[PATCH] arm64: mm: fix pmd_leaf()

[Muchun Song]

On Mon, Apr 4, 2022 at 5:20 PM Will Deacon <will at kernel.org> wrote:
>
> On Sun, Apr 03, 2022 at 10:49:28AM +0800, Muchun Song wrote:
> > The pmd_leaf() is used to test a leaf mapped PMD, however, it misses
> > the PROT_NONE mapped PMD on arm64.  Fix it.  A real world issue [1]
> > caused by this was reported by Qian Cai.
> >
> > Link: https://patchwork.kernel.org/comment/24798260/ [1]
> > Fixes: 8aa82df3c123 ("arm64: mm: add p?d_leaf() definitions")
> > Reported-by: Qian Cai <quic_qiancai at quicinc.com>
> > Signed-off-by: Muchun Song <songmuchun at bytedance.com>
> > ---
> >  arch/arm64/include/asm/pgtable.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index 94e147e5456c..09eaae46a19b 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -535,7 +535,7 @@ extern pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
> >                                PMD_TYPE_TABLE)
> >  #define pmd_sect(pmd)                ((pmd_val(pmd) & PMD_TYPE_MASK) == \
> >                                PMD_TYPE_SECT)
> > -#define pmd_leaf(pmd)                pmd_sect(pmd)
> > +#define pmd_leaf(pmd)                (pmd_present(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT))
> >  #define pmd_bad(pmd)         (!pmd_table(pmd))
> >
> >  #define pmd_leaf_size(pmd)   (pmd_cont(pmd) ? CONT_PMD_SIZE : PMD_SIZE)
>
> A bunch of the users of pmd_leaf() already check pmd_present() -- is it
> documented that we need to handle this check inside the macro? afaict x86
> doesn't do this either.
>

arm64 is different from x86 here. pmd_leaf() could return true for
the none pmd without pmd_present() check, the check of
pmd_present() aims to exclude the pmd_none() case.  However,
it could work properly on x86 without pmd_present() or pmd_none().
So we don't see pmd_present() or pmd_none() check in pmd_leaf().
For this reason, I think this check is necessary.

BTW, there are some users of pmd_leaf() (e.g. apply_to_pmd_range,
walk_pmd_range, ptdump_pmd_entry) which do not check pmd_present()
or pmd_none() before the call of pmd_leaf().  So it is also necessary
to add the check.

Thanks.