Re: [PATCH 16/23] pefile: Parse a PE binary to find a key and asignature contained therein

[Kees Cook]

On Tue, Oct 30, 2012 at 5:59 PM, David Howells <dhowells@xxxxxxxxxx> wrote:
> Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
>> This multiplication can push the cursor out of bounds. (n_data_dirents
>> is unverified).
>> ...
>> Both of these cases of n_sections multiplications can wrap.
>> Ultimately, you can end up with cursor close to zero, but n_sections
>> being giant.
>
> Good points.  I wonder if I should limit these to some low number, or just
> check that they don't exceed header_size, which also needs checking as you
> said.

I think header_size can be bounded by datalen? I didn't investigate
that one very deeply. For the multiplications, I think you can just do
a check before chkaddr:

ctx->n_sections = pe->sections;
if (datalen / ctx->n_sections > sizeof(*sec))
    return -ELIBBAD;
chkaddr(0, cursor, sizeof(*sec) * ctx->n_sections);

Or extend chkaddr to do the check for you:

chkaddr(0, cursor, sizeof(*sec), ctx->n_sections);

with other callers just using "1" for the final argument.

Either way, you won't have to choose arbitrary limits.

-Kees

-- 
Kees Cook
Chrome OS Security
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