[PATCH] mm: vmalloc: Ensure vmap_block is initialised before adding to queue

[Will Deacon <will@kernel.org>]

Commit 8c61291fd850 ("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.

Cc: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
Cc: Hailong.Liu <hailong.liu@oppo.com>
Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Fixes: 8c61291fd850 ("mm: fix incorrect vbq reference in purge_fragmented_block")
Signed-off-by: Will Deacon <will@kernel.org>
---

I _think_ the insertion into the free list is ok, as the vb shouldn't be
considered for purging if it's clean. It would be great if somebody more
familiar with this code could confirm either way, however.

 mm/vmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 6b783baf12a1..64c0a2c8a73c 100644
--- a/mm/vmalloc.c
+++ b/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);
-- 
2.46.0.76.ge559c4bf1a-goog