Re: [win-pv-devel] [PATCH for 8.1] Fix hash table overflow

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx [mailto:win-pv-devel-
> bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf Of Paul Durrant
> Sent: 09 September 2015 13:46
> To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Paul Durrant
> Subject: [win-pv-devel] [PATCH for 8.1] Fix hash table overflow
> 
> There is a flaw in HashTableHash() which means that, for example, an Array
> value of 0xff added to an Accumulator value of 0xff will lead to more than
> 4 bits of Overflow. The 5th bit is missed by the mask and is hence not
> folded back into the lower order bits of the Accumulator. The upshot of the
> this is an ASSERTion failure for a debug build or an array overflow in the
> caller for a non-debug build.
> This patch fixes this issue by increasing the overflow mask to 8 bits
> instead of 4 (although 5 bit would actually be sufficient).
> 
> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>

Sorry, that should also be:

Reported-by: Rafał Wojdyła <omeg@xxxxxxxxxxxxxxxxxxxxxx>

> ---
>  src/xenbus/hash_table.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/xenbus/hash_table.c b/src/xenbus/hash_table.c
> index a9c1b79..c7c6101 100644
> --- a/src/xenbus/hash_table.c
> +++ b/src/xenbus/hash_table.c
> @@ -90,7 +90,7 @@ HashTableHash(
> 
>          Accumulator = (Accumulator << 4) + Array[Index];
> 
> -        Overflow = Accumulator & 0x00000f00;
> +        Overflow = Accumulator & 0x0000ff00;
>          if (Overflow != 0) {
>              Accumulator ^= Overflow >> 8;
>              Accumulator ^= Overflow;
> --
> 2.1.1
> 
> 
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