[PATCH 7/7] x86/entry/64: extend register clearing on syscall entry to lower registers

Dominik Brodowski
2018-03-30T09:38:02Z

To reduce the chance that random user space content leaks down the call
chain in registers, also clear lower registers on syscall entry:

For 64bit syscalls, extend the register clearing in PUSH_AND_CLEAR_REGS
to %dx and %cx. This should not hurt at all, also on the other callers
of that macro. We do not need to clear %rdi and %rsi for syscall entry,
as those registers are used to pass the parameters to do_syscall_64().

For the 32bit compat syscalls, do_int80_syscall_32() and
do_fast_syscall_32() each only take one parameter. Therefore, extend the
register clearing to %dx, %cx, and %si in entry_SYSCALL_compat and
entry_INT80_compat.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
---
arch/x86/entry/calling.h | 2 ++
arch/x86/entry/entry_64_compat.S | 6 ++++++
2 files changed, 8 insertions(+)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index be63330c5511..593812a4c29e 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -114,7 +114,9 @@ For 32-bit we have the following conventions - kernel is built with
pushq %rsi /* pt_regs->si */
.endif
pushq \rdx /* pt_regs->dx */
+ xorl %edx, %edx /* nosepc dx */
pushq %rcx /* pt_regs->cx */
+ xorl %ecx, %ecx /* nosepc cx */
pushq \rax /* pt_regs->ax */
pushq %r8 /* pt_regs->r8 */
xorl %r8d, %r8d /* nospec r8 */
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 08425c42f8b7..23e0945959e5 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -220,8 +220,11 @@ GLOBAL(entry_SYSCALL_compat_after_hwframe)
pushq %rax /* pt_regs->orig_ax */
pushq %rdi /* pt_regs->di */
pushq %rsi /* pt_regs->si */
+ xorl %esi, %esi /* nosepc si */
pushq %rdx /* pt_regs->dx */
+ xorl %edx, %edx /* nosepc dx */
pushq %rbp /* pt_regs->cx (stashed in bp) */
+ xorl %ecx, %ecx /* nosepc cx */
pushq $-ENOSYS /* pt_regs->ax */
pushq $0 /* pt_regs->r8 = 0 */
xorl %r8d, %r8d /* nospec r8 */
@@ -365,8 +368,11 @@ ENTRY(entry_INT80_compat)

pushq (%rdi) /* pt_regs->di */
pushq %rsi /* pt_regs->si */
+ xorl %esi, %esi /* nosepc si */
pushq %rdx /* pt_regs->dx */
+ xorl %edx, %edx /* nosepc dx */
pushq %rcx /* pt_regs->cx */
+ xorl %ecx, %ecx /* nosepc cx */
pushq $-ENOSYS /* pt_regs->ax */
pushq $0 /* pt_regs->r8 = 0 */
xorl %r8d, %r8d /* nospec r8 */
--
2.16.3