linux-kernel - [PATCH 4.15 053/202] [Variant 1/Spectre-v1] arm64: entry: Ensure branch through syscall table is bounded under speculation

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Message-Id: <20180215151716.007874324@linuxfoundation.org>
Date:   Thu, 15 Feb 2018 16:15:53 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: [PATCH 4.15 053/202] [Variant 1/Spectre-v1] arm64: entry: Ensure branch through syscall table is bounded under speculation

4.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Will Deacon <will.deacon@....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@....com>
Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/arm64/include/asm/assembler.h |   11 +++++++++++
 arch/arm64/kernel/entry.S          |    2 ++
 2 files changed, 13 insertions(+)

--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -116,6 +116,17 @@
 	.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
  */
 	.macro	nops, num
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -378,6 +378,7 @@ alternative_insn eret, nop, ARM64_UNMAP_
  * x7 is reserved for the system call number in 32-bit mode.
  */
 wsc_nr	.req	w25		// number of system calls
+xsc_nr	.req	x25		// number of system calls (zero-extended)
 wscno	.req	w26		// syscall number
 xscno	.req	x26		// syscall number (zero-extended)
 stbl	.req	x27		// syscall table pointer
@@ -932,6 +933,7 @@ el0_svc_naked:					// compat entry point
 	b.ne	__sys_trace
 	cmp     wscno, wsc_nr			// check upper syscall limit
 	b.hs	ni_sys
+	mask_nospec64 xscno, xsc_nr, x19	// enforce bounds for syscall number
 	ldr	x16, [stbl, xscno, lsl #3]	// address in the syscall table
 	blr	x16				// call sys_* routine
 	b	ret_fast_syscall