Re: [PATCH v3 1/2] libfdt: prevent integer overflow in fdt_next_tag

[David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>]

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On Wed, Oct 05, 2022 at 04:29:30PM -0700, Tadeusz Struk wrote:
> Since fdt_next_tag() in a public API function all input parameters,
> including the fdt blob should not be trusted. It is possible to forge
> a blob with invalid property length that will cause integer overflow
> during offset calculation. To prevent that, validate the property length
> read from the blob before doing calculations.
> 
> Signed-off-by: Tadeusz Struk <tadeusz.struk-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>

Reviewed-by: David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>

> ---
> v2:
> * Use len local variable to avoid multiple calls to fdt32_to_cpu(*lenp)
> * Add can_assume(VALID_DTB) to the new checks
> v3:
> * Use unsigned integer for prop len and offset validation
> ---
>  libfdt/fdt.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/libfdt/fdt.c b/libfdt/fdt.c
> index 90a39e8..20c6415 100644
> --- a/libfdt/fdt.c
> +++ b/libfdt/fdt.c
> @@ -162,7 +162,7 @@ const void *fdt_offset_ptr(const void *fdt, int offset, unsigned int len)
>  uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
>  {
>  	const fdt32_t *tagp, *lenp;
> -	uint32_t tag;
> +	uint32_t tag, len, sum;
>  	int offset = startoffset;
>  	const char *p;
>  
> @@ -188,12 +188,19 @@ uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
>  		lenp = fdt_offset_ptr(fdt, offset, sizeof(*lenp));
>  		if (!can_assume(VALID_DTB) && !lenp)
>  			return FDT_END; /* premature end */
> +
> +		len = fdt32_to_cpu(*lenp);
> +		sum = len + offset;
> +		if (!can_assume(VALID_DTB) &&
> +		    (INT_MAX <= sum || sum < (uint32_t) offset))
> +			return FDT_END; /* premature end */
> +
>  		/* skip-name offset, length and value */
> -		offset += sizeof(struct fdt_property) - FDT_TAGSIZE
> -			+ fdt32_to_cpu(*lenp);
> +		offset += sizeof(struct fdt_property) - FDT_TAGSIZE + len;
> +
>  		if (!can_assume(LATEST) &&
> -		    fdt_version(fdt) < 0x10 && fdt32_to_cpu(*lenp) >= 8 &&
> -		    ((offset - fdt32_to_cpu(*lenp)) % 8) != 0)
> +		    fdt_version(fdt) < 0x10 && len >= 8 &&
> +		    ((offset - len) % 8) != 0)
>  			offset += 4;
>  		break;
>  

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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