Re: [PATCH 5/7] uprobes: Do not (ab)use TIF_SINGLESTEP/user_*_single_step() for single-stepping

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Oleg Nesterov <oleg@redhat.com> [2012-09-03 17:26:13]:

> user_enable/disable_single_step() was designed for ptrace, it assumes
> a single user and does unnecessary and wrong things for uprobes. For
> example:
> 
> 	- arch_uprobe_enable_step() can't trust TIF_SINGLESTEP, an
> 	  application itself can set X86_EFLAGS_TF which must be
> 	  preserved after arch_uprobe_disable_step().
> 
> 	- we do not want to set TIF_SINGLESTEP/TIF_FORCED_TF in
> 	  arch_uprobe_enable_step(), this only makes sense for ptrace.
> 
> 	- otoh we leak TIF_SINGLESTEP if arch_uprobe_disable_step()
> 	  doesn't do user_disable_single_step(), the application will
> 	  be killed after the next syscall.
> 
> 	- arch_uprobe_enable_step() does access_process_vm() we do
> 	  not need/want.
> 
> Change arch_uprobe_enable/disable_step() to set/clear X86_EFLAGS_TF
> directly, this is much simpler and more correct. However, we need to
> clear TIF_BLOCKSTEP/DEBUGCTLMSR_BTF before executing the probed insn,
> add set_task_blockstep(false).
> 
> Note: with or without this patch, there is another (hopefully minor)
> problem. A probed "pushf" insn can see the wrong X86_EFLAGS_TF set by
> uprobes. Perhaps we should change _disable to update the stack, or
> teach arch_uprobe_skip_sstep() to emulate this insn.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

> ---
>  arch/x86/include/asm/processor.h |    2 ++
>  arch/x86/kernel/step.c           |    2 +-
>  arch/x86/kernel/uprobes.c        |   32 ++++++++++++++++++--------------
>  3 files changed, 21 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index d048cad..433d2e5 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -759,6 +759,8 @@ static inline void update_debugctlmsr(unsigned long debugctlmsr)
>  	wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctlmsr);
>  }
> 
> +extern void set_task_blockstep(struct task_struct *task, bool on);
> +
>  /*
>   * from system description table in BIOS. Mostly for MCA use, but
>   * others may find it useful:
> diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
> index f89cdc6..cd3b243 100644
> --- a/arch/x86/kernel/step.c
> +++ b/arch/x86/kernel/step.c
> @@ -157,7 +157,7 @@ static int enable_single_step(struct task_struct *child)
>  	return 1;
>  }
> 
> -static void set_task_blockstep(struct task_struct *task, bool on)
> +void set_task_blockstep(struct task_struct *task, bool on)
>  {
>  	unsigned long debugctl;
> 
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 309a0e0..3b4aae6 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -683,26 +683,30 @@ bool arch_uprobe_skip_sstep(struct arch_uprobe *auprobe, struct pt_regs *regs)
> 
>  void arch_uprobe_enable_step(struct arch_uprobe *auprobe)
>  {
> -	struct uprobe_task	*utask		= current->utask;
> -	struct arch_uprobe_task	*autask		= &utask->autask;
> +	struct task_struct *task = current;

Any particular reason to use task instead of current?

> +	struct arch_uprobe_task	*autask	= &task->utask->autask;
> +	struct pt_regs *regs = task_pt_regs(task);
> 
>  	autask->restore_flags = 0;
> -	if (!test_tsk_thread_flag(current, TIF_SINGLESTEP) &&
> -			!(auprobe->fixups & UPROBE_FIX_SETF))
> +	if (!(regs->flags & X86_EFLAGS_TF) &&
> +	    !(auprobe->fixups & UPROBE_FIX_SETF))
>  		autask->restore_flags |= UPROBE_CLEAR_TF;
> -	/*
> -	 * The state of TIF_BLOCKSTEP is not saved. With the TF flag set we
> -	 * would to examine the opcode and the flags to make it right. Without
> -	 * TF block stepping makes no sense.
> -	 */
> -	user_enable_single_step(current);
> +
> +	regs->flags |= X86_EFLAGS_TF;
> +	if (test_tsk_thread_flag(task, TIF_BLOCKSTEP))
> +		set_task_blockstep(task, false);
>  }
> 
>  void arch_uprobe_disable_step(struct arch_uprobe *auprobe)
>  {
> -	struct uprobe_task *utask		= current->utask;
> -	struct arch_uprobe_task	*autask		= &utask->autask;
> -
> +	struct task_struct *task = current;
> +	struct arch_uprobe_task	*autask	= &task->utask->autask;
> +	struct pt_regs *regs = task_pt_regs(task);
> +	/*
> +	 * The state of TIF_BLOCKSTEP was not saved so we can get an extra
> +	 * SIGTRAP if we do not clear TF. We need to examine the opcode to
> +	 * make it right.
> +	 */
>  	if (autask->restore_flags & UPROBE_CLEAR_TF)
> -		user_disable_single_step(current);
> +		regs->flags &= ~X86_EFLAGS_TF;
>  }
> -- 
> 1.5.5.1
>