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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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