Re: [PATCH] staging: android: ashmem: Disallow ashmem memory from being remapped

[Joel Fernandes]

On Mon, Jan 27, 2020 at 1:00 PM 'Todd Kjos' via kernel-team
<kernel-team@xxxxxxxxxxx> wrote:
>
> From: Suren Baghdasaryan <surenb@xxxxxxxxxx>
>
> When ashmem file is being mmapped the resulting vma->vm_file points to the
> backing shmem file with the generic fops that do not check ashmem
> permissions like fops of ashmem do. Fix that by disallowing mapping
> operation for backing shmem file.

Looks good, but I think the commit message is confusing. I had to read
the code a couple times to understand what's going on since there are
no links to a PoC for the security issue, in the commit message. I
think a better message could have been:

 When ashmem file is mmapped, the resulting vma->vm_file points to the
 backing shmem file with the generic fops that do not check ashmem
 permissions like fops of ashmem do. If an mremap is done on the ashmem
 region, then the permission checks will be skipped. Fix that by disallowing
 mapping operation on the backing shmem file.

Or did I miss something?

thanks!

- Joel



>
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: stable <stable@xxxxxxxxxxxxxxx> # 4.4,4.9,4.14,4.18,5.4
> Signed-off-by: Todd Kjos <tkjos@xxxxxxxxxx>
> ---
>  drivers/staging/android/ashmem.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
>
> diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
> index 74d497d39c5a..c6695354b123 100644
> --- a/drivers/staging/android/ashmem.c
> +++ b/drivers/staging/android/ashmem.c
> @@ -351,8 +351,23 @@ static inline vm_flags_t calc_vm_may_flags(unsigned long prot)
>                _calc_vm_trans(prot, PROT_EXEC,  VM_MAYEXEC);
>  }
>
> +static int ashmem_vmfile_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +       /* do not allow to mmap ashmem backing shmem file directly */
> +       return -EPERM;
> +}
> +
> +static unsigned long
> +ashmem_vmfile_get_unmapped_area(struct file *file, unsigned long addr,
> +                               unsigned long len, unsigned long pgoff,
> +                               unsigned long flags)
> +{
> +       return current->mm->get_unmapped_area(file, addr, len, pgoff, flags);
> +}
> +
>  static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
>  {
> +       static struct file_operations vmfile_fops;
>         struct ashmem_area *asma = file->private_data;
>         int ret = 0;
>
> @@ -393,6 +408,19 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
>                 }
>                 vmfile->f_mode |= FMODE_LSEEK;
>                 asma->file = vmfile;
> +               /*
> +                * override mmap operation of the vmfile so that it can't be
> +                * remapped which would lead to creation of a new vma with no
> +                * asma permission checks. Have to override get_unmapped_area
> +                * as well to prevent VM_BUG_ON check for f_ops modification.
> +                */
> +               if (!vmfile_fops.mmap) {
> +                       vmfile_fops = *vmfile->f_op;
> +                       vmfile_fops.mmap = ashmem_vmfile_mmap;
> +                       vmfile_fops.get_unmapped_area =
> +                                       ashmem_vmfile_get_unmapped_area;
> +               }
> +               vmfile->f_op = &vmfile_fops;
>         }
>         get_file(asma->file);
>
> --
> 2.25.0.341.g760bfbb309-goog
>
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