From: Suren Baghdasaryan surenb@google.com
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.
Reported-by: Jann Horn jannh@google.com Signed-off-by: Suren Baghdasaryan surenb@google.com Cc: stable stable@vger.kernel.org # 4.4,4.9,4.14,4.18,5.4 Signed-off-by: Todd Kjos tkjos@google.com --- 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);
On Mon, Jan 27, 2020 at 1:00 PM 'Todd Kjos' via kernel-team kernel-team@android.com wrote:
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
On Mon, Jan 27, 2020 at 2:30 PM Joel Fernandes joelaf@google.com wrote:
Sent v2 with the suggested change.
On Mon, Jan 27, 2020 at 3:57 PM 'Todd Kjos' via kernel-team kernel-team@android.com wrote:
Sorry for the delay. The suggestion makes sense to me. Thanks!
linux-stable-mirror@lists.linaro.org
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Joel Fernandes -
Suren Baghdasaryan -
Todd Kjos