[PATCH v2 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels

[Dan Williams]

At entry userspace may have populated the extra registers outside the
syscall calling convention with values that could be useful in a
speculative execution attack. Clear them 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.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Reported-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 arch/x86/entry/calling.h  |   17 +++++++++++++++++
 arch/x86/entry/entry_64.S |    1 +
 2 files changed, 18 insertions(+)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 3f48f695d5e6..daee2d19e73d 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -147,6 +147,23 @@ For 32-bit we have the following conventions - kernel is built with
 	UNWIND_HINT_REGS offset=\offset
 	.endm
 
+	/*
+	 * Sanitize extra registers of values that a speculation attack
+	 * might want to exploit. In the CONFIG_FRAME_POINTER=y case,
+	 * the expectation is that %ebp will be clobbered before it
+	 * could be used.
+	 */
+	.macro CLEAR_EXTRA_REGS_NOSPEC
+	xorq %r15, %r15
+	xorq %r14, %r14
+	xorq %r13, %r13
+	xorq %r12, %r12
+	xorl %ebx, %ebx
+#ifndef CONFIG_FRAME_POINTER
+	xorl %ebp, %ebp
+#endif
+	.endm
+
 	.macro POP_EXTRA_REGS
 	popq %r15
 	popq %r14
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index c752abe89d80..5de9a5922026 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -244,6 +244,7 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
 	pushq	%r15				/* pt_regs->r15 */
 	UNWIND_HINT_REGS
 
+	CLEAR_EXTRA_REGS_NOSPEC
 	TRACE_IRQS_OFF
 
 	/* IRQs are off. */