Re: [PATCH] hugetlb: check for undefined shift on 32 bit architectures

[Jesper Juhl]

On Wed, 15 Feb 2023, Mike Kravetz wrote:

> Users can specify the hugetlb page size in the mmap, shmget and
> memfd_create system calls.  This is done by using 6 bits within the
> flags argument to encode the base-2 logarithm of the desired page size.
> The routine hstate_sizelog() uses the log2 value to find the
> corresponding hugetlb hstate structure.  Converting the log2 value
> (page_size_log) to potential hugetlb page size is the simple statement:
>
> 	1UL << page_size_log
>
> Because only 6 bits are used for page_size_log, the left shift can not
> be greater than 63.  This is fine on 64 bit architectures where a long
> is 64 bits.  However, if a value greater than 31 is passed on a 32 bit
> architecture (where long is 32 bits) the shift will result in undefined
> behavior.  This was generally not an issue as the result of the
> undefined shift had to exactly match hugetlb page size to proceed.
>
> Recent improvements in runtime checking have resulted in this undefined
> behavior throwing errors such as reported below.
>
> Fix by comparing page_size_log to BITS_PER_LONG before doing shift.
>
> Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/CA+G9fYuei_Tr-vN9GS7SfFyU1y9hNysnf=PB7kT0=yv4MiPgVg@xxxxxxxxxxxxxx/
> Fixes: 42d7395feb56 ("mm: support more pagesizes for MAP_HUGETLB/SHM_HUGETLB")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

Looks good to me.

Reviewed-by: Jesper Juhl <jesperjuhl76@xxxxxxxxx>