[U-Boot] [PATCH 3/6] gzip: correctly bounds-check output buffer

Kees Cook
2013-11-08T15:21:28Z

On Fri, Nov 8, 2013 at 4:04 AM, Michal Simek <monstr@monstr.eu> wrote:
> Hi Kees,
>
> On 08/16/2013 04:59 PM, Kees Cook wrote:
>> The output buffer size must not be reset by the gzip decoder or there
>> is a risk of overflowing memory during decompression.
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> Acked-by: Simon Glass <sjg@chromium.org>
>> ---
>> lib/gunzip.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/lib/gunzip.c b/lib/gunzip.c
>> index 9959781..35abfb3 100644
>> --- a/lib/gunzip.c
>> +++ b/lib/gunzip.c
>> @@ -89,13 +89,13 @@ int zunzip(void *dst, int dstlen, unsigned char *src, unsigned long *lenp,
>> s.avail_out = dstlen;
>> do {
>> r = inflate(&s, Z_FINISH);
>> - if (r != Z_STREAM_END && r != Z_BUF_ERROR && stoponerr == 1) {
>> + if (stoponerr == 1 && r != Z_STREAM_END &&
>> + (s.avail_out == 0 || r != Z_BUF_ERROR)) {
>> printf("Error: inflate() returned %d\n", r);
>> inflateEnd(&s);
>> return -1;
>> }
>> s.avail_in = *lenp - offset - (int)(s.next_out - (unsigned char*)dst);
>> - s.avail_out = dstlen;
>> } while (r == Z_BUF_ERROR);
>> *lenp = s.next_out - (unsigned char *) dst;
>> inflateEnd(&s);
>>
>
> I have done u-boot upgrade to v2013.10 version and I see the problem with this patch
> when I am trying to boot my zynq image.
>
> After reverting this patch everything works as expected.

Eek, sorry this is causing you trouble!

> Here is the image I am using.
> http://www.monstr.eu/20131108-image.ub

Is there any way you can extract just the gzipped kernel from this
image? I'm not sure how to get at it from this .ub file.

> Below is the bootlog.
>
> Do you have any idea what can be wrong?
> [...]
> Uncompressing Kernel Image ... Error: inflate() returned -5
> GUNZIP: uncompress, out-of-mem or overwrite error - must RESET board to recover
> resetting ...

Either my change is failing to detect end-of-buffer correctly, or it
_is_, in which case this has uncovered an unsafe caller of gunzip.
This is after the "Uncompressing" message, so it's this caller:

case IH_COMP_GZIP:
printf(" Uncompressing %s ... ", type_name);
if (gunzip(load_buf, unc_len, image_buf, &image_len) != 0) {
puts("GUNZIP: uncompress, out-of-mem or overwrite "
"error - must RESET board to recover\n");
if (boot_progress)
bootstage_error(BOOTSTAGE_ID_DECOMP_IMAGE);
return BOOTM_ERR_RESET;
}

*load_end = load + image_len;
break;

If the uncompressed length of the kernel image is larger than
"unc_len", then this is catching a legitimate memory overflow. This is
entirely controlled by CONFIG_SYS_BOOTM_LEN. Is it possible this is
set too low for your build?

-Kees

--
Kees Cook
Chrome OS Security