staging: android: ashmem: Disallow ashmem memory from being remapped

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.
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>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Link: https://lore.kernel.org/r/20200127235616.48920-1-tkjos@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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);