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H A D | paravirt_types.h | 8c86f29b Fri Sep 02 16:37:50 CDT 2022 Bill Wendling <morbo@google.com> 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: 035f7f87b729 ("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
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