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

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Message-Id: <9B09A3E9-09E6-44E4-9B4F-30975C6947B7@linux.dev>
Date:   Thu, 16 Feb 2023 11:01:13 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Linux Memory Management List <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Sasha Levin <sashal@...nel.org>,
        Anders Roxell <anders.roxell@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] hugetlb: check for undefined shift on 32 bit
 architectures



> On Feb 16, 2023, at 09:35, Mike Kravetz <mike.kravetz@...cle.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@...aro.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@...r.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>

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

Thanks.