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Subject[tip:x86/asm] syscalls/x86: Extend register clearing on syscall entry to lower registers
Commit-ID:  6dc936f175cc6d12a8eb14d29b87e9238e460383
Gitweb: https://git.kernel.org/tip/6dc936f175cc6d12a8eb14d29b87e9238e460383
Author: Dominik Brodowski <linux@dominikbrodowski.net>
AuthorDate: Thu, 5 Apr 2018 11:53:06 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 5 Apr 2018 16:59:39 +0200

syscalls/x86: Extend register clearing on syscall entry to lower registers

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

For 64-bit 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 32-bit 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.

Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180405095307.3730-8-linux@dominikbrodowski.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
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..352e70cd33e8 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 /* nospec dx */
pushq %rcx /* pt_regs->cx */
+ xorl %ecx, %ecx /* nospec 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..9af927e59d49 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 /* nospec si */
pushq %rdx /* pt_regs->dx */
+ xorl %edx, %edx /* nospec dx */
pushq %rbp /* pt_regs->cx (stashed in bp) */
+ xorl %ecx, %ecx /* nospec 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 /* nospec si */
pushq %rdx /* pt_regs->dx */
+ xorl %edx, %edx /* nospec dx */
pushq %rcx /* pt_regs->cx */
+ xorl %ecx, %ecx /* nospec cx */
pushq $-ENOSYS /* pt_regs->ax */
pushq $0 /* pt_regs->r8 = 0 */
xorl %r8d, %r8d /* nospec r8 */
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