_USED_REGS enabled

From: Bill Wendling
Date: Fri Sep 02 2022 - 17:38:10 EST


The ZERO_CALL_USED_REGS feature may zero out caller-saved registers
before returning.

In spurious_kernel_fault(), the "pte_offset_kernel()" call results in
this assembly code:

.Ltmp151:
#APP
# ALT: oldnstr
.Ltmp152:
.Ltmp153:
.Ltmp154:
.section .discard.retpoline_safe,"",@progbits
.quad .Ltmp154
.text

callq *pv_ops+536(%rip)

.Ltmp155:
.section .parainstructions,"a",@progbits
.p2align 3, 0x0
.quad .Ltmp153
.byte 67
.byte .Ltmp155-.Ltmp153
.short 1
.text
.Ltmp156:
# ALT: padding
.zero (-(((.Ltmp157-.Ltmp158)-(.Ltmp156-.Ltmp152))>0))*((.Ltmp157-.Ltmp158)-(.Ltmp156-.Ltmp152)),144
.Ltmp159:
.section .altinstructions,"a",@progbits
.Ltmp160:
.long .Ltmp152-.Ltmp160
.Ltmp161:
.long .Ltmp158-.Ltmp161
.short 33040
.byte .Ltmp159-.Ltmp152
.byte .Ltmp157-.Ltmp158
.text

.section .altinstr_replacement,"ax",@progbits
# ALT: replacement 1
.Ltmp158:
movq %rdi, %rax
.Ltmp157:
.text
#NO_APP
.Ltmp162:
testb $-128, %dil

The "testb" here is using %dil, but the %rdi register was cleared before
returning from "callq *pv_ops+536(%rip)". Adding the proper constraints
results in the use of a different register:

movq %r11, %rdi

# Similar to above.

testb $-128, %r11b

Link: https://github.com/KSPP/linux/issues/192
Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx>
Reported-and-tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
---
arch/x86/include/asm/paravirt_types.h | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
index f04157456a49..b1ab5d94881b 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -414,8 +414,17 @@ int paravirt_disable_iospace(void);
"=c" (__ecx)
#define PVOP_CALL_CLOBBERS PVOP_VCALL_CLOBBERS, "=a" (__eax)

-/* void functions are still allowed [re]ax for scratch */
+/*
+ * void functions are still allowed [re]ax for scratch.
+ *
+ * The ZERO_CALL_USED REGS feature may end up zeroing out callee-saved
+ * registers. Make sure we model this with the appropriate clobbers.
+ */
+#ifdef CONFIG_ZERO_CALL_USED_REGS
+#define PVOP_VCALLEE_CLOBBERS "=a" (__eax), PVOP_VCALL_CLOBBERS
+#else
#define PVOP_VCALLEE_CLOBBERS "=a" (__eax)
+#endif
#define PVOP_CALLEE_CLOBBERS PVOP_VCALLEE_CLOBBERS

#define EXTRA_CLOBBERS , "r8", "r9", "r10", "r11"
--
2.37.2.789.g6183377224-goog