Re: [PATCH 12/15] maccess: always use strict semantics for probe_kernel_read

[Masami Hiramatsu]

Hi Christoph,

At first, thank you for your work on cleaning up these functions!

On Wed,  6 May 2020 08:22:20 +0200
Christoph Hellwig <hch@xxxxxx> wrote:

> Except for historical confusion in the kprobes/uprobes and bpf tracers
> there is no good reason to ever allow user memory accesses from
> probe_kernel_read.

Yes, thus now trace_kprobe supports "ustring" type for accessing
user space memory. (If the address spaces are overwrapped, we have
no way to distinguish whether an address is kernel or user)

>  Make the tracers fall back to a probe_user_read
> if the probe_kernel_read falls to keep the core API clean.

For trace_kprobes doesn't need to fall back. User must specify
the probe should be read from user space or kernel space. This is
because it has  fetch_store_string_user() and probe_mem_read_user()
variants.

Thank you,


> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  arch/parisc/lib/memcpy.c    |  3 +--
>  arch/um/kernel/maccess.c    |  3 +--
>  arch/x86/mm/maccess.c       |  5 +----
>  include/linux/uaccess.h     |  4 +---
>  kernel/trace/bpf_trace.c    | 20 +++++++++++++------
>  kernel/trace/trace_kprobe.c | 11 ++++++++++-
>  mm/maccess.c                | 39 ++++++-------------------------------
>  7 files changed, 34 insertions(+), 51 deletions(-)
> 
> diff --git a/arch/parisc/lib/memcpy.c b/arch/parisc/lib/memcpy.c
> index 5ef648bd33119..9fe662b3b5604 100644
> --- a/arch/parisc/lib/memcpy.c
> +++ b/arch/parisc/lib/memcpy.c
> @@ -57,8 +57,7 @@ void * memcpy(void * dst,const void *src, size_t count)
>  EXPORT_SYMBOL(raw_copy_in_user);
>  EXPORT_SYMBOL(memcpy);
>  
> -bool probe_kernel_read_allowed(void *dst, const void *unsafe_src, size_t size,
> -		bool strict)
> +bool probe_kernel_read_allowed(void *dst, const void *unsafe_src, size_t size)
>  {
>  	if ((unsigned long)unsafe_src < PAGE_SIZE)
>  		return false;
> diff --git a/arch/um/kernel/maccess.c b/arch/um/kernel/maccess.c
> index 90a1bec923158..734f3d7e57c0f 100644
> --- a/arch/um/kernel/maccess.c
> +++ b/arch/um/kernel/maccess.c
> @@ -7,8 +7,7 @@
>  #include <linux/kernel.h>
>  #include <os.h>
>  
> -bool probe_kernel_read_allowed(void *dst, const void *src, size_t size,
> -		bool strict)
> +bool probe_kernel_read_allowed(void *dst, const void *src, size_t size)
>  {
>  	void *psrc = (void *)rounddown((unsigned long)src, PAGE_SIZE);
>  
> diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c
> index 5c323ab187b27..a1bd81677aa72 100644
> --- a/arch/x86/mm/maccess.c
> +++ b/arch/x86/mm/maccess.c
> @@ -26,10 +26,7 @@ static __always_inline bool invalid_probe_range(u64 vaddr)
>  }
>  #endif
>  
> -bool probe_kernel_read_allowed(void *dst, const void *unsafe_src, size_t size,
> -		bool strict)
> +bool probe_kernel_read_allowed(void *dst, const void *unsafe_src, size_t size)
>  {
> -	if (!strict)
> -		return true;
>  	return !invalid_probe_range((unsigned long)unsafe_src);
>  }
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 09d6e358883cc..99e2c2a41164a 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -301,11 +301,9 @@ copy_struct_from_user(void *dst, size_t ksize, const void __user *src,
>  	return 0;
>  }
>  
> -bool probe_kernel_read_allowed(void *dst, const void *unsafe_src,
> -		size_t size, bool strict);
> +bool probe_kernel_read_allowed(void *dst, const void *unsafe_src, size_t size);
>  
>  extern long probe_kernel_read(void *dst, const void *src, size_t size);
> -extern long probe_kernel_read_strict(void *dst, const void *src, size_t size);
>  extern long probe_user_read(void *dst, const void __user *src, size_t size);
>  
>  extern long notrace probe_kernel_write(void *dst, const void *src, size_t size);
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index ffe841433caa1..f694befe8ec9b 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -183,12 +183,20 @@ bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr,
>  	int ret = security_locked_down(LOCKDOWN_BPF_READ);
>  
>  	if (unlikely(ret < 0))
> -		goto out;
> -	ret = compat ? probe_kernel_read(dst, unsafe_ptr, size) :
> -	      probe_kernel_read_strict(dst, unsafe_ptr, size);
> -	if (unlikely(ret < 0))
> -out:
> -		memset(dst, 0, size);
> +		goto fail;
> +
> +	ret = probe_kernel_read(dst, unsafe_ptr, size);
> +	if (unlikely(ret < 0)) {
> +		if (compat)
> +			ret = probe_user_read(dst,
> +				(__force const void __user *)unsafe_ptr, size);
> +		if (unlikely(ret < 0))
> +			goto fail;
> +	}
> +
> +	return 0;
> +fail:
> +	memset(dst, 0, size);
>  	return ret;
>  }
>  
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 525d12137325c..1300c9fd5c755 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -1203,6 +1203,9 @@ fetch_store_strlen(unsigned long addr)
>  
>  	do {
>  		ret = probe_kernel_read(&c, (u8 *)addr + len, 1);
> +		if (ret)
> +			ret = probe_user_read(&c,
> +				(__force u8 __user *)addr + len, 1);
>  		len++;
>  	} while (c && ret == 0 && len < MAX_STRING_SIZE);
>  
> @@ -1275,7 +1278,13 @@ fetch_store_string_user(unsigned long addr, void *dest, void *base)
>  static nokprobe_inline int
>  probe_mem_read(void *dest, void *src, size_t size)
>  {
> -	return probe_kernel_read(dest, src, size);
> +	int ret;
> +
> +	ret = probe_kernel_read(dest, src, size);
> +	if (ret)
> +		ret = probe_user_read(dest, (__force const void __user *)src,
> +				size);
> +	return ret;
>  }
>  
>  static nokprobe_inline int
> diff --git a/mm/maccess.c b/mm/maccess.c
> index cbd9d668aa46e..811f49e8de113 100644
> --- a/mm/maccess.c
> +++ b/mm/maccess.c
> @@ -6,36 +6,14 @@
>  #include <linux/mm.h>
>  #include <linux/uaccess.h>
>  
> -static long __probe_kernel_read(void *dst, const void *src, size_t size,
> -		bool strict);
> -
>  bool __weak probe_kernel_read_allowed(void *dst, const void *unsafe_src,
> -		size_t size, bool strict)
> +		size_t size)
>  {
>  	return true;
>  }
>  
>  /**
> - * probe_kernel_read(): safely attempt to read from any location
> - * @dst: pointer to the buffer that shall take the data
> - * @src: address to read from
> - * @size: size of the data chunk
> - *
> - * Same as probe_kernel_read_strict() except that for architectures with
> - * not fully separated user and kernel address spaces this function also works
> - * for user address tanges.
> - *
> - * DO NOT USE THIS FUNCTION - it is broken on architectures with entirely
> - * separate kernel and user address spaces, and also a bad idea otherwise.
> - */
> -long probe_kernel_read(void *dst, const void *src, size_t size)
> -{
> -	return __probe_kernel_read(dst, src, size, false);
> -}
> -EXPORT_SYMBOL_GPL(probe_kernel_read);
> -
> -/**
> - * probe_kernel_read_strict(): safely attempt to read from kernel-space
> + * probe_kernel_read(): safely attempt to read from kernel-space
>   * @dst: pointer to the buffer that shall take the data
>   * @src: address to read from
>   * @size: size of the data chunk
> @@ -48,18 +26,12 @@ EXPORT_SYMBOL_GPL(probe_kernel_read);
>   * probe_kernel_read() suitable for use within regions where the caller
>   * already holds mmap_sem, or other locks which nest inside mmap_sem.
>   */
> -long probe_kernel_read_strict(void *dst, const void *src, size_t size)
> -{
> -	return __probe_kernel_read(dst, src, size, true);
> -}
> -
> -static long __probe_kernel_read(void *dst, const void *src, size_t size,
> -		bool strict)
> +long probe_kernel_read(void *dst, const void *src, size_t size)
>  {
>  	long ret;
>  	mm_segment_t old_fs = get_fs();
>  
> -	if (!probe_kernel_read_allowed(dst, src, size, strict))
> +	if (!probe_kernel_read_allowed(dst, src, size))
>  		return -EFAULT;
>  
>  	set_fs(KERNEL_DS);
> @@ -73,6 +45,7 @@ static long __probe_kernel_read(void *dst, const void *src, size_t size,
>  		return -EFAULT;
>  	return 0;
>  }
> +EXPORT_SYMBOL_GPL(probe_kernel_read);
>  
>  /**
>   * probe_user_read(): safely attempt to read from a user-space location
> @@ -180,7 +153,7 @@ long strncpy_from_kernel_unsafe(char *dst, const void *unsafe_addr, long count)
>  
>  	if (unlikely(count <= 0))
>  		return 0;
> -	if (!probe_kernel_read_allowed(dst, unsafe_addr, count, true))
> +	if (!probe_kernel_read_allowed(dst, unsafe_addr, count))
>  		return -EFAULT;
>  
>  	set_fs(KERNEL_DS);
> -- 
> 2.26.2
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>