Re: [PATCH v2 11/11] uprobes/x86: Make arch_uretprobe_is_alive(RP_CHECK_CALL) more clever

[Anton Arapov <arapov@gmail.com>]

On Tue, Jul 07, 2015 at 03:23:13AM +0200, Oleg Nesterov wrote:
> The previous change documents that cleanup_return_instances() can't
> always detect the dead frames, the stack can grow. But there is one
> special case which imho worth fixing: arch_uretprobe_is_alive() can
> return true when the stack didn't actually grow, but the next "call"
> insn uses the already invalidated frame.
> 
> Test-case:
> 
> 	#include <stdio.h>
> 	#include <setjmp.h>
> 
> 	jmp_buf jmp;
> 	int nr = 1024;
> 
> 	void func_2(void)
> 	{
> 		if (--nr == 0)
> 			return;
> 		longjmp(jmp, 1);
> 	}
> 
> 	void func_1(void)
> 	{
> 		setjmp(jmp);
> 		func_2();
> 	}
> 
> 	int main(void)
> 	{
> 		func_1();
> 		return 0;
> 	}
> 
> If you ret-probe func_1() and func_2() prepare_uretprobe() hits the
> MAX_URETPROBE_DEPTH limit and "return" from func_2() is not reported.
> 
> When we know that the new call is not chained, we can do the more
> strict check. In this case "sp" points to the new ret-addr, so every
> frame which uses the same "sp" must be dead. The only complication is
> that arch_uretprobe_is_alive() needs to know was it chained or not, so
> we add the new RP_CHECK_CHAIN_CALL enum and change prepare_uretprobe()
> to pass RP_CHECK_CALL only if !chained.
> 
> Note: arch_uretprobe_is_alive() could also re-read *sp and check if
> this word is still trampoline_vaddr. This could obviously improve the
> logic, but I would like to avoid another copy_from_user() especially
> in the case when we can't avoid the false "alive == T" positives.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Anton Arapov <arapov@gmail.com>


> ---
>  arch/x86/kernel/uprobes.c |    5 ++++-
>  include/linux/uprobes.h   |    1 +
>  kernel/events/uprobes.c   |   14 +++++++-------
>  3 files changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 67eb168..a5c59f2 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -997,5 +997,8 @@ arch_uretprobe_hijack_return_addr(unsigned long trampoline_vaddr, struct pt_regs
>  bool arch_uretprobe_is_alive(struct return_instance *ret, enum rp_check ctx,
>  				struct pt_regs *regs)
>  {
> -	return regs->sp <= ret->stack;
> +	if (ctx == RP_CHECK_CALL) /* sp was just decremented by "call" insn */
> +		return regs->sp < ret->stack;
> +	else
> +		return regs->sp <= ret->stack;
>  }
> diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> index c0a5402..0bdc72f 100644
> --- a/include/linux/uprobes.h
> +++ b/include/linux/uprobes.h
> @@ -104,6 +104,7 @@ struct return_instance {
>  
>  enum rp_check {
>  	RP_CHECK_CALL,
> +	RP_CHECK_CHAIN_CALL,
>  	RP_CHECK_RET,
>  };
>  
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index df5661a..0f370ef 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1511,10 +1511,11 @@ static unsigned long get_trampoline_vaddr(void)
>  	return trampoline_vaddr;
>  }
>  
> -static void cleanup_return_instances(struct uprobe_task *utask, struct pt_regs *regs)
> +static void cleanup_return_instances(struct uprobe_task *utask, bool chained,
> +					struct pt_regs *regs)
>  {
>  	struct return_instance *ri = utask->return_instances;
> -	enum rp_check ctx = RP_CHECK_CALL;
> +	enum rp_check ctx = chained ? RP_CHECK_CHAIN_CALL : RP_CHECK_CALL;
>  
>  	while (ri && !arch_uretprobe_is_alive(ri, ctx, regs)) {
>  		ri = free_ret_instance(ri);
> @@ -1528,7 +1529,7 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
>  	struct return_instance *ri;
>  	struct uprobe_task *utask;
>  	unsigned long orig_ret_vaddr, trampoline_vaddr;
> -	bool chained = false;
> +	bool chained;
>  
>  	if (!get_xol_area())
>  		return;
> @@ -1554,14 +1555,15 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
>  		goto fail;
>  
>  	/* drop the entries invalidated by longjmp() */
> -	cleanup_return_instances(utask, regs);
> +	chained = (orig_ret_vaddr == trampoline_vaddr);
> +	cleanup_return_instances(utask, chained, regs);
>  
>  	/*
>  	 * We don't want to keep trampoline address in stack, rather keep the
>  	 * original return address of first caller thru all the consequent
>  	 * instances. This also makes breakpoint unwrapping easier.
>  	 */
> -	if (orig_ret_vaddr == trampoline_vaddr) {
> +	if (chained) {
>  		if (!utask->return_instances) {
>  			/*
>  			 * This situation is not possible. Likely we have an
> @@ -1570,8 +1572,6 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
>  			uprobe_warn(current, "handle tail call");
>  			goto fail;
>  		}
> -
> -		chained = true;
>  		orig_ret_vaddr = utask->return_instances->orig_ret_vaddr;
>  	}
>  
> -- 
> 1.5.5.1
>