Re: [PATCH] arm64: mm: fix pmd_leaf()

[Aneesh Kumar K V]

On 4/4/22 5:10 PM, Muchun Song wrote:
> On Mon, Apr 4, 2022 at 5:20 PM Will Deacon <will@xxxxxxxxxx> 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@xxxxxxxxxxx>
> > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > > ---
> > >   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.


ppc64 also doesn't do a pmd_present check.

> >
>
> 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.


I would expect pmd_leaf check to return true, if the said pmd page table 
entry can point to a leaf page table entry which can also be a not 
present page table entry?

-aneesh