Re: [RFC PATCH 1/5] mm: mremap: Fix new_addr being used as a hint with MREMAP_DONTUNMAP

[Marco Vanotti <mvanotti@google.com>]

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On Tue, Dec 10, 2024 at 6:31 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.
>
> This patch only fixes the behavior that was inadvertently added
> with MREMAP_DONTUNMAP.
>
> v2:
>   - Addressed comment from Marco Vanotti to consolidate these checks
>     into existing MREMAP_FIXED blocks.
>
> Signed-off-by: Brian Geffon <bgeffon@google.com>
> Reported-by: Marco Vanotti <mvanotti@google.com>
Reviewed-By: Marco Vanotti <mvanotti@google.com>
> ---
>  mm/mremap.c | 29 +++++++++++++++++++----------
>  1 file changed, 19 insertions(+), 10 deletions(-)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 60473413836b..62aec72bbe42 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -912,16 +912,6 @@ 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;
> -
> -       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;
> -
>         /*
>          * move_vma() need us to stay 4 maps below the threshold, otherwise
>          * it will bail out at the very beginning.
> @@ -940,6 +930,25 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
>                 return -ENOMEM;
>
>         if (flags & MREMAP_FIXED) {
> +               /*
> +                * 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.
> +                */
> +               if (offset_in_page(new_addr))
> +                       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;
>                 /*
>                  * In mremap_to().
>                  * VMA is moved to dst address, and munmap dst first.
> --
> 2.47.0.338.g60cca15819-goog
>

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