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

[Pry Mar <pryorm09@xxxxxxxxx>]

On 12/4/19, Jan Beulich <jbeulich@xxxxxxxx> 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 */
>
>
This patch worked building xen-4.12.1 in Buster with python3.

I can now boot Focal kernel-5.3 which is LZ4 compressed. The domU can
boot as pv, pvh, using direct kernel (kernel/ramdisk), or pygrub.

I looked into booting as pvgrub2, thinking the code is universal, but
no. Thats why I CC'd Juergen. I wanted to do a kernel direct boot with
scripted pvgrub2 kernel.

I expect that once I build stubdom including this patch, then pvgrub
will work too.

Thanks for the attention to this bug, now solved.
PryMar56

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