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

[Jan Beulich <jbeulich@xxxxxxxx>]

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 */


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