FAILED: patch "[PATCH] x86/entry/64: Clear extra registers beyond syscall arguments," failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: Dan Williams <[email protected]>
Date: Mon, 5 Feb 2018 17:18:05 -0800
Subject: [PATCH] x86/entry/64: Clear extra registers beyond syscall arguments,
to reduce speculation attack surface

At entry userspace may have (maliciously) populated the extra registers outside the syscall calling convention with arbitrary values that could be useful in a speculative execution (Spectre style) attack.

Clear these registers to minimize the kernel's attack surface.

Note, this only clears the extra registers and not the unused registers for syscalls less than 6 arguments, since those registers are likely to be clobbered well before their values could be put to use under speculation.

Note, Linus found that the XOR instructions can be executed with minimized cost if interleaved with the PUSH instructions, and Ingo's analysis found that R10 and R11 should be included in the register clearing beyond the typical 'extra' syscall calling convention registers.

index c752abe89d80..065a71b90808 100644
@@ -235,13 +235,26 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
 	pushq	%r8				/* pt_regs->r8 */
 	pushq	%r9				/* pt_regs->r9 */
 	pushq	%r10				/* pt_regs->r10 */
+	/*
+	 * Clear extra registers that a speculation attack might
+	 * otherwise want to exploit. Interleave XOR with PUSH
+	 * for better uop scheduling:
+	 */
+	xorq	%r10, %r10			/* nospec   r10 */
 	pushq	%r11				/* pt_regs->r11 */
+	xorq	%r11, %r11			/* 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 */
+	xorq	%r12, %r12			/* nospec   r12 */
 	pushq	%r13				/* pt_regs->r13 */
+	xorq	%r13, %r13			/* nospec   r13 */
 	pushq	%r14				/* pt_regs->r14 */
+	xorq	%r14, %r14			/* nospec   r14 */
 	pushq	%r15				/* pt_regs->r15 */
+	xorq	%r15, %r15			/* nospec   r15 */
 	UNWIND_HINT_REGS
 
 	TRACE_IRQS_OFF