Subject: Re: [PATCH] asm/sections: Check for overflow in memory_contains()

From: Russell King - ARM Linux admin <[email-masked]>
Date: Tue, 17 Dec 2019 10:28:31 +0000

On Tue, Dec 17, 2019 at 11:22:38AM +0100, Vincent Whitchurch wrote:
> ARM uses memory_contains() from its stacktrace code via this function:
> 
>  static inline bool in_entry_text(unsigned long addr)
>  {
>  	return memory_contains(__entry_text_start, __entry_text_end,
>  			       (void *)addr, 1);
>  }
> 
> addr is taken from the stack and can be a completely invalid.  If addr
> is 0xffffffff, there is an overflow in the pointer arithmetic in
> memory_contains() and in_entry_text() incorrectly returns true.
> 
> Fix this by adding an overflow check.  The check is done on unsigned
> longs to avoid undefined behaviour.
> 
> Signed-off-by: Vincent Whitchurch <[email-masked]>
> ---
>  include/asm-generic/sections.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
> index d1779d442aa5..e6e1b381c5df 100644
> --- a/include/asm-generic/sections.h
> +++ b/include/asm-generic/sections.h
> @@ -105,7 +105,15 @@ static inline int arch_is_kernel_initmem_freed(unsigned long addr)
>  static inline bool memory_contains(void *begin, void *end, void *virt,
>  				   size_t size)
>  {
> -	return virt >= begin && virt + size <= end;
> +	unsigned long membegin = (unsigned long)begin;
> +	unsigned long memend = (unsigned long)end;
> +	unsigned long objbegin = (unsigned long)virt;
> +	unsigned long objend = objbegin + size;
> +
> +	if (objend < objbegin)
> +		return false;
> +
> +	return objbegin >= membegin && objend <= memend;

Would merely changing to:

	return virt >= begin && virt <= end - size;

be sufficient ?  Is end - size possible to underflow?

>  }
>  
>  /**
> -- 
> 2.20.0
> 
> 
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> 

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