x86/paravirt: add extra clobbers with ZERO_CALL_USED_REGS enabled

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@google.com>
Reported-and-tested-by: Nathan Chancellor <nathan@kernel.org>
Fixes: 035f7f87b7 ("randstruct: Enable Clang support")
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/lkml/fa6df43b-8a1a-8ad1-0236-94d2a0b588fa@suse.com/
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220902213750.1124421-3-morbo@google.com

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"