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



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