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uprobes/x86: 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>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
This commit is contained in:
Oleg Nesterov 2012-09-03 15:24:17 +02:00
parent 95cf00fa5d
commit 9bd1190a11
3 changed files with 21 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
arch/x86/include/asm/processor.h
View File

@ -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:

2
arch/x86/kernel/step.c
View File

@ -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;

32
arch/x86/kernel/uprobes.c
View File

@ -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;
}