From 9bd1190a11c9d2c59d35cb999b8d170ad52aab5f Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Mon, 3 Sep 2012 15:24:17 +0200 Subject: [PATCH] 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 Acked-by: Srikar Dronamraju --- 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 d048cad9bcad..433d2e5c98a7 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 f89cdc6ccd5b..cd3b2438a980 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 309a0e02b124..3b4aae68efe0 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; } -- 2.20.1