fs: aio: set VMA_DONTCOPY_BIT in mmap to fix NULL-pointer-dereference error

[BUG]
Recently, our internal syzkaller testing uncovered a null pointer
dereference issue:
BUG: kernel NULL pointer dereference, address: 0000000000000000
...
[   51.111664]  filemap_read_folio+0x25/0xe0
[   51.112410]  filemap_fault+0xad7/0x1250
[   51.113112]  __do_fault+0x4b/0x460
[   51.113699]  do_pte_missing+0x5bc/0x1db0
[   51.114250]  ? __pte_offset_map+0x23/0x170
[   51.114822]  __handle_mm_fault+0x9f8/0x1680
[   51.115408]  handle_mm_fault+0x24c/0x570
[   51.115958]  do_user_addr_fault+0x226/0xa50
...
Crash analysis showed the file involved was an AIO ring file.

[CAUSE]
	PARENT process		CHILD process
t=0	io_setup(1, &ctx)
	[access ctx addr]
	fork()
	io_destroy
	  vm_munmap // not affect child vma
	  percpu_ref_put
	  ...
	    put_aio_ring_file
t=1				[access ctx addr]	// pagefault
				...
				  __do_fault
				    filemap_fault
				      max_idx = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE)
t=2	      truncate_setsize
	        truncate_pagecache
t=3				      filemap_get_folio	// no folio, create folio
				      __filemap_get_folio(..., FGP_CREAT, ...)	// page_not_uptodate
				      filemap_read_folio(file, mapping->a_ops->read_folio, folio) // oops!

At t=0, the parent process calls io_setup and then fork. The child process
gets its own VMA but without any PTEs. The parent then calls io_destroy.
Before i_size is truncated to 0, at t=1 the child process accesses this AIO
ctx address and triggers a pagefault. After the max_idx check passes, at
t=2 the parent calls truncate_setsize and truncate_pagecache. At t=3 the
child fails to obtain the folio, falls into the "page_not_uptodate" path,
and hits this problem because AIO does not implement "read_folio".

[Fix]
Fix this by marking the AIO ring buffer VMA with VM_DONTCOPY so
that fork()'s dup_mmap() skips it entirely. This is the correct
semantic because:

1) The child's ioctx_table is already reset to NULL by mm_init_aio() during
fork(), so the child has no AIO context and no way to perform any AIO
operations on this mapping.
2) The AIO ring VMA is only meaningful in conjunction with its associated
kioctx, which is never inherited across fork(). So child process with no
AIO context has no legitimate reason to access the ring buffer. Delivering
SIGSEGV on such an erroneous access is preferable to a kernel crash.

Signed-off-by: Zizhi Wo <[email protected]>
Link: https://patch.msgid.link/[email protected]
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: Zizhi Wo

fs/aio.c

@@ -447,7 +447,7 @@ static const struct vm_operations_struct aio_ring_vm_ops = {
 
 static int aio_ring_mmap_prepare(struct vm_area_desc *desc)
 {
-	vma_desc_set_flags(desc, VMA_DONTEXPAND_BIT);
+	vma_desc_set_flags(desc, VMA_DONTEXPAND_BIT, VMA_DONTCOPY_BIT);
 	desc->vm_ops = &aio_ring_vm_ops;
 	return 0;
 }