x86/entry/64: Clear extra registers beyond syscall arguments, to reduce speculation...

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@linux-foundation.org>
Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: <stable@vger.kernel.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/151787988577.7847.16733592218894189003.stgit@dwillia2-desk3.amr.corp.intel.com
[ Made small improvements to the changelog and the code comments. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index c752abe89d80787395179f2f3c698d7480cbd34a..065a71b90808025beb744e3ebe2fb9993ac84ba8 100644 (file)
--- 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