[PATCH 06/39] arm64: entry: Ensure branch through syscall table is bounded under speculation

[Alex Shi]

From: Will Deacon <will.deacon at arm.com>

commit 6314d90e6493 upstream.

In a similar manner to array_index_mask_nospec, this patch introduces an
assembly macro (mask_nospec64) which can be used to bound a value under
speculation. This macro is then used to ensure that the indirect branch
through the syscall table is bounded under speculation, with out-of-range
addresses speculating as calls to sys_io_setup (0).

Reviewed-by: Mark Rutland <mark.rutland at arm.com>
Signed-off-by: Will Deacon <will.deacon at arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Signed-off-by: Alex Shi <alex.shi at linaro.org>
---
 arch/arm64/include/asm/assembler.h | 11 +++++++++++
 arch/arm64/kernel/entry.S          |  1 +
 2 files changed, 12 insertions(+)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index 229b37ddf52c..81a5f416a27a 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -101,6 +101,17 @@
 	hint	#20
 	.endm
 
+/*
+ * Sanitise a 64-bit bounded index wrt speculation, returning zero if out
+ * of bounds.
+ */
+	.macro	mask_nospec64, idx, limit, tmp
+	sub	\tmp, \idx, \limit
+	bic	\tmp, \tmp, \idx
+	and	\idx, \idx, \tmp, asr #63
+	csdb
+	.endm
+
 /*
  * NOP sequence
  */
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 274898465a3f..166cd6626ca7 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -694,6 +694,7 @@ el0_svc_naked:					// compat entry point
 	b.ne	__sys_trace
 	cmp     scno, sc_nr                     // check upper syscall limit
 	b.hs	ni_sys
+	mask_nospec64 scno, sc_nr, x19		// enforce bounds for syscall number
 	ldr	x16, [stbl, scno, lsl #3]	// address in the syscall table
 	blr	x16				// call sys_* routine
 	b	ret_fast_syscall
-- 
2.16.2.440.gc6284da