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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@osandov.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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