mm: vmalloc: prevent use after free in _vm_unmap_aliases

A potential use after free can occur in _vm_unmap_aliases where an already
freed vmap_area could be accessed, Consider the following scenario:

Process 1						Process 2

__vm_unmap_aliases					__vm_unmap_aliases
	purge_fragmented_blocks_allcpus				rcu_read_lock()
		rcu_read_lock()
			list_del_rcu(&vb->free_list)
									list_for_each_entry_rcu(vb .. )
	__purge_vmap_area_lazy
		kmem_cache_free(va)
										va_start = vb->va->va_start

Here Process 1 is in purge path and it does list_del_rcu on vmap_block and
later frees the vmap_area, since Process 2 was holding the rcu lock at
this time vmap_block will still be present in and Process 2 accesse it and
thereby it tries to access vmap_area of that vmap_block which was already
freed by Process 1 and this results in use after free.

Fix this by adding a check for vb->dirty before accessing vmap_area
structure since vb->dirty will be set to VMAP_BBMAP_BITS in purge path
checking for this will prevent the use after free.

Link: https://lkml.kernel.org/r/1616062105-23263-1-git-send-email-vjitta@codeaurora.org
Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmalloc.c

@@ -2040,7 +2040,7 @@ static void _vm_unmap_aliases(unsigned long start, unsigned long end, int flush)
 		rcu_read_lock();
 		list_for_each_entry_rcu(vb, &vbq->free, free_list) {
 			spin_lock(&vb->lock);
-			if (vb->dirty) {
+			if (vb->dirty && vb->dirty != VMAP_BBMAP_BITS) {
 				unsigned long va_start = vb->va->va_start;
 				unsigned long s, e;