linux-kernel - [PATCH] ARM/ARM64: don't enter kgdb when userspace executes a kgdb break instruction.

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Message-ID: <20140730071245.GA8954@mew.web-pass.com>
Date:	Wed, 30 Jul 2014 00:12:45 -0700
From:	Omar Sandoval <osandov@...ndov.com>
To:	linux-arm-kernel@...ts.infradead.org, linux@....linux.org.uk,
	catalin.marinas@....com, will.deacon@....com
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] ARM/ARM64: 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 on ARM, or
brk #KGDB_{DYN,COMPILED}_DGB_BRK_IMM on ARM64.

Signed-off-by: Omar Sandoval <osandov@...ndov.com>
---
The following program reproduces the fixed problem on ARM:
.globl _start
_start:
	udf	#65006	@ KGDB_BREAKINST

And on ARM64:
.globl _start
_start:
        brk     #0x400	@ KGDB_DYN_DGB_BRK_IMM

 arch/arm/kernel/kgdb.c                  | 4 ++++
 arch/arm64/include/asm/debug-monitors.h | 4 +++-
 arch/arm64/kernel/debug-monitors.c      | 3 ++-
 arch/arm64/kernel/kgdb.c                | 4 ++++
 4 files changed, 13 insertions(+), 2 deletions(-)

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
 };
 
diff --git a/arch/arm64/include/asm/debug-monitors.h b/arch/arm64/include/asm/debug-monitors.h
index 6e9b5b3..e1d27ce 100644
--- a/arch/arm64/include/asm/debug-monitors.h
+++ b/arch/arm64/include/asm/debug-monitors.h
@@ -105,8 +105,10 @@ void unregister_step_hook(struct step_hook *hook);
 
 struct break_hook {
 	struct list_head node;
-	u32 esr_val;
 	u32 esr_mask;
+	u32 esr_val;
+	u64 pstate_mask;
+	u64 pstate_val;
 	int (*fn)(struct pt_regs *regs, unsigned int esr);
 };
 
diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index a7fb874..594fac4 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -303,7 +303,8 @@ static int call_break_hook(struct pt_regs *regs, unsigned int esr)
 
 	read_lock(&break_hook_lock);
 	list_for_each_entry(hook, &break_hook, node)
-		if ((esr & hook->esr_mask) == hook->esr_val)
+		if ((esr & hook->esr_mask) == hook->esr_val &&
+		    (regs->pstate & hook->pstate_mask) == hook->pstate_val)
 			fn = hook->fn;
 	read_unlock(&break_hook_lock);
 
diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
index 75c9cf1..80eb035 100644
--- a/arch/arm64/kernel/kgdb.c
+++ b/arch/arm64/kernel/kgdb.c
@@ -236,12 +236,16 @@ static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned int esr)
 static struct break_hook kgdb_brkpt_hook = {
 	.esr_mask	= 0xffffffff,
 	.esr_val	= DBG_ESR_VAL_BRK(KGDB_DYN_DGB_BRK_IMM),
+	.pstate_mask	= PSR_MODE_MASK,
+	.pstate_val	= PSR_MODE_EL1h,
 	.fn		= kgdb_brk_fn
 };
 
 static struct break_hook kgdb_compiled_brkpt_hook = {
 	.esr_mask	= 0xffffffff,
 	.esr_val	= DBG_ESR_VAL_BRK(KDBG_COMPILED_DBG_BRK_IMM),
+	.pstate_mask	= PSR_MODE_MASK,
+	.pstate_val	= PSR_MODE_EL1h,
 	.fn		= kgdb_compiled_brk_fn
 };
 
-- 
2.0.3

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