Re: [Xen-devel] bug: unable to LZ4 decompress ub1910 installer kernel when launching domU

[Jeremi Piotrowski <jeremi.piotrowski@xxxxxxxxx>]

On Wed, Dec 04, 2019 at 02:27:26PM +0100, Jan Beulich wrote:
> On 04.12.2019 08:14, Jeremi Piotrowski wrote:
> > Any suggestions how to proceed?
> 
> Actually here's a better version (I think).
> 
> Jan
> 
> lz4: refine commit 9143a6c55ef7 for the 64-bit case
> 
> I clearly went too far there: While the LZ4_WILDCOPY() instances indeed
> need prior guarding, LZ4_SECURECOPY() needs this only in the 32-bit case
> (where it simply aliases LZ4_WILDCOPY()). "cpy" can validly point
> (slightly) below "op" in these cases, due to
> 
>               cpy = op + length - (STEPSIZE - 4);
> 
> where length can be as low as 0 and STEPSIZE is 8. However, instead of
> removing the check via "#if !LZ4_ARCH64", refine it such that it would
> also properly work in the 64-bit case, aborting decompression instead
> of continuing on bogus input.
> 
> Reported-by: Mark Pryor <pryorm09@xxxxxxxxx>
> Reported-by: Jeremi Piotrowski <jeremi.piotrowski@xxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- unstable.orig/xen/common/lz4/decompress.c
> +++ unstable/xen/common/lz4/decompress.c
> @@ -147,7 +147,7 @@ static int INIT lz4_uncompress(const uns
>                               goto _output_error;
>                       continue;
>               }
> -             if (unlikely((unsigned long)cpy < (unsigned long)op))
> +             if (unlikely((unsigned long)cpy < (unsigned long)op - (STEPSIZE 
> - 4)))
>                       goto _output_error;
>               LZ4_SECURECOPY(ref, op, cpy);
>               op = cpy; /* correction */
> @@ -279,7 +279,7 @@ static int lz4_uncompress_unknownoutputs
>                               goto _output_error;
>                       continue;
>               }
> -             if (unlikely((unsigned long)cpy < (unsigned long)op))
> +             if (unlikely((unsigned long)cpy < (unsigned long)op - (STEPSIZE 
> - 4)))
>                       goto _output_error;
>               LZ4_SECURECOPY(ref, op, cpy);
>               op = cpy; /* correction */
> 

Thanks Jan, this works. I tested it with direct kernel boot. Like Pry
wrote pvgrub2 is another story, seems there is no support for lz4
compressed kernels in it , and ubuntu patches in support to the grub that
they ship.

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