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

[Naresh Kamboju <naresh.kamboju@linaro.org>]

On Thu, 16 Feb 2023 at 07:06, Mike Kravetz <mike.kravetz@oracle.com> 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.

This proposed fix tested and confirmed that reported issues were fixed.

>
> Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> Link: https://lore.kernel.org/lkml/CA+G9fYuei_Tr-vN9GS7SfFyU1y9hNysnf=PB7kT0=yv4MiPgVg@mail.gmail.com/
> Fixes: 42d7395feb56 ("mm: support more pagesizes for MAP_HUGETLB/SHM_HUGETLB")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>

> ---
>  include/linux/hugetlb.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index df6dd624ccfe..8b45720f9475 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -781,7 +781,10 @@ static inline struct hstate *hstate_sizelog(int page_size_log)
>         if (!page_size_log)
>                 return &default_hstate;
>
> -       return size_to_hstate(1UL << page_size_log);
> +       if (page_size_log < BITS_PER_LONG)
> +               return size_to_hstate(1UL << page_size_log);
> +
> +       return NULL;
>  }
>
>  static inline struct hstate *hstate_vma(struct vm_area_struct *vma)
> --
> 2.39.1


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