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x86/entry/64/compat: Preserve r8-r11 in int $0x80

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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
This commit is contained in:
Andy Lutomirski 2018-04-17 07:36:36 -07:00 committed by Thomas Gleixner
parent 316d097c4c
commit 8bb2610bc4
2 changed files with 25 additions and 18 deletions
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8
arch/x86/entry/entry_64_compat.S
View File

@ -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 */

35
tools/testing/selftests/x86/test_syscall_vdso.c
View File

@ -100,12 +100,19 @@ asm (
" shl $32, %r8\n"
" orq $0x7f7f7f7f, %r8\n"
" movq %r8, %r9\n"
" movq %r8, %r10\n"
" movq %r8, %r11\n"
" movq %r8, %r12\n"
" movq %r8, %r13\n"
" movq %r8, %r14\n"
" movq %r8, %r15\n"
" incq %r9\n"
" movq %r9, %r10\n"
" incq %r10\n"
" movq %r10, %r11\n"
" incq %r11\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,
* but R8..11 are clobbered (cleared to 0).
* IOW: they are the only registers which aren't
* preserved across INT80 syscall.
* Starting in Linux 4.17 (and any kernel that
* backports the change), R8..11 are preserved.
* Historically (and probably unintentionally), they
* were clobbered or zeroed.
*/
if (*r64 == 0 && num <= 11)
continue;
}
printf("[FAIL]\tR%d has changed:%016llx\n", num, *r64);
err++;