aio: fix NULL deref when a fault races ring truncation

The aio ring lives in a file on the aio pseudo filesystem and the ring
VMA uses filemap_fault(). aio_ctx_aops doesn't have a ->read_folio
because aio_setup_ring() puts all folios of the ring into the page
cache. So a fault on the ring only works as long as it finds its folio
in the page cache. If it doesn't, filemap_fault() calls ->read_folio
without checking and we jump to NULL.

We already fixed this twice this year. A forked child could inherit the
ring mapping, fixed by commit c03ce4173c7b ("fs: aio: set
VMA_DONTCOPY_BIT in mmap to fix NULL-pointer-dereference error"). And a
partial mremap() could leave part of the ring mapped, fixed by
commit 3adf7ae18bf4 ("fs: aio: reject partial mremap to avoid
Null-pointer-dereference error"). Both times a VMA outlived the ring and
the ring file was truncated under it.

There's a third way to get there. kill_ioctx() and the error path of
ioctx_alloc() unmap the ring with vm_munmap() and ignore the return
value. If vm_munmap() has to wait for mmap_lock and a fatal signal is
pending it returns -EINTR and the VMA stays. free_ioctx() then calls
put_aio_ring_file() which truncates the ring file to zero. A fault that
passed the i_size check in filemap_fault() before that doesn't find its
folio anymore, allocates a new one and calls ->read_folio:

    fault on the ring                 io_destroy() with SIGKILL pending
    =================                 =================================
                                      vm_munmap() returns -EINTR
    filemap_fault()
      i_size check passes
                                      free_ioctx()
                                        put_aio_ring_file()
                                          truncate_setsize(inode, 0)
      no folio, allocate a new one
      filemap_read_folio()
        ->read_folio == NULL

All that's needed is a second process that shares the mm. If a CLONE_VM
child calls io_destroy() and gets SIGKILLed while it waits for mmap_lock
the ring stays mapped for the threads of the parent. Any user can do
that.

Instead of plugging the routes one by one give the ring a ->read_folio
that unlocks the folio and returns -EIO. All folios of a live ring are
in the page cache so a fault can only end up there if the ring was
truncated. The fault now gets SIGBUS. Nothing else reads the ring file
and the aio filesystem uses noop_backing_dev_info so there's no
readahead either.

Making vm_munmap() unkillable in those two places would only close this
one route and bring back what commit ae7987835643 ("mm: make vm_munmap
killable") fixed.

Fixes: 36bc08cc0170 ("fs/aio: Add support to aio ring pages migration")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Link: https://patch.msgid.link/20261029-work-fs-ai-fixes-v1-1-88f22792f6f7@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/fs/aio.c b/fs/aio.c
index 4f32b45cb8a0..893db6961ad3 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -528,7 +528,18 @@ out:
 #define aio_migrate_folio NULL
 #endif
 
+/*
+ * filemap_fault() reads a folio that it did not find in the page cache. All
+ * folios of a live ring are there, so the ring was truncated under the fault.
+ */
+static int aio_read_folio(struct file *file, struct folio *folio)
+{
+	folio_unlock(folio);
+	return -EIO;
+}
+
 static const struct address_space_operations aio_ctx_aops = {
+	.read_folio	= aio_read_folio,
 	.dirty_folio	= noop_dirty_folio,
 	.migrate_folio	= aio_migrate_folio,
 };