Re: [PATCH] mm: fix potential pte_unmap_unlock pte error

[Shijie Luo]

On 2020/10/15 20:58, osalvador@xxxxxxx wrote:
> On 2020-10-15 14:15, Shijie Luo wrote:
> > When flags don't have MPOL_MF_MOVE or MPOL_MF_MOVE_ALL bits, code breaks
> >  and passing origin pte - 1 to pte_unmap_unlock seems like not a good 
> > idea.
> >
> > Signed-off-by: Shijie Luo <luoshijie1@xxxxxxxxxx>
> > Signed-off-by: linmiaohe <linmiaohe@xxxxxxxxxx>
> > ---
> >  mm/mempolicy.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 3fde772ef5ef..01f088630d1d 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -571,7 +571,11 @@ static int queue_pages_pte_range(pmd_t *pmd,
> > unsigned long addr,
> >          } else
> >              break;
> >      }
> > -    pte_unmap_unlock(pte - 1, ptl);
> > +
> > +    if (addr >= end)
> > +        pte = pte - 1;
> > +
> > +    pte_unmap_unlock(pte, ptl);
>
> But this is still wrong, isn't it?
> Unless I am missing something, this is "only" important under 
> CONFIG_HIGHPTE.
>
> We have:
>
> pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
>
> which under CONFIG_HIGHPTE does a kmap_atomoc.
>
> Now, we either break the loop in the first pass because of 
> !(MPOL_MF_MOVE | MPOL_MF_MOVE_ALL),
> or we keep incrementing pte by every pass.
> Either way is wrong, because the pointer kunmap_atomic gets will not 
> be the same (since we incremented pte).
>
> Or is the loop meant to be running only once, so pte - 1 will bring us 
> back to the original pte?
>
> .

Thanks for your reply, if we break the loop in the first pass, the pte 
pointer will not be incremented,

pte - 1 equals original pte - 1,  because we only increase pte pointer 
when not break the loop.