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

[Marco Vanotti <mvanotti@google.com>]

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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?
> +       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
>

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