linux-kernel - [PATCH] ARM: don't enter kgdb when userspace executes a kgdb break instruction.

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Date:	Thu, 31 Jul 2014 20:05:20 -0700
From:	Omar Sandoval <osandov@...ndov.com>
To:	will.deacon@....com, linux-arm-kernel@...ts.infradead.org,
	linux@....linux.org.uk, linux-kernel@...r.kernel.org,
	Catalin.Marinas@....com
Subject: [PATCH] ARM: don't enter kgdb when userspace executes a kgdb break
 instruction.

The kgdb breakpoint hooks (kgdb_brk_fn and kgdb_compiled_brk_fn) should only be
entered when a kgdb break instruction is executed from the kernel. Otherwise,
if kgdb is enabled, a userspace program can cause the kernel to drop into the
debugger by executing either KGDB_BREAKINST or KGDB_COMPILED_BREAK.

Signed-off-by: Omar Sandoval <osandov@...ndov.com>
---
On a kernel running with kgdb enabled, this program reproduces the problem:
.globl _start
_start:
	udf	#65006	@ KGDB_BREAKINST

The same problem has been fixed in ARM64.

 arch/arm/kernel/kgdb.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm/kernel/kgdb.c b/arch/arm/kernel/kgdb.c
index 778c2f7..a74b53c 100644
--- a/arch/arm/kernel/kgdb.c
+++ b/arch/arm/kernel/kgdb.c
@@ -160,12 +160,16 @@ static int kgdb_compiled_brk_fn(struct pt_regs *regs, unsigned int instr)
 static struct undef_hook kgdb_brkpt_hook = {
 	.instr_mask		= 0xffffffff,
 	.instr_val		= KGDB_BREAKINST,
+	.cpsr_mask		= MODE_MASK,
+	.cpsr_val		= SVC_MODE,
 	.fn			= kgdb_brk_fn
 };
 
 static struct undef_hook kgdb_compiled_brkpt_hook = {
 	.instr_mask		= 0xffffffff,
 	.instr_val		= KGDB_COMPILED_BREAK,
+	.cpsr_mask		= MODE_MASK,
+	.cpsr_val		= SVC_MODE,
 	.fn			= kgdb_compiled_brk_fn
 };
 
-- 
2.0.3

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