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

commit 8e1eb3fa009aa7c0b944b3c8b26b07de0efb3200 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 16e2d72e79a097c2a897228be3fbeb95b13d0cd8..5771ae24ba5869add5e889241a1304a470af75e2 100644 (file)
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -231,13 +231,26 @@ 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