[PATCH v3] x86,entry: Use PUSH_AND_CLEAR_REGS for compat

From: Peter Zijlstra
Date: Fri Apr 29 2022 - 05:13:21 EST


Since the upper regs don't exist for ia32 code, preserving them
doesn't hurt and it simplifies the code.

If there was any attack surface on this, that attack surface already
exists for INT80 and needs to be otherwise dealt with.

Notably:

- SYSENTER: didn't clear si, dx, cx.
- SYSCALL, INT80: *do* clear si since the C functions don't take a
second argument.

Add a clear_rsi argument to CLEAR_REGS for these 3 sites, and have
SYSENTER clear everything (no code relies on those registers not being
cleared and selftests pass).

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index a4c061fb7c6e..526eba74b84c 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -63,13 +63,15 @@ For 32-bit we have the following conventions - kernel is built with
* for assembly code:
*/

-.macro PUSH_REGS rdx=%rdx rax=%rax save_ret=0
+.macro PUSH_REGS rdx=%rdx rax=%rax save_ret=0 save_rdi=1
.if \save_ret
pushq %rsi /* pt_regs->si */
movq 8(%rsp), %rsi /* temporarily store the return address in %rsi */
movq %rdi, 8(%rsp) /* pt_regs->di (overwriting original return address) */
.else
+ .if \save_rdi
pushq %rdi /* pt_regs->di */
+ .endif
pushq %rsi /* pt_regs->si */
.endif
pushq \rdx /* pt_regs->dx */
@@ -92,13 +94,16 @@ For 32-bit we have the following conventions - kernel is built with
.endif
.endm

-.macro CLEAR_REGS
+.macro CLEAR_REGS clear_rsi=0
/*
* Sanitize registers of values that a speculation attack might
* otherwise want to exploit. The lower registers are likely clobbered
* well before they could be put to use in a speculative execution
* gadget.
*/
+ .if \clear_rsi
+ xorl %esi, %esi /* nospec si */
+ .endif
xorl %edx, %edx /* nospec dx */
xorl %ecx, %ecx /* nospec cx */
xorl %r8d, %r8d /* nospec r8 */
@@ -111,12 +116,11 @@ For 32-bit we have the following conventions - kernel is built with
xorl %r13d, %r13d /* nospec r13 */
xorl %r14d, %r14d /* nospec r14 */
xorl %r15d, %r15d /* nospec r15 */
-
.endm

-.macro PUSH_AND_CLEAR_REGS rdx=%rdx rax=%rax save_ret=0
- PUSH_REGS rdx=\rdx, rax=\rax, save_ret=\save_ret
- CLEAR_REGS
+.macro PUSH_AND_CLEAR_REGS rdx=%rdx rax=%rax save_ret=0 save_rdi=1 clear_rsi=0
+ PUSH_REGS rdx=\rdx, rax=\rax, save_ret=\save_ret save_rdi=\save_rdi
+ CLEAR_REGS clear_rsi=\clear_rsi
.endm

.macro POP_REGS pop_rdi=1 skip_r11rcx=0
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 4fdb007cddbd..4ca8d6bfbe6b 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -83,32 +83,7 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
movl %eax, %eax

pushq %rax /* pt_regs->orig_ax */
- pushq %rdi /* pt_regs->di */
- pushq %rsi /* pt_regs->si */
- pushq %rdx /* pt_regs->dx */
- pushq %rcx /* pt_regs->cx */
- pushq $-ENOSYS /* pt_regs->ax */
- pushq $0 /* pt_regs->r8 = 0 */
- xorl %r8d, %r8d /* nospec r8 */
- pushq $0 /* pt_regs->r9 = 0 */
- xorl %r9d, %r9d /* nospec r9 */
- pushq $0 /* pt_regs->r10 = 0 */
- xorl %r10d, %r10d /* nospec r10 */
- pushq $0 /* pt_regs->r11 = 0 */
- xorl %r11d, %r11d /* nospec r11 */
- pushq %rbx /* pt_regs->rbx */
- xorl %ebx, %ebx /* nospec rbx */
- pushq %rbp /* pt_regs->rbp (will be overwritten) */
- xorl %ebp, %ebp /* nospec rbp */
- pushq $0 /* pt_regs->r12 = 0 */
- xorl %r12d, %r12d /* nospec r12 */
- pushq $0 /* pt_regs->r13 = 0 */
- xorl %r13d, %r13d /* nospec r13 */
- pushq $0 /* pt_regs->r14 = 0 */
- xorl %r14d, %r14d /* nospec r14 */
- pushq $0 /* pt_regs->r15 = 0 */
- xorl %r15d, %r15d /* nospec r15 */
-
+ PUSH_AND_CLEAR_REGS rax=$-ENOSYS clear_rsi=1
UNWIND_HINT_REGS

cld
@@ -225,35 +200,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_safe_stack, SYM_L_GLOBAL)
SYM_INNER_LABEL(entry_SYSCALL_compat_after_hwframe, SYM_L_GLOBAL)
movl %eax, %eax /* discard orig_ax high bits */
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 */
- pushq $0 /* pt_regs->r9 = 0 */
- xorl %r9d, %r9d /* nospec r9 */
- pushq $0 /* pt_regs->r10 = 0 */
- xorl %r10d, %r10d /* nospec r10 */
- pushq $0 /* pt_regs->r11 = 0 */
- xorl %r11d, %r11d /* nospec r11 */
- pushq %rbx /* pt_regs->rbx */
- xorl %ebx, %ebx /* nospec rbx */
- pushq %rbp /* pt_regs->rbp (will be overwritten) */
- xorl %ebp, %ebp /* nospec rbp */
- pushq $0 /* pt_regs->r12 = 0 */
- xorl %r12d, %r12d /* nospec r12 */
- pushq $0 /* pt_regs->r13 = 0 */
- xorl %r13d, %r13d /* nospec r13 */
- pushq $0 /* pt_regs->r14 = 0 */
- xorl %r14d, %r14d /* nospec r14 */
- pushq $0 /* pt_regs->r15 = 0 */
- xorl %r15d, %r15d /* nospec r15 */
-
+ PUSH_AND_CLEAR_REGS rax=$-ENOSYS clear_rsi=1
UNWIND_HINT_REGS

movq %rsp, %rdi
@@ -381,35 +328,7 @@ SYM_CODE_START(entry_INT80_compat)
pushq 1*8(%rdi) /* regs->orig_ax */
pushq (%rdi) /* pt_regs->di */
.Lint80_keep_stack:
-
- 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 $-ENOSYS /* pt_regs->ax */
- pushq %r8 /* pt_regs->r8 */
- xorl %r8d, %r8d /* nospec r8 */
- pushq %r9 /* pt_regs->r9 */
- xorl %r9d, %r9d /* nospec r9 */
- pushq %r10 /* pt_regs->r10*/
- xorl %r10d, %r10d /* nospec r10 */
- pushq %r11 /* pt_regs->r11 */
- xorl %r11d, %r11d /* nospec r11 */
- pushq %rbx /* pt_regs->rbx */
- xorl %ebx, %ebx /* nospec rbx */
- pushq %rbp /* pt_regs->rbp */
- xorl %ebp, %ebp /* nospec rbp */
- pushq %r12 /* pt_regs->r12 */
- xorl %r12d, %r12d /* nospec r12 */
- pushq %r13 /* pt_regs->r13 */
- xorl %r13d, %r13d /* nospec r13 */
- pushq %r14 /* pt_regs->r14 */
- xorl %r14d, %r14d /* nospec r14 */
- pushq %r15 /* pt_regs->r15 */
- xorl %r15d, %r15d /* nospec r15 */
-
+ PUSH_AND_CLEAR_REGS rax=$-ENOSYS save_rdi=0 clear_rsi=1
UNWIND_HINT_REGS

cld