x86/entry/64/compat: Preserve r8-r11 in int $0x80
32-bit user code that uses int $80 doesn't care about r8-r11. There is, however, some 64-bit user code that intentionally uses int $0x80 to invoke 32-bit system calls. From what I've seen, basically all such code assumes that r8-r15 are all preserved, but the kernel clobbers r8-r11. Since I doubt that there's any code that depends on int $0x80 zeroing r8-r11, change the kernel to preserve them. I suspect that very little user code is broken by the old clobber, since r8-r11 are only rarely allocated by gcc, and they're clobbered by function calls, so they only way we'd see a problem is if the same function that invokes int $0x80 also spills something important to one of these registers. The current behavior seems to date back to the historical commit "[PATCH] x86-64 merge for 2.6.4". Before that, all regs were preserved. I can't find any explanation of why this change was made. Update the test_syscall_vdso_32 testcase as well to verify the new behavior, and it strengthens the test to make sure that the kernel doesn't accidentally permute r8..r15. Suggested-by: Denys Vlasenko <dvlasenk@redhat.com> Signed-off-by: Andy Lutomirski <luto@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Borislav Petkov <bp@alien8.de> Cc: Dominik Brodowski <linux@dominikbrodowski.net> Link: https://lkml.kernel.org/r/d4c4d9985fbe64f8c9e19291886453914b48caee.1523975710.git.luto@kernel.org
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@ -84,13 +84,13 @@ ENTRY(entry_SYSENTER_compat)
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pushq %rdx /* pt_regs->dx */
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pushq %rcx /* pt_regs->cx */
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pushq $-ENOSYS /* pt_regs->ax */
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pushq $0 /* pt_regs->r8 = 0 */
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pushq %r8 /* pt_regs->r8 */
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xorl %r8d, %r8d /* nospec r8 */
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pushq $0 /* pt_regs->r9 = 0 */
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pushq %r9 /* pt_regs->r9 */
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xorl %r9d, %r9d /* nospec r9 */
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pushq $0 /* pt_regs->r10 = 0 */
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pushq %r10 /* pt_regs->r10 */
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xorl %r10d, %r10d /* nospec r10 */
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pushq $0 /* pt_regs->r11 = 0 */
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pushq %r11 /* pt_regs->r11 */
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xorl %r11d, %r11d /* nospec r11 */
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pushq %rbx /* pt_regs->rbx */
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xorl %ebx, %ebx /* nospec rbx */
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@ -100,12 +100,19 @@ asm (
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" shl $32, %r8\n"
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" orq $0x7f7f7f7f, %r8\n"
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" movq %r8, %r9\n"
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" movq %r8, %r10\n"
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" movq %r8, %r11\n"
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" movq %r8, %r12\n"
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" movq %r8, %r13\n"
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" movq %r8, %r14\n"
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" movq %r8, %r15\n"
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" incq %r9\n"
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" movq %r9, %r10\n"
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" incq %r10\n"
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" movq %r10, %r11\n"
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" incq %r11\n"
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" movq %r11, %r12\n"
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" incq %r12\n"
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" movq %r12, %r13\n"
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" incq %r13\n"
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" movq %r13, %r14\n"
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" incq %r14\n"
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" movq %r14, %r15\n"
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" incq %r15\n"
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" ret\n"
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" .code32\n"
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" .popsection\n"
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@ -128,12 +135,13 @@ int check_regs64(void)
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int err = 0;
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int num = 8;
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uint64_t *r64 = ®s64.r8;
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uint64_t expected = 0x7f7f7f7f7f7f7f7fULL;
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if (!kernel_is_64bit)
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return 0;
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do {
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if (*r64 == 0x7f7f7f7f7f7f7f7fULL)
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if (*r64 == expected++)
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continue; /* register did not change */
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if (syscall_addr != (long)&int80) {
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/*
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@ -147,18 +155,17 @@ int check_regs64(void)
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continue;
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}
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} else {
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/* INT80 syscall entrypoint can be used by
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/*
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* INT80 syscall entrypoint can be used by
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* 64-bit programs too, unlike SYSCALL/SYSENTER.
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* Therefore it must preserve R12+
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* (they are callee-saved registers in 64-bit C ABI).
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*
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* This was probably historically not intended,
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* but R8..11 are clobbered (cleared to 0).
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* IOW: they are the only registers which aren't
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* preserved across INT80 syscall.
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* Starting in Linux 4.17 (and any kernel that
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* backports the change), R8..11 are preserved.
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* Historically (and probably unintentionally), they
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* were clobbered or zeroed.
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*/
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if (*r64 == 0 && num <= 11)
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continue;
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}
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printf("[FAIL]\tR%d has changed:%016llx\n", num, *r64);
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err++;
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