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 <stable@xxxxxxxxxxxxxxx>. thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From 8e1eb3fa009aa7c0b944b3c8b26b07de0efb3200 Mon Sep 17 00:00:00 2001 From: Dan Williams <dan.j.williams@xxxxxxxxx> 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. Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Reported-by: Andi Kleen <ak@xxxxxxxxxxxxxxx> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Brian Gerst <brgerst@xxxxxxxxx> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/151787988577.7847.16733592218894189003.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx [ Made small improvements to the changelog and the code comments. ] Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index c752abe89d80..065a71b90808 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -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