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Subject[PATCH 4.14 121/125] arm64: ptrace: Override SPSR.SS when single-stepping is enabled
Date
From: Will Deacon <will@kernel.org>

commit 3a5a4366cecc25daa300b9a9174f7fdd352b9068 upstream.

Luis reports that, when reverse debugging with GDB, single-step does not
function as expected on arm64:

| I've noticed, under very specific conditions, that a PTRACE_SINGLESTEP
| request by GDB won't execute the underlying instruction. As a consequence,
| the PC doesn't move, but we return a SIGTRAP just like we would for a
| regular successful PTRACE_SINGLESTEP request.

The underlying problem is that when the CPU register state is restored
as part of a reverse step, the SPSR.SS bit is cleared and so the hardware
single-step state can transition to the "active-pending" state, causing
an unexpected step exception to be taken immediately if a step operation
is attempted.

In hindsight, we probably shouldn't have exposed SPSR.SS in the pstate
accessible by the GPR regset, but it's a bit late for that now. Instead,
simply prevent userspace from configuring the bit to a value which is
inconsistent with the TIF_SINGLESTEP state for the task being traced.

Cc: <stable@vger.kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Keno Fischer <keno@juliacomputing.com>
Link: https://lore.kernel.org/r/1eed6d69-d53d-9657-1fc9-c089be07f98c@linaro.org
Reported-by: Luis Machado <luis.machado@linaro.org>
Tested-by: Luis Machado <luis.machado@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/arm64/include/asm/debug-monitors.h | 2 ++
arch/arm64/kernel/debug-monitors.c | 20 ++++++++++++++++----
arch/arm64/kernel/ptrace.c | 4 ++--
3 files changed, 20 insertions(+), 6 deletions(-)

--- a/arch/arm64/include/asm/debug-monitors.h
+++ b/arch/arm64/include/asm/debug-monitors.h
@@ -119,6 +119,8 @@ void disable_debug_monitors(enum dbg_act

void user_rewind_single_step(struct task_struct *task);
void user_fastforward_single_step(struct task_struct *task);
+void user_regs_reset_single_step(struct user_pt_regs *regs,
+ struct task_struct *task);

void kernel_enable_single_step(struct pt_regs *regs);
void kernel_disable_single_step(void);
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -150,17 +150,20 @@ postcore_initcall(debug_monitors_init);
/*
* Single step API and exception handling.
*/
-static void set_regs_spsr_ss(struct pt_regs *regs)
+static void set_user_regs_spsr_ss(struct user_pt_regs *regs)
{
regs->pstate |= DBG_SPSR_SS;
}
-NOKPROBE_SYMBOL(set_regs_spsr_ss);
+NOKPROBE_SYMBOL(set_user_regs_spsr_ss);

-static void clear_regs_spsr_ss(struct pt_regs *regs)
+static void clear_user_regs_spsr_ss(struct user_pt_regs *regs)
{
regs->pstate &= ~DBG_SPSR_SS;
}
-NOKPROBE_SYMBOL(clear_regs_spsr_ss);
+NOKPROBE_SYMBOL(clear_user_regs_spsr_ss);
+
+#define set_regs_spsr_ss(r) set_user_regs_spsr_ss(&(r)->user_regs)
+#define clear_regs_spsr_ss(r) clear_user_regs_spsr_ss(&(r)->user_regs)

/* EL1 Single Step Handler hooks */
static LIST_HEAD(step_hook);
@@ -397,6 +400,15 @@ void user_fastforward_single_step(struct
clear_regs_spsr_ss(task_pt_regs(task));
}

+void user_regs_reset_single_step(struct user_pt_regs *regs,
+ struct task_struct *task)
+{
+ if (test_tsk_thread_flag(task, TIF_SINGLESTEP))
+ set_user_regs_spsr_ss(regs);
+ else
+ clear_user_regs_spsr_ss(regs);
+}
+
/* Kernel API */
void kernel_enable_single_step(struct pt_regs *regs)
{
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -1496,8 +1496,8 @@ static int valid_native_regs(struct user
*/
int valid_user_regs(struct user_pt_regs *regs, struct task_struct *task)
{
- if (!test_tsk_thread_flag(task, TIF_SINGLESTEP))
- regs->pstate &= ~DBG_SPSR_SS;
+ /* https://lore.kernel.org/lkml/20191118131525.GA4180@willie-the-truck */
+ user_regs_reset_single_step(regs, task);

if (is_compat_thread(task_thread_info(task)))
return valid_compat_regs(regs);

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