arm64: ptrace: Override SPSR.SS when single-stepping is enabled
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>
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@ -109,6 +109,8 @@ void disable_debug_monitors(enum dbg_active_el el);
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void user_rewind_single_step(struct task_struct *task);
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void user_fastforward_single_step(struct task_struct *task);
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void user_regs_reset_single_step(struct user_pt_regs *regs,
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struct task_struct *task);
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void kernel_enable_single_step(struct pt_regs *regs);
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void kernel_disable_single_step(void);
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@ -141,17 +141,20 @@ postcore_initcall(debug_monitors_init);
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/*
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* Single step API and exception handling.
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*/
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static void set_regs_spsr_ss(struct pt_regs *regs)
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static void set_user_regs_spsr_ss(struct user_pt_regs *regs)
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{
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regs->pstate |= DBG_SPSR_SS;
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}
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NOKPROBE_SYMBOL(set_regs_spsr_ss);
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NOKPROBE_SYMBOL(set_user_regs_spsr_ss);
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static void clear_regs_spsr_ss(struct pt_regs *regs)
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static void clear_user_regs_spsr_ss(struct user_pt_regs *regs)
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{
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regs->pstate &= ~DBG_SPSR_SS;
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}
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NOKPROBE_SYMBOL(clear_regs_spsr_ss);
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NOKPROBE_SYMBOL(clear_user_regs_spsr_ss);
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#define set_regs_spsr_ss(r) set_user_regs_spsr_ss(&(r)->user_regs)
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#define clear_regs_spsr_ss(r) clear_user_regs_spsr_ss(&(r)->user_regs)
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static DEFINE_SPINLOCK(debug_hook_lock);
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static LIST_HEAD(user_step_hook);
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@ -402,6 +405,15 @@ void user_fastforward_single_step(struct task_struct *task)
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clear_regs_spsr_ss(task_pt_regs(task));
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}
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void user_regs_reset_single_step(struct user_pt_regs *regs,
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struct task_struct *task)
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{
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if (test_tsk_thread_flag(task, TIF_SINGLESTEP))
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set_user_regs_spsr_ss(regs);
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else
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clear_user_regs_spsr_ss(regs);
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}
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/* Kernel API */
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void kernel_enable_single_step(struct pt_regs *regs)
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{
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@ -1947,8 +1947,8 @@ static int valid_native_regs(struct user_pt_regs *regs)
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*/
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int valid_user_regs(struct user_pt_regs *regs, struct task_struct *task)
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{
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if (!test_tsk_thread_flag(task, TIF_SINGLESTEP))
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regs->pstate &= ~DBG_SPSR_SS;
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/* https://lore.kernel.org/lkml/20191118131525.GA4180@willie-the-truck */
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user_regs_reset_single_step(regs, task);
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if (is_compat_thread(task_thread_info(task)))
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return valid_compat_regs(regs);
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