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arm64: ptrace: Override SPSR.SS when single-stepping is enabled

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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>
This commit is contained in:
Will Deacon 2020-02-13 12:06:26 +00:00
parent ac2081cdc4
commit 3a5a4366ce
3 changed files with 20 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
arch/arm64/include/asm/debug-monitors.h
View File

@ -109,6 +109,8 @@ void disable_debug_monitors(enum dbg_active_el el);
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);

20
arch/arm64/kernel/debug-monitors.c
View File

@ -141,17 +141,20 @@ 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)
static DEFINE_SPINLOCK(debug_hook_lock);
static LIST_HEAD(user_step_hook);
@ -402,6 +405,15 @@ void user_fastforward_single_step(struct task_struct *task)
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)
{

4
arch/arm64/kernel/ptrace.c
View File

@ -1947,8 +1947,8 @@ static int valid_native_regs(struct user_pt_regs *regs)
*/
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);