mm: allow drivers to prevent new writable mappings
This patch (of 6):
The i_mmap_writable field counts existing writable mappings of an
address_space. To allow drivers to prevent new writable mappings, make
this counter signed and prevent new writable mappings if it is negative.
This is modelled after i_writecount and DENYWRITE.
This will be required by the shmem-sealing infrastructure to prevent any
new writable mappings after the WRITE seal has been set. In case there
exists a writable mapping, this operation will fail with EBUSY.
Note that we rely on the fact that iff you already own a writable mapping,
you can increase the counter without using the helpers. This is the same
that we do for i_writecount.
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Ryan Lortie <desrt@desrt.ca>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Daniel Mack <zonque@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/mmap.c b/mm/mmap.c
index 64c9d73..c1f2ea4 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -221,7 +221,7 @@
if (vma->vm_flags & VM_DENYWRITE)
atomic_inc(&file_inode(file)->i_writecount);
if (vma->vm_flags & VM_SHARED)
- mapping->i_mmap_writable--;
+ mapping_unmap_writable(mapping);
flush_dcache_mmap_lock(mapping);
if (unlikely(vma->vm_flags & VM_NONLINEAR))
@@ -622,7 +622,7 @@
if (vma->vm_flags & VM_DENYWRITE)
atomic_dec(&file_inode(file)->i_writecount);
if (vma->vm_flags & VM_SHARED)
- mapping->i_mmap_writable++;
+ atomic_inc(&mapping->i_mmap_writable);
flush_dcache_mmap_lock(mapping);
if (unlikely(vma->vm_flags & VM_NONLINEAR))
@@ -1577,6 +1577,17 @@
if (error)
goto free_vma;
}
+ if (vm_flags & VM_SHARED) {
+ error = mapping_map_writable(file->f_mapping);
+ if (error)
+ goto allow_write_and_free_vma;
+ }
+
+ /* ->mmap() can change vma->vm_file, but must guarantee that
+ * vma_link() below can deny write-access if VM_DENYWRITE is set
+ * and map writably if VM_SHARED is set. This usually means the
+ * new file must not have been exposed to user-space, yet.
+ */
vma->vm_file = get_file(file);
error = file->f_op->mmap(file, vma);
if (error)
@@ -1616,8 +1627,12 @@
vma_link(mm, vma, prev, rb_link, rb_parent);
/* Once vma denies write, undo our temporary denial count */
- if (vm_flags & VM_DENYWRITE)
- allow_write_access(file);
+ if (file) {
+ if (vm_flags & VM_SHARED)
+ mapping_unmap_writable(file->f_mapping);
+ if (vm_flags & VM_DENYWRITE)
+ allow_write_access(file);
+ }
file = vma->vm_file;
out:
perf_event_mmap(vma);
@@ -1646,14 +1661,17 @@
return addr;
unmap_and_free_vma:
- if (vm_flags & VM_DENYWRITE)
- allow_write_access(file);
vma->vm_file = NULL;
fput(file);
/* Undo any partial mapping done by a device driver. */
unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);
charged = 0;
+ if (vm_flags & VM_SHARED)
+ mapping_unmap_writable(file->f_mapping);
+allow_write_and_free_vma:
+ if (vm_flags & VM_DENYWRITE)
+ allow_write_access(file);
free_vma:
kmem_cache_free(vm_area_cachep, vma);
unacct_error: