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SubjectRe: [PATCH] hugetlb: check for undefined shift on 32 bit architectures
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> On Feb 16, 2023, at 09:35, 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.
>
> 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>

Acked-by: Muchun Song <songmuchun@bytedance.com>

Thanks.

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