From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754007AbeDQOgr (ORCPT ); Tue, 17 Apr 2018 10:36:47 -0400 Received: from mail.kernel.org ([198.145.29.99]:37736 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752744AbeDQOgm (ORCPT ); Tue, 17 Apr 2018 10:36:42 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 189DB206B2 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=luto@kernel.org From: Andy Lutomirski To: x86@kernel.org, LKML Cc: Borislav Petkov , Dominik Brodowski , Denys Vlasenko , Andy Lutomirski Subject: [PATCH] x86/entry/64/compat: Preserve r8-r11 in int $0x80 Date: Tue, 17 Apr 2018 07:36:36 -0700 Message-Id: X-Mailer: git-send-email 2.14.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. This patch also updates the test_syscall_vdso_32 testcase 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 Signed-off-by: Andy Lutomirski --- Ingo, Thomas: this could be a -stable candidate, but it's apparently not severe enough for many people to have noticed. arch/x86/entry/entry_64_compat.S | 8 +++--- tools/testing/selftests/x86/test_syscall_vdso.c | 35 +++++++++++++++---------- 2 files changed, 25 insertions(+), 18 deletions(-) diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S index 08425c42f8b7..e4b94b7494c6 100644 --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -368,13 +368,13 @@ ENTRY(entry_INT80_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 */ diff --git a/tools/testing/selftests/x86/test_syscall_vdso.c b/tools/testing/selftests/x86/test_syscall_vdso.c index 40370354d4c1..c9c3281077bc 100644 --- a/tools/testing/selftests/x86/test_syscall_vdso.c +++ b/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" - " 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 = ®s64.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++; -- 2.14.3