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

arch/x86/entry/entry_64_compat.S

@@ -84,13 +84,13 @@ ENTRY(entry_SYSENTER_compat)
 	pushq	%rdx			/* pt_regs->dx */
 	pushq	%rcx			/* pt_regs->cx */
 	pushq	$-ENOSYS		/* pt_regs->ax */
-	pushq   $0			/* pt_regs->r8  = 0 */
+	pushq   %r8			/* pt_regs->r8 */
 	xorl	%r8d, %r8d		/* nospec   r8 */
-	pushq   $0			/* pt_regs->r9  = 0 */
+	pushq   %r9			/* pt_regs->r9 */
 	xorl	%r9d, %r9d		/* nospec   r9 */
-	pushq   $0			/* pt_regs->r10 = 0 */
+	pushq   %r10			/* pt_regs->r10 */
 	xorl	%r10d, %r10d		/* nospec   r10 */
-	pushq   $0			/* pt_regs->r11 = 0 */
+	pushq   %r11			/* pt_regs->r11 */
 	xorl	%r11d, %r11d		/* nospec   r11 */
 	pushq   %rbx                    /* pt_regs->rbx */
 	xorl	%ebx, %ebx		/* nospec   rbx */

tools/testing/selftests/x86/test_syscall_vdso.c

@@ -100,12 +100,19 @@ asm (
 	"	shl	$32, %r8\n"
 	"	orq	$0x7f7f7f7f, %r8\n"
 	"	movq	%r8, %r9\n"
-	"	movq	%r8, %r10\n"
+	"	incq	%r9\n"
-	"	movq	%r8, %r11\n"
+	"	movq	%r9, %r10\n"
-	"	movq	%r8, %r12\n"
+	"	incq	%r10\n"
-	"	movq	%r8, %r13\n"
+	"	movq	%r10, %r11\n"
-	"	movq	%r8, %r14\n"
+	"	incq	%r11\n"
-	"	movq	%r8, %r15\n"
+	"	movq	%r11, %r12\n"
+	"	incq	%r12\n"
+	"	movq	%r12, %r13\n"
+	"	incq	%r13\n"
+	"	movq	%r13, %r14\n"
+	"	incq	%r14\n"
+	"	movq	%r14, %r15\n"
+	"	incq	%r15\n"
 	"	ret\n"
 	"	.code32\n"
 	"	.popsection\n"
@@ -128,12 +135,13 @@ int check_regs64(void)
 	int err = 0;
 	int num = 8;
 	uint64_t *r64 = &regs64.r8;
+	uint64_t expected = 0x7f7f7f7f7f7f7f7fULL;
 
 	if (!kernel_is_64bit)
 		return 0;
 
 	do {
-		if (*r64 == 0x7f7f7f7f7f7f7f7fULL)
+		if (*r64 == expected++)
 			continue; /* register did not change */
 		if (syscall_addr != (long)&int80) {
 			/*
@@ -147,18 +155,17 @@ int check_regs64(void)
 				continue;
 			}
 		} else {
-			/* INT80 syscall entrypoint can be used by
+			/*
+			 * INT80 syscall entrypoint can be used by
 			 * 64-bit programs too, unlike SYSCALL/SYSENTER.
 			 * Therefore it must preserve R12+
 			 * (they are callee-saved registers in 64-bit C ABI).
 			 *
-			 * This was probably historically not intended,
+			 * Starting in Linux 4.17 (and any kernel that
-			 * but R8..11 are clobbered (cleared to 0).
+			 * backports the change), R8..11 are preserved.
-			 * IOW: they are the only registers which aren't
+			 * Historically (and probably unintentionally), they
-			 * preserved across INT80 syscall.
+			 * were clobbered or zeroed.
 			 */
-			if (*r64 == 0 && num <= 11)
-				continue;
 		}
 		printf("[FAIL]\tR%d has changed:%016llx\n", num, *r64);
 		err++;