Re: [RFC,PATCH] kprobes - optimized kprobes might crash before setting kernel stack

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2011/02/15 0:12), Jiri Olsa wrote:
> hi,
> 
> you can crash the kernel using kprobe tracer via:
> 
> echo "p system_call_after_swapgs" > ./kprobe_events
> echo 1 > ./events/kprobes/enable

Ah, thank you very much!

> The reason is that at the system_call_after_swapgs label,
> the kernel stack is not set up. If optimized kprobes are
> enabled, the user space stack is being used in this case
> (see optimized kprobe template) and this might result in a crash.

Verified here, and also it didn't occur when turning optimization
off by sysctl. So this is a bug of kprobe jump optimization, not
kprobes itself.

> Looks like there are several places like this over the entry_$(BIT)
> code. First I thought it'd be ok to localize those places, but
> I haven't found any reasonable/maintainable way to disable only those
> places.

Hmm, agreed.

> So I switched off the whole entry code from optimizing, but this
> also switch many safe places (attached patch - tested on x86_64).

I'm OK for this solution. I think possible another solution is using
interrupt stack in optprobe template too. Anyway in short term, this
solution will be good.

> Also not sure this crash falls in to the area of that once such
> probe is used, user should know consequences..

User can see that those probe is not optimized via sysfs.

Thanks again,

> 
> any ideas?
> 
> wbr,
> jirka
> 
> 
> Signed-off-by: Jiri Olsa <jolsa@redhat.com>
> ---
>  arch/x86/ia32/ia32entry.S         |    2 ++
>  arch/x86/kernel/entry_32.S        |    6 ++++--
>  arch/x86/kernel/entry_64.S        |    6 ++++--
>  arch/x86/kernel/kprobes.c         |    8 ++++++++
>  arch/x86/kernel/vmlinux.lds.S     |    1 +
>  include/asm-generic/sections.h    |    1 +
>  include/asm-generic/vmlinux.lds.h |    6 ++++++
>  7 files changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
> index 0ed7896..50f1630 100644
> --- a/arch/x86/ia32/ia32entry.S
> +++ b/arch/x86/ia32/ia32entry.S
> @@ -25,6 +25,8 @@
>  #define sysretl_audit ia32_ret_from_sys_call
>  #endif
>  
> +	.section .entry.text, "ax"
> +
>  #define IA32_NR_syscalls ((ia32_syscall_end - ia32_sys_call_table)/8)
>  
>  	.macro IA32_ARG_FIXUP noebp=0
> diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
> index c8b4efa..051b4e2 100644
> --- a/arch/x86/kernel/entry_32.S
> +++ b/arch/x86/kernel/entry_32.S
> @@ -65,6 +65,8 @@
>  #define sysexit_audit	syscall_exit_work
>  #endif
>  
> +	.section .entry.text, "ax"
> +
>  /*
>   * We use macros for low-level operations which need to be overridden
>   * for paravirtualization.  The following will never clobber any registers:
> @@ -788,7 +790,7 @@ ENDPROC(ptregs_clone)
>   */
>  .section .init.rodata,"a"
>  ENTRY(interrupt)
> -.text
> +.entry.text
>  	.p2align 5
>  	.p2align CONFIG_X86_L1_CACHE_SHIFT
>  ENTRY(irq_entries_start)
> @@ -807,7 +809,7 @@ vector=FIRST_EXTERNAL_VECTOR
>        .endif
>        .previous
>  	.long 1b
> -      .text
> +      .entry.text
>  vector=vector+1
>      .endif
>    .endr
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index aed1ffb..0a0ed79 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -61,6 +61,8 @@
>  #define __AUDIT_ARCH_LE	   0x40000000
>  
>  	.code64
> +	.section .entry.text, "ax"
> +
>  #ifdef CONFIG_FUNCTION_TRACER
>  #ifdef CONFIG_DYNAMIC_FTRACE
>  ENTRY(mcount)
> @@ -744,7 +746,7 @@ END(stub_rt_sigreturn)
>   */
>  	.section .init.rodata,"a"
>  ENTRY(interrupt)
> -	.text
> +	.section .entry.text
>  	.p2align 5
>  	.p2align CONFIG_X86_L1_CACHE_SHIFT
>  ENTRY(irq_entries_start)
> @@ -763,7 +765,7 @@ vector=FIRST_EXTERNAL_VECTOR
>        .endif
>        .previous
>  	.quad 1b
> -      .text
> +      .section .entry.text
>  vector=vector+1
>      .endif
>    .endr
> diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
> index d91c477..d03bc1e 100644
> --- a/arch/x86/kernel/kprobes.c
> +++ b/arch/x86/kernel/kprobes.c
> @@ -1276,6 +1276,14 @@ static int __kprobes can_optimize(unsigned long paddr)
>  	if (!kallsyms_lookup_size_offset(paddr, &size, &offset))
>  		return 0;
>  
> +	/*
> +	 * Do not optimize in the entry code due to the unstable
> +	 * stack handling.
> +	 */
> +	if ((paddr >= (unsigned long ) __entry_text_start) &&
> +	    (paddr <  (unsigned long ) __entry_text_end))
> +		return 0;
> +
>  	/* Check there is enough space for a relative jump. */
>  	if (size - offset < RELATIVEJUMP_SIZE)
>  		return 0;
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index e9f7a3c..0381e1f 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -105,6 +105,7 @@ SECTIONS
>  		SCHED_TEXT
>  		LOCK_TEXT
>  		KPROBES_TEXT
> +		ENTRY_TEXT
>  		IRQENTRY_TEXT
>  		*(.fixup)
>  		*(.gnu.warning)
> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
> index b3bfabc..c1a1216 100644
> --- a/include/asm-generic/sections.h
> +++ b/include/asm-generic/sections.h
> @@ -11,6 +11,7 @@ extern char _sinittext[], _einittext[];
>  extern char _end[];
>  extern char __per_cpu_load[], __per_cpu_start[], __per_cpu_end[];
>  extern char __kprobes_text_start[], __kprobes_text_end[];
> +extern char __entry_text_start[], __entry_text_end[];
>  extern char __initdata_begin[], __initdata_end[];
>  extern char __start_rodata[], __end_rodata[];
>  
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index fe77e33..906c3ce 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -424,6 +424,12 @@
>  		*(.kprobes.text)					\
>  		VMLINUX_SYMBOL(__kprobes_text_end) = .;
>  
> +#define ENTRY_TEXT							\
> +		ALIGN_FUNCTION();					\
> +		VMLINUX_SYMBOL(__entry_text_start) = .;			\
> +		*(.entry.text)						\
> +		VMLINUX_SYMBOL(__entry_text_end) = .;
> +
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>  #define IRQENTRY_TEXT							\
>  		ALIGN_FUNCTION();					\


-- 
Masami HIRAMATSU
2nd Dept. Linux Technology Center
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com