[PATCH v4] x86,entry: Use PUSH_AND_CLEAR_REGS for compat

[Peter Zijlstra]

On Fri, Apr 29, 2022 at 11:12:56PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 29, 2022 at 08:00:37PM +0800, Lai Jiangshan wrote:
> > On Fri, Apr 29, 2022 at 5:13 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > > Notably:
> > >
> > >  - SYSENTER: didn't clear si, dx, cx.
> > >  - SYSCALL, INT80: *do* clear si since the C functions don't take a
> > >    second argument.
> > >
> > 
> > If CLEAR_REGS for SYSCALL, INT80 clears si, it is better, IMO, to
> > make CLEAR_REGS clear si unconditionally.
> 
> Well, I didn't want to add the overhead to 64bit native syscalls, but
> Linus just suggested the same thing elsewhere. So yeah.
> 
> He also suggested cleaning up INT80 like below to get rid of the
> save_rdi wart.

Which then results in...

---
Subject: x86,entry: Use PUSH_AND_CLEAR_REGS for compat
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Sat, 9 Apr 2022 00:38:27 +0200

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:

 - 32bit SYSENTER: didn't clear si, dx, cx.
 - 32bit SYSCALL, INT80: *do* clear si since the C functions don't
   take a second argument.
 - 64bit: don't clear si since the C functions take a second argument.

SYSENTER should be clearing all those 3 registers, nothing uses them
and selftests pass. Unconditionally clear rsi since it simplifies
code.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/entry/calling.h         |    1 
 arch/x86/entry/entry_64_compat.S |   87 +--------------------------------------
 2 files changed, 4 insertions(+), 84 deletions(-)

--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -99,6 +99,7 @@ For 32-bit we have the following convent
 	 * well before they could be put to use in a speculative execution
 	 * gadget.
 	 */
+	xorl	%esi,  %esi	/* nospec si  */
 	xorl	%edx,  %edx	/* nospec dx  */
 	xorl	%ecx,  %ecx	/* nospec cx  */
 	xorl	%r8d,  %r8d	/* nospec r8  */
--- 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_af
 	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
 	UNWIND_HINT_REGS
 
 	cld
@@ -225,35 +200,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_saf
 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
 	UNWIND_HINT_REGS
 
 	movq	%rsp, %rdi
@@ -380,35 +327,7 @@ SYM_CODE_START(entry_INT80_compat)
 	pushq	0*8(%rax)		/* regs->orig_ax */
 .Lint80_keep_stack:
 
-	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	$-ENOSYS		/* pt_regs->ax */
-	pushq   %r8			/* pt_regs->r8 */
-	xorl	%r8d, %r8d		/* nospec   r8 */
-	pushq   %r9			/* pt_regs->r9 */
-	xorl	%r9d, %r9d		/* nospec   r9 */
-	pushq   %r10			/* pt_regs->r10*/
-	xorl	%r10d, %r10d		/* nospec   r10 */
-	pushq   %r11			/* pt_regs->r11 */
-	xorl	%r11d, %r11d		/* nospec   r11 */
-	pushq   %rbx                    /* pt_regs->rbx */
-	xorl	%ebx, %ebx		/* nospec   rbx */
-	pushq   %rbp                    /* pt_regs->rbp */
-	xorl	%ebp, %ebp		/* nospec   rbp */
-	pushq   %r12                    /* pt_regs->r12 */
-	xorl	%r12d, %r12d		/* nospec   r12 */
-	pushq   %r13                    /* pt_regs->r13 */
-	xorl	%r13d, %r13d		/* nospec   r13 */
-	pushq   %r14                    /* pt_regs->r14 */
-	xorl	%r14d, %r14d		/* nospec   r14 */
-	pushq   %r15                    /* pt_regs->r15 */
-	xorl	%r15d, %r15d		/* nospec   r15 */
-
+	PUSH_AND_CLEAR_REGS rax=$-ENOSYS
 	UNWIND_HINT_REGS
 
 	cld