Re: [PATCH] arm64: hugetlb: avoid potential NULL dereference

Itaru Kitayama
2020-05-06T02:00:21Z

Hi Mark,
I looked in the mm and arch/arm64 directories and all the calls to
pmd_alloc() are tested
whether it returns a NULL or not, so your patch just fixes the hugetlb
code to follow the standard.

On Tue, May 5, 2020 at 11:31 PM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Tue, May 05, 2020 at 10:12:02PM +0900, Itaru Kitayama wrote:
> > The overhead is just negligible in the paths?
>
> Sorry, I'm not sure I understand your question. Are you asking if this
> is likely to affect performance?
>
> I don't expect there to be any measureable overhead here. Practically
> speaking the difference is a CBZ + RET, and other factors will dominate
> here.
>
> Regardless, this is trivial error handling that was missing from the
> original patch.
>
> Thanks,
> Mark.
>
> > On Tue, May 5, 2020 at 21:59 Mark Rutland <mark.rutland@arm.com> wrote:
> >
> > > The static analyzer in GCC 10 spotted that in huge_pte_alloc() we may
> > > pass a NULL pmdp into pte_alloc_map() when pmd_alloc() returns NULL:
> > >
> > > | CC arch/arm64/mm/pageattr.o
> > > | CC arch/arm64/mm/hugetlbpage.o
> > > | from arch/arm64/mm/hugetlbpage.c:10:
> > > | arch/arm64/mm/hugetlbpage.c: In function ‘huge_pte_alloc’:
> > > | ./arch/arm64/include/asm/pgtable-types.h:28:24: warning: dereference of
> > > NULL ‘pmdp’ [CWE-690] [-Wanalyzer-null-dereference]
> > > | ./arch/arm64/include/asm/pgtable.h:436:26: note: in expansion of macro
> > > ‘pmd_val’
> > > | arch/arm64/mm/hugetlbpage.c:242:10: note: in expansion of macro
> > > ‘pte_alloc_map’
> > > | |arch/arm64/mm/hugetlbpage.c:232:10:
> > > | |./arch/arm64/include/asm/pgtable-types.h:28:24:
> > > | ./arch/arm64/include/asm/pgtable.h:436:26: note: in expansion of macro
> > > ‘pmd_val’
> > > | arch/arm64/mm/hugetlbpage.c:242:10: note: in expansion of macro
> > > ‘pte_alloc_map’
> > >
> > > This can only occur when the kernel cannot allocate a page, and so is
> > > unlikely to happen in practice before other systems start failing.
> > >
> > > We can avoid this by bailing out if pmd_alloc() fails, as we do earlier
> > > in the function if pud_alloc() fails.
> > >
> > > Fixes: 66b3923a1a0f77a5 ("arm64: hugetlb: add support for PTE contiguous
> > > bit)"
> > > Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> > > Reported-by: Kyrill Tkachov <kyrylo.tkachov@arm.com>
> > > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > > Cc: Will Deacon <will@kernel.org>
> > > ---
> > > arch/arm64/mm/hugetlbpage.c | 2 ++
> > > 1 file changed, 2 insertions(+)
> > >
> > > diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> > > index bbeb6a5a6ba6..0be3355e3499 100644
> > > --- a/arch/arm64/mm/hugetlbpage.c
> > > +++ b/arch/arm64/mm/hugetlbpage.c
> > > @@ -230,6 +230,8 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
> > > ptep = (pte_t *)pudp;
> > > } else if (sz == (CONT_PTE_SIZE)) {
> > > pmdp = pmd_alloc(mm, pudp, addr);
> > > + if (!pmdp)
> > > + return NULL;
> > >
> > > WARN_ON(addr & (sz - 1));
> > > /*
> > > --
> > > 2.11.0
> > >
> > >
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> > >