linux-kernel - [PATCH] arm: uprobes: Don't hook on thumb instructions

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Message-Id: <20200518125948.25315-1-fredrik@strupe.net>
Date:   Mon, 18 May 2020 14:59:48 +0200
From:   Fredrik Strupe <fredrik@...upe.net>
To:     Russell King <linux@...linux.org.uk>, Rabin Vincent <rabin@....in>,
        "David A . Long" <dave.long@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Fredrik Strupe <fredrik@...upe.net>
Subject: [PATCH] arm: uprobes: Don't hook on thumb instructions

Since uprobes is not supported for thumb, check that the thumb bit is
not set when matching the uprobes instruction hooks.

The Arm UDF instructions used for uprobes triggering
(UPROBE_SWBP_ARM_INSN and UPROBE_SS_ARM_INSN) coincidentally share the
same encoding as a pair of unallocated 32-bit thumb instructions (not
UDF) when the condition code is 0b1111 (0xf). This in effect makes it
possible to trigger the uprobes functionality from thumb, and at that
using two unallocated instructions which are not permanently undefined.

Signed-off-by: Fredrik Strupe <fredrik@...upe.net>
Fixes: c7edc9e326d5 ("ARM: add uprobes support")
---
 arch/arm/probes/uprobes/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/probes/uprobes/core.c b/arch/arm/probes/uprobes/core.c
index c4b49b322e8a..f5f790c6e5f8 100644
--- a/arch/arm/probes/uprobes/core.c
+++ b/arch/arm/probes/uprobes/core.c
@@ -204,7 +204,7 @@ unsigned long uprobe_get_swbp_addr(struct pt_regs *regs)
 static struct undef_hook uprobes_arm_break_hook = {
 	.instr_mask	= 0x0fffffff,
 	.instr_val	= (UPROBE_SWBP_ARM_INSN & 0x0fffffff),
-	.cpsr_mask	= MODE_MASK,
+	.cpsr_mask	= (PSR_T_BIT | MODE_MASK),
 	.cpsr_val	= USR_MODE,
 	.fn		= uprobe_trap_handler,
 };
@@ -212,7 +212,7 @@ static struct undef_hook uprobes_arm_break_hook = {
 static struct undef_hook uprobes_arm_ss_hook = {
 	.instr_mask	= 0x0fffffff,
 	.instr_val	= (UPROBE_SS_ARM_INSN & 0x0fffffff),
-	.cpsr_mask	= MODE_MASK,
+	.cpsr_mask	= (PSR_T_BIT | MODE_MASK),
 	.cpsr_val	= USR_MODE,
 	.fn		= uprobe_trap_handler,
 };
-- 
2.20.1