Re: [PATCH v3 bpf-next 2/8] x86/alternative: introduce text_poke_set

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Thu, May 19, 2022 at 08:15:42PM -0700, Song Liu wrote:
> Introduce a memset like API for text_poke. This will be used to fill the
> unused RX memory with illegal instructions.
> 
> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Song Liu <song@kernel.org>
> ---
>  arch/x86/include/asm/text-patching.h |  1 +
>  arch/x86/kernel/alternative.c        | 67 +++++++++++++++++++++++-----
>  2 files changed, 58 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
> index d20ab0921480..1cc15528ce29 100644
> --- a/arch/x86/include/asm/text-patching.h
> +++ b/arch/x86/include/asm/text-patching.h
> @@ -45,6 +45,7 @@ extern void *text_poke(void *addr, const void *opcode, size_t len);
>  extern void text_poke_sync(void);
>  extern void *text_poke_kgdb(void *addr, const void *opcode, size_t len);
>  extern void *text_poke_copy(void *addr, const void *opcode, size_t len);
> +extern void *text_poke_set(void *addr, int c, size_t len);
>  extern int poke_int3_handler(struct pt_regs *regs);
>  extern void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate);
>  
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index d374cb3cf024..7563b5bc8328 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -994,7 +994,21 @@ static inline void unuse_temporary_mm(temp_mm_state_t prev_state)
>  __ro_after_init struct mm_struct *poking_mm;
>  __ro_after_init unsigned long poking_addr;
>  
> -static void *__text_poke(void *addr, const void *opcode, size_t len)
> +static void text_poke_memcpy(void *dst, const void *src, size_t len)
> +{
> +	memcpy(dst, src, len);
> +}
> +
> +static void text_poke_memset(void *dst, const void *src, size_t len)
> +{
> +	int c = *(const int *)src;
> +
> +	memset(dst, c, len);
> +}
> +
> +typedef void text_poke_f(void *dst, const void *src, size_t len);
> +
> +static void *__text_poke(text_poke_f func, void *addr, const void *src, size_t len)
>  {
>  	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
>  	struct page *pages[2] = {NULL};
> @@ -1059,7 +1073,7 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
>  	prev = use_temporary_mm(poking_mm);
>  
>  	kasan_disable_current();
> -	memcpy((u8 *)poking_addr + offset_in_page(addr), opcode, len);
> +	func((u8 *)poking_addr + offset_in_page(addr), src, len);
>  	kasan_enable_current();
>  
>  	/*
> @@ -1087,11 +1101,13 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
>  			   (cross_page_boundary ? 2 : 1) * PAGE_SIZE,
>  			   PAGE_SHIFT, false);
>  
> -	/*
> -	 * If the text does not match what we just wrote then something is
> -	 * fundamentally screwy; there's nothing we can really do about that.
> -	 */
> -	BUG_ON(memcmp(addr, opcode, len));
> +	if (func == text_poke_memcpy) {
> +		/*
> +		 * If the text does not match what we just wrote then something is
> +		 * fundamentally screwy; there's nothing we can really do about that.
> +		 */
> +		BUG_ON(memcmp(addr, src, len));

Maybe something like this?

	} else if (func == text_poke_memset) {
		WARN_ON or BUG_ON(memchr_inv(addr, *((const int *)src), len));
	}

Thanks,
Hyeonggon

>  
>  	local_irq_restore(flags);
>  	pte_unmap_unlock(ptep, ptl);
> @@ -1118,7 +1134,7 @@ void *text_poke(void *addr, const void *opcode, size_t len)
>  {
>  	lockdep_assert_held(&text_mutex);
>  
> -	return __text_poke(addr, opcode, len);
> +	return __text_poke(text_poke_memcpy, addr, opcode, len);
>  }
>  
>  /**
> @@ -1137,7 +1153,7 @@ void *text_poke(void *addr, const void *opcode, size_t len)
>   */
>  void *text_poke_kgdb(void *addr, const void *opcode, size_t len)
>  {
> -	return __text_poke(addr, opcode, len);
> +	return __text_poke(text_poke_memcpy, addr, opcode, len);
>  }
>  
>  /**
> @@ -1167,7 +1183,38 @@ void *text_poke_copy(void *addr, const void *opcode, size_t len)
>  
>  		s = min_t(size_t, PAGE_SIZE * 2 - offset_in_page(ptr), len - patched);
>  
> -		__text_poke((void *)ptr, opcode + patched, s);
> +		__text_poke(text_poke_memcpy, (void *)ptr, opcode + patched, s);
> +		patched += s;
> +	}
> +	mutex_unlock(&text_mutex);
> +	return addr;
> +}
> +
> +/**
> + * text_poke_set - memset into (an unused part of) RX memory
> + * @addr: address to modify
> + * @c: the byte to fill the area with
> + * @len: length to copy, could be more than 2x PAGE_SIZE
> + *
> + * This is useful to overwrite unused regions of RX memory with illegal
> + * instructions.
> + */
> +void *text_poke_set(void *addr, int c, size_t len)
> +{
> +	unsigned long start = (unsigned long)addr;
> +	size_t patched = 0;
> +
> +	if (WARN_ON_ONCE(core_kernel_text(start)))
> +		return NULL;
> +
> +	mutex_lock(&text_mutex);
> +	while (patched < len) {
> +		unsigned long ptr = start + patched;
> +		size_t s;
> +
> +		s = min_t(size_t, PAGE_SIZE * 2 - offset_in_page(ptr), len - patched);
> +
> +		__text_poke(text_poke_memset, (void *)ptr, (void *)&c, s);
>  		patched += s;
>  	}
>  	mutex_unlock(&text_mutex);
> -- 
> 2.30.2
> 
>