Re: [PATCH 14/23] tracing/kprobes: handle mixed kernel/userspace probes better

[Masami Hiramatsu]

On Thu, 21 May 2020 17:22:52 +0200
Christoph Hellwig <hch@xxxxxx> wrote:

> Instead of using the dangerous probe_kernel_read and strncpy_from_unsafe
> helpers, rework probes to try a user probe based on the address if the
> architecture has a common address space for kernel and userspace.
> 

This looks good to me.

Acked-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Thank you!

> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  kernel/trace/trace_kprobe.c | 72 ++++++++++++++++++++++---------------
>  1 file changed, 43 insertions(+), 29 deletions(-)
> 
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 4325f9e7fadaa..4aeaef53ba970 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -1200,6 +1200,15 @@ static const struct file_operations kprobe_profile_ops = {
>  
>  /* Kprobe specific fetch functions */
>  
> +/* Return the length of string -- including null terminal byte */
> +static nokprobe_inline int
> +fetch_store_strlen_user(unsigned long addr)
> +{
> +	const void __user *uaddr =  (__force const void __user *)addr;
> +
> +	return strnlen_user_nofault(uaddr, MAX_STRING_SIZE);
> +}
> +
>  /* Return the length of string -- including null terminal byte */
>  static nokprobe_inline int
>  fetch_store_strlen(unsigned long addr)
> @@ -1207,30 +1216,27 @@ fetch_store_strlen(unsigned long addr)
>  	int ret, len = 0;
>  	u8 c;
>  
> +#ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
> +	if (addr < TASK_SIZE)
> +		return fetch_store_strlen_user(addr);
> +#endif
> +
>  	do {
> -		ret = probe_kernel_read(&c, (u8 *)addr + len, 1);
> +		ret = probe_kernel_read_strict(&c, (u8 *)addr + len, 1);
>  		len++;
>  	} while (c && ret == 0 && len < MAX_STRING_SIZE);
>  
>  	return (ret < 0) ? ret : len;
>  }
>  
> -/* Return the length of string -- including null terminal byte */
> -static nokprobe_inline int
> -fetch_store_strlen_user(unsigned long addr)
> -{
> -	const void __user *uaddr =  (__force const void __user *)addr;
> -
> -	return strnlen_user_nofault(uaddr, MAX_STRING_SIZE);
> -}
> -
>  /*
> - * Fetch a null-terminated string. Caller MUST set *(u32 *)buf with max
> - * length and relative data location.
> + * Fetch a null-terminated string from user. Caller MUST set *(u32 *)buf
> + * with max length and relative data location.
>   */
>  static nokprobe_inline int
> -fetch_store_string(unsigned long addr, void *dest, void *base)
> +fetch_store_string_user(unsigned long addr, void *dest, void *base)
>  {
> +	const void __user *uaddr =  (__force const void __user *)addr;
>  	int maxlen = get_loc_len(*(u32 *)dest);
>  	void *__dest;
>  	long ret;
> @@ -1240,11 +1246,7 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
>  
>  	__dest = get_loc_data(dest, base);
>  
> -	/*
> -	 * Try to get string again, since the string can be changed while
> -	 * probing.
> -	 */
> -	ret = strncpy_from_unsafe(__dest, (void *)addr, maxlen);
> +	ret = strncpy_from_user_nofault(__dest, uaddr, maxlen);
>  	if (ret >= 0)
>  		*(u32 *)dest = make_data_loc(ret, __dest - base);
>  
> @@ -1252,35 +1254,37 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
>  }
>  
>  /*
> - * Fetch a null-terminated string from user. Caller MUST set *(u32 *)buf
> - * with max length and relative data location.
> + * Fetch a null-terminated string. Caller MUST set *(u32 *)buf with max
> + * length and relative data location.
>   */
>  static nokprobe_inline int
> -fetch_store_string_user(unsigned long addr, void *dest, void *base)
> +fetch_store_string(unsigned long addr, void *dest, void *base)
>  {
> -	const void __user *uaddr =  (__force const void __user *)addr;
>  	int maxlen = get_loc_len(*(u32 *)dest);
>  	void *__dest;
>  	long ret;
>  
> +#ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
> +	if ((unsigned long)addr < TASK_SIZE)
> +		return fetch_store_string_user(addr, dest, base);
> +#endif
> +
>  	if (unlikely(!maxlen))
>  		return -ENOMEM;
>  
>  	__dest = get_loc_data(dest, base);
>  
> -	ret = strncpy_from_user_nofault(__dest, uaddr, maxlen);
> +	/*
> +	 * Try to get string again, since the string can be changed while
> +	 * probing.
> +	 */
> +	ret = strncpy_from_user_nofault(__dest, (void *)addr, maxlen);
>  	if (ret >= 0)
>  		*(u32 *)dest = make_data_loc(ret, __dest - base);
>  
>  	return ret;
>  }
>  
> -static nokprobe_inline int
> -probe_mem_read(void *dest, void *src, size_t size)
> -{
> -	return probe_kernel_read(dest, src, size);
> -}
> -
>  static nokprobe_inline int
>  probe_mem_read_user(void *dest, void *src, size_t size)
>  {
> @@ -1289,6 +1293,16 @@ probe_mem_read_user(void *dest, void *src, size_t size)
>  	return probe_user_read(dest, uaddr, size);
>  }
>  
> +static nokprobe_inline int
> +probe_mem_read(void *dest, void *src, size_t size)
> +{
> +#ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
> +	if ((unsigned long)src < TASK_SIZE)
> +		return probe_mem_read_user(dest, src, size);
> +#endif
> +	return probe_kernel_read_strict(dest, src, size);
> +}
> +
>  /* Note that we don't verify it, since the code does not come from user space */
>  static int
>  process_fetch_insn(struct fetch_insn *code, struct pt_regs *regs, void *dest,
> -- 
> 2.26.2
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>