Re: [PATCH 1/2] mremap: Fix new_addr being used as a hint with MREMAP_DONTUNMAP

[Brian Geffon <bgeffon@google.com>]

On Fri, Dec 6, 2024 at 10:35 AM Marco Vanotti <mvanotti@google.com> wrote:
>
> On Fri, Dec 6, 2024 at 12:20 PM Brian Geffon <bgeffon@google.com> wrote:
> >
> > Two non-mutually exclusive paths can land in mremap_to, MREMAP_FIXED
> > and MREMAP_DONTUNMAP which are called from mremap(). In the case of
> > MREMAP_FIXED we must validate the new_addr to ensure that the new
> > address is valid. In the case of MREMAP_DONTUNMAP without MREMAP_FIXED
> > a new address is specified as a hint, just like it would be in the
> > case of mmap. In this second case we don't need to perform any checks
> > because get_unmapped_area() will align new_addr, just like it would in
> > the case of mmap.
> >
> > Signed-off-by: Brian Geffon <bgeffon@google.com>
> > Reported-by: Marco Vanotti <mvanotti@google.com>
> > ---
> >  mm/mremap.c | 26 +++++++++++++++++++-------
> >  1 file changed, 19 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index 60473413836b..286ffdb883df 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -912,15 +912,27 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
> >         unsigned long ret;
> >         unsigned long map_flags = 0;
> >
> > -       if (offset_in_page(new_addr))
> > -               return -EINVAL;
> > +       /*
> > +        * Two non-mutually exclusive paths can land in mremap_to, MREMAP_FIXED
> > +        * and MREMAP_DONTUNMAP which are called from mremap(). In the case of
> > +        * MREMAP_FIXED we must validate the new_addr to ensure that the new
> > +        * address is valid. In the case of MREMAP_DONTUNMAP without MREMAP_FIXED
> > +        * a new address is specified as a hint, just like it would be in the
> > +        * case of mmap. In this second case we don't need to perform any checks
> > +        * because get_unmapped_area() will align new_addr, just like it would in
> > +        * the case of mmap.
> > +        */
> A few lines below we also check for MREMAP_FIXED before calling
> do_unmap, can't we do the validation there?

I don't have a strong preference either way. I can mail a v2 with this
suggestion.

> > +       if (flags & MREMAP_FIXED) {
> > +               if (offset_in_page(new_addr))
> > +                       return -EINVAL;
> >
> > -       if (new_len > TASK_SIZE || new_addr > TASK_SIZE - new_len)
> > -               return -EINVAL;
> > +               if (new_len > TASK_SIZE || new_addr > TASK_SIZE - new_len)
> > +                       return -EINVAL;
> >
> > -       /* Ensure the old/new locations do not overlap */
> > -       if (addr + old_len > new_addr && new_addr + new_len > addr)
> > -               return -EINVAL;
> > +               /* Ensure the old/new locations do not overlap */
> > +               if (addr + old_len > new_addr && new_addr + new_len > addr)
> > +                       return -EINVAL;
> > +       }
> >
> >         /*
> >          * move_vma() need us to stay 4 maps below the threshold, otherwise
> > --
> > 2.47.0.338.g60cca15819-goog
> >