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@xxxxxxxxxx>
Reviewed-by: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
> ---
> 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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