linux-kernel - [PATCH v2] arm: ptrace: Fix mask for thumb breakpoint hook

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Message-Id: <20200413173841.29651-1-fredrik@strupe.net>
Date:   Mon, 13 Apr 2020 19:38:41 +0200
From:   Fredrik Strupe <fredrik@...upe.net>
To:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Fredrik Strupe <fredrik@...upe.net>,
        Oleg Nesterov <oleg@...hat.com>,
        Russell King <linux@...linux.org.uk>
Subject: [PATCH v2] arm: ptrace: Fix mask for thumb breakpoint hook

call_undef_hook() in traps.c applies the same instr_mask for both 16-bit
and 32-bit thumb instructions. If instr_mask then is only 16 bits wide
(0xffff as opposed to 0xffffffff), the first half-word of 32-bit thumb
instructions will be masked out. This makes the function match 32-bit
thumb instructions where the second half-word is equal to instr_val,
regardless of the first half-word.

The result in this case is that all undefined 32-bit thumb instructions
with the second half-word equal to de01 (udf #1) work as breakpoints
and will raise a SIGTRAP instead of a SIGILL, instead of just the one
intended 16-bit instruction. An example of such an instruction is
eaa0b650, which is unallocated according to Arm ARM and should raise a
SIGILL, but instead raises a SIGTRAP.

This patch fixes the issue by setting all the bits in instr_mask, which
will still match the intended 16-bit thumb instruction (where the
upper half is always 0), but not any 32-bit thumb instructions.

Signed-off-by: Fredrik Strupe <fredrik@...upe.net>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Russell King <linux@...linux.org.uk>
---
 arch/arm/kernel/ptrace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index b606cded90cd..4cc6a7eff635 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -219,8 +219,8 @@ static struct undef_hook arm_break_hook = {
 };

 static struct undef_hook thumb_break_hook = {
-	.instr_mask	= 0xffff,
-	.instr_val	= 0xde01,
+	.instr_mask	= 0xffffffff,
+	.instr_val	= 0x0000de01,
 	.cpsr_mask	= PSR_T_BIT,
 	.cpsr_val	= PSR_T_BIT,
 	.fn		= break_trap,
-- 
2.20.1