Re: [PATCH] kprobes: Reduce preempt disable scope in check_kprobe_access_safe()

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 21 Nov 2024 08:38:08 +0100
Thomas Weißschuh <thomas.weissschuh@linutronix.de> wrote:

> Commit a189d0350f387 ("kprobes: disable preempt for module_text_address() and kernel_text_address()")
> introduced a preempt_disable() region to protect against concurrent
> module unloading. However this region also includes the call to
> jump_label_text_reserved() which takes a long time;
> up to 400us, iterating over approx 6000 jump tables.

Oops, that's too long.

> 
> The scope protected by preempt_disable() is largen than necessary.
> core_kernel_text() does not need to be protected as it does not interact
> with module code at all.
> Only the scope from __module_text_address() to try_module_get() needs to
> be protected.
> By limiting the critical section to __module_text_address() and
> try_module_get() the function responsible for the latency spike remains
> preemptible.

Yes, after getting the module refcount, no need to take care of
preemption.

> 
> This works fine even when !CONFIG_MODULES as in that case
> try_module_get() will always return true and that block can be optimized
> away.
> 
> Limit the critical section to __module_text_address() and
> try_module_get(). Use guard(preempt)() for easier error handling.
> 
> While at it also remove a spurious *probed_mod = NULL in an error
> path. On errors the output parameter is never inspected by the caller.
> Some error paths were clearing the parameters, some didn't.
> Align them for clarity.
> 
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

Looks good to me. let me pick it.

Thank you,

> ---
>  kernel/kprobes.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index da59c68df8412c4662d39d3e286d516e8cee9a69..369020170e832a34ea9c05eda6693bded02ec505 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -39,6 +39,7 @@
>  #include <linux/static_call.h>
>  #include <linux/perf_event.h>
>  #include <linux/execmem.h>
> +#include <linux/cleanup.h>
>  
>  #include <asm/sections.h>
>  #include <asm/cacheflush.h>
> @@ -1570,16 +1571,25 @@ static int check_kprobe_address_safe(struct kprobe *p,
>  	if (ret)
>  		return ret;
>  	jump_label_lock();
> -	preempt_disable();
>  
>  	/* Ensure the address is in a text area, and find a module if exists. */
>  	*probed_mod = NULL;
>  	if (!core_kernel_text((unsigned long) p->addr)) {
> +		guard(preempt)();
>  		*probed_mod = __module_text_address((unsigned long) p->addr);
>  		if (!(*probed_mod)) {
>  			ret = -EINVAL;
>  			goto out;
>  		}
> +
> +		/*
> +		 * We must hold a refcount of the probed module while updating
> +		 * its code to prohibit unexpected unloading.
> +		 */
> +		if (unlikely(!try_module_get(*probed_mod))) {
> +			ret = -ENOENT;
> +			goto out;
> +		}
>  	}
>  	/* Ensure it is not in reserved area. */
>  	if (in_gate_area_no_mm((unsigned long) p->addr) ||
> @@ -1588,21 +1598,13 @@ static int check_kprobe_address_safe(struct kprobe *p,
>  	    static_call_text_reserved(p->addr, p->addr) ||
>  	    find_bug((unsigned long)p->addr) ||
>  	    is_cfi_preamble_symbol((unsigned long)p->addr)) {
> +		module_put(*probed_mod);
>  		ret = -EINVAL;
>  		goto out;
>  	}
>  
>  	/* Get module refcount and reject __init functions for loaded modules. */
>  	if (IS_ENABLED(CONFIG_MODULES) && *probed_mod) {
> -		/*
> -		 * We must hold a refcount of the probed module while updating
> -		 * its code to prohibit unexpected unloading.
> -		 */
> -		if (unlikely(!try_module_get(*probed_mod))) {
> -			ret = -ENOENT;
> -			goto out;
> -		}
> -
>  		/*
>  		 * If the module freed '.init.text', we couldn't insert
>  		 * kprobes in there.
> @@ -1610,13 +1612,11 @@ static int check_kprobe_address_safe(struct kprobe *p,
>  		if (within_module_init((unsigned long)p->addr, *probed_mod) &&
>  		    !module_is_coming(*probed_mod)) {
>  			module_put(*probed_mod);
> -			*probed_mod = NULL;
>  			ret = -ENOENT;
>  		}
>  	}
>  
>  out:
> -	preempt_enable();
>  	jump_label_unlock();
>  
>  	return ret;
> 
> ---
> base-commit: adc218676eef25575469234709c2d87185ca223a
> change-id: 20241120-kprobes-preempt-09826f5e5d9c
> 
> Best regards,
> -- 
> Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>