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

[Todd Kjos]

On Mon, Jan 27, 2020 at 2:30 PM Joel Fernandes <joelaf@xxxxxxxxxx> wrote:
>
> 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.

Sent v2 with the suggested change.

>
> 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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> >