mm: vmalloc: ensure vmap_block is initialised before adding to queue

Commit 8c61291 ("mm: fix incorrect vbq reference in
purge_fragmented_block") extended the 'vmap_block' structure to contain a
'cpu' field which is set at allocation time to the id of the initialising
CPU.

When a new 'vmap_block' is being instantiated by new_vmap_block(), the
partially initialised structure is added to the local 'vmap_block_queue'
xarray before the 'cpu' field has been initialised.  If another CPU is
concurrently walking the xarray (e.g.  via vm_unmap_aliases()), then it
may perform an out-of-bounds access to the remote queue thanks to an
uninitialised index.

This has been observed as UBSAN errors in Android:

 | Internal error: UBSAN: array index out of bounds: 00000000f2005512 [#1] PREEMPT SMP
 |
 | Call trace:
 |  purge_fragmented_block+0x204/0x21c
 |  _vm_unmap_aliases+0x170/0x378
 |  vm_unmap_aliases+0x1c/0x28
 |  change_memory_common+0x1dc/0x26c
 |  set_memory_ro+0x18/0x24
 |  module_enable_ro+0x98/0x238
 |  do_init_module+0x1b0/0x310

Move the initialisation of 'vb->cpu' in new_vmap_block() ahead of the
addition to the xarray.

Link: https://lkml.kernel.org/r/20240812171606.17486-1-will@kernel.org
Fixes: 8c61291 ("mm: fix incorrect vbq reference in purge_fragmented_block")
Signed-off-by: Will Deacon <will@kernel.org>
Reviewed-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
Cc: Hailong.Liu <hailong.liu@oppo.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmalloc.c

@@ -2626,6 +2626,7 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
 	vb->dirty_max = 0;
 	bitmap_set(vb->used_map, 0, (1UL << order));
 	INIT_LIST_HEAD(&vb->free_list);
+	vb->cpu = raw_smp_processor_id();
 
 	xa = addr_to_vb_xa(va->va_start);
 	vb_idx = addr_to_vb_idx(va->va_start);
@@ -2642,7 +2643,6 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
 	 * integrity together with list_for_each_rcu from read
 	 * side.
 	 */
-	vb->cpu = raw_smp_processor_id();
 	vbq = per_cpu_ptr(&vmap_block_queue, vb->cpu);
 	spin_lock(&vbq->lock);
 	list_add_tail_rcu(&vb->free_list, &vbq->free);