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