Re: [PATCH] x86 trace: Fix page fault tracing bug

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, Feb 28, 2014 at 04:33:40PM +0100, Jiri Olsa wrote:
> The trace_do_page_fault function trigger tracepoint
> and then handles the actual page fault.
> 
> This could lead to error if the tracepoint caused page
> fault. The original cr2 value gets lost and the original
> page fault handler kills current process with SIGSEGV.
> 
> This happens if you record page faults with callchain
> data, the user part of it will cause tracepoint handler
> to page fault:
> 
>   # perf record -g -e exceptions:page_fault_user ls
> 
> Fixing this by saving the original cr2 value
> and using it after tracepoint handler is done.
> 
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Seiji Aguchi <seiji.aguchi@hds.com>
> ---
>  arch/x86/mm/fault.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 9d591c8..52fad6c 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1016,11 +1016,11 @@ static inline bool smap_violation(int error_code, struct pt_regs *regs)
>   * routines.
>   */
>  static void __kprobes
> -__do_page_fault(struct pt_regs *regs, unsigned long error_code)
> +__do_page_fault(struct pt_regs *regs, unsigned long error_code,
> +		unsigned long address)
>  {
>  	struct vm_area_struct *vma;
>  	struct task_struct *tsk;
> -	unsigned long address;
>  	struct mm_struct *mm;
>  	int fault;
>  	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
> @@ -1028,9 +1028,6 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code)
>  	tsk = current;
>  	mm = tsk->mm;
>  
> -	/* Get the faulting address: */
> -	address = read_cr2();
> -
>  	/*
>  	 * Detect and handle instructions that would cause a page fault for
>  	 * both a tracked kernel page and a userspace page.
> @@ -1248,9 +1245,14 @@ dotraplinkage void __kprobes
>  do_page_fault(struct pt_regs *regs, unsigned long error_code)
>  {
>  	enum ctx_state prev_state;
> +	unsigned long address;
>  
>  	prev_state = exception_enter();
> -	__do_page_fault(regs, error_code);
> +
> +	/* Get the faulting address: */
> +	address = read_cr2();
> +

As Peter already stated, the address = read_cr2() needs to go before the
exception_enter() call.

> +	__do_page_fault(regs, error_code, address);
>  	exception_exit(prev_state);
>  }
>  
> @@ -1267,9 +1269,18 @@ dotraplinkage void __kprobes
>  trace_do_page_fault(struct pt_regs *regs, unsigned long error_code)
>  {
>  	enum ctx_state prev_state;
> +	unsigned long address;
>  
>  	prev_state = exception_enter();
> +
> +	/*
> +	 * The tracepoint processing could trigger another page
> +	 * fault (user space callchain reading) and destroy the
> +	 * original cr2 value, so read the faulting address now.
> +	 */
> +	address = read_cr2();

Same here.

After that, you can add:

Acked-by: Steven Rostedt <rostedt@goodmis.org>

-- Steve

> +
>  	trace_page_fault_entries(regs, error_code);
> -	__do_page_fault(regs, error_code);
> +	__do_page_fault(regs, error_code, address);
>  	exception_exit(prev_state);
>  }
> -- 
> 1.7.11.7
> 
> --
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