[PATCH] iommu/iova: Clear the slab cache pointers when destroying them

[Davidlohr Bueso <dave@stgolabs.net>]

Both iova_cache_get() failure path and iova_cache_put() destroy the two
slab caches without clearing the pointers, leaving them dangling with
iova_cache_users at zero. The next iova_cache_get() then re-enters the
respective block, and if it fails early enough to reach 'out_err' before
re-creating both caches, it calls kmem_cache_destroy() a second time on
whichever cache is still stale:

  BUG: KASAN: slab-use-after-free in iova_cache_get+0x216/0x280
  Read of size 1 at addr ffff888001b9fdc0 by task swapper/0/1
  CPU: 1 UID: 0 PID: 1 Comm: swapper/0 Not tainted 7.2.0-rc1 #2
  Call Trace:
   <TASK>
   dump_stack_lvl+0x53/0x70
   print_report+0xce/0x620
   kasan_report+0xce/0x100
   __kasan_check_byte+0x36/0x50
   kmem_cache_destroy+0x1b/0x1c0
   iova_cache_get+0x216/0x280
   ...
  Freed by task 1:
   kasan_save_stack+0x33/0x60
   kasan_save_track+0x14/0x30
   kasan_save_free_info+0x3b/0x60
   __kasan_slab_free+0x43/0x70
   kmem_cache_free+0xbe/0x3c0
   kobject_put+0x14d/0x280
   iova_cache_put+0x8e/0xd0

Clear both pointers after destroying them, in both places.

Fixes: 84e6f56be9c6 ("iommu/iova: use named kmem_cache for iova magazines")
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
---
 drivers/iommu/iova.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index b710e5ad37e2..0e8ea04b824f 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -984,6 +984,8 @@ int iova_cache_get(void)
 out_err:
 	kmem_cache_destroy(iova_cache);
 	kmem_cache_destroy(iova_magazine_cache);
+	iova_cache = NULL;
+	iova_magazine_cache = NULL;
 	mutex_unlock(&iova_cache_mutex);
 	return err;
 }
@@ -1001,6 +1003,8 @@ void iova_cache_put(void)
 		cpuhp_remove_multi_state(CPUHP_IOMMU_IOVA_DEAD);
 		kmem_cache_destroy(iova_cache);
 		kmem_cache_destroy(iova_magazine_cache);
+		iova_cache = NULL;
+		iova_magazine_cache = NULL;
 	}
 	mutex_unlock(&iova_cache_mutex);
 }
-- 
2.39.5