linux-kernel - [PATCH 4.9 15/66] arm64: entry: Ensure branch through syscall table is bounded under speculation

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Date:   Tue, 17 Apr 2018 17:58:48 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Greg Hackmann <ghackmann@...gle.com>
Subject: [PATCH 4.9 15/66] arm64: entry: Ensure branch through syscall table is bounded under speculation

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

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

From: Mark Rutland <mark.rutland@....com>


From: Will Deacon <will.deacon@....com>

commit 6314d90e64936c584f300a52ef173603fb2461b5 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>
[v4.9: use existing scno & sc_nr definitions]
Signed-off-by: Mark Rutland <mark.rutland@....com> [v4.9 backport]
Tested-by: Greg Hackmann <ghackmann@...gle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/arm64/include/asm/assembler.h |   11 +++++++++++
 arch/arm64/kernel/entry.S          |    1 +
 2 files changed, 12 insertions(+)

--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -94,6 +94,17 @@
 	.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
@@ -795,6 +795,7 @@ 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