drm/i915: Hold rcu_read_lock when iterating over the radixtree (objects)

Kasan spotted

    [IGT] gem_tiled_pread_pwrite: exiting, ret=0
    ==================================================================
    BUG: KASAN: use-after-free in __i915_gem_object_reset_page_iter+0x15c/0x170 [i915]
    Read of size 8 at addr ffff8801359da310 by task kworker/3:2/182

    CPU: 3 PID: 182 Comm: kworker/3:2 Tainted: G     U          4.14.0-rc6-CI-Custom_3340+ #1
    Hardware name: Intel Corp. Geminilake/GLK RVP1 DDR4 (05), BIOS GELKRVPA.X64.0062.B30.1708222146 08/22/2017
    Workqueue: events __i915_gem_free_work [i915]
    Call Trace:
     dump_stack+0x68/0xa0
     print_address_description+0x78/0x290
     ? __i915_gem_object_reset_page_iter+0x15c/0x170 [i915]
     kasan_report+0x23d/0x350
     __asan_report_load8_noabort+0x19/0x20
     __i915_gem_object_reset_page_iter+0x15c/0x170 [i915]
     ? i915_gem_object_truncate+0x100/0x100 [i915]
     ? lock_acquire+0x380/0x380
     __i915_gem_object_put_pages+0x30d/0x530 [i915]
     __i915_gem_free_objects+0x551/0xbd0 [i915]
     ? lock_acquire+0x13e/0x380
     __i915_gem_free_work+0x4e/0x70 [i915]
     process_one_work+0x6f6/0x1590
     ? pwq_dec_nr_in_flight+0x2b0/0x2b0
     worker_thread+0xe6/0xe90
     ? pci_mmcfg_check_reserved+0x110/0x110
     kthread+0x309/0x410
     ? process_one_work+0x1590/0x1590
     ? kthread_create_on_node+0xb0/0xb0
     ret_from_fork+0x27/0x40

    Allocated by task 1801:
     save_stack_trace+0x1b/0x20
     kasan_kmalloc+0xee/0x190
     kasan_slab_alloc+0x12/0x20
     kmem_cache_alloc+0xdc/0x2e0
     radix_tree_node_alloc.constprop.12+0x48/0x330
     __radix_tree_create+0x274/0x480
     __radix_tree_insert+0xa2/0x610
     i915_gem_object_get_sg+0x224/0x670 [i915]
     i915_gem_object_get_page+0xb5/0x1c0 [i915]
     i915_gem_pread_ioctl+0x822/0xf60 [i915]
     drm_ioctl_kernel+0x13f/0x1c0
     drm_ioctl+0x6cf/0x980
     do_vfs_ioctl+0x184/0xf30
     SyS_ioctl+0x41/0x70
     entry_SYSCALL_64_fastpath+0x1c/0xb1

    Freed by task 37:
     save_stack_trace+0x1b/0x20
     kasan_slab_free+0xaf/0x190
     kmem_cache_free+0xbf/0x340
     radix_tree_node_rcu_free+0x79/0x90
     rcu_process_callbacks+0x46d/0xf40
     __do_softirq+0x21c/0x8d3

    The buggy address belongs to the object at ffff8801359da0f0
    which belongs to the cache radix_tree_node of size 576
    The buggy address is located 544 bytes inside of
    576-byte region [ffff8801359da0f0, ffff8801359da330)
    The buggy address belongs to the page:
    page:ffffea0004d67600 count:1 mapcount:0 mapping:          (null) index:0x0 compound_mapcount: 0
    flags: 0x8000000000008100(slab|head)
    raw: 8000000000008100 0000000000000000 0000000000000000 0000000100110011
    raw: ffffea0004b52920 ffffea0004b38020 ffff88015b416a80 0000000000000000
    page dumped because: kasan: bad access detected

    Memory state around the buggy address:
     ffff8801359da200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
     ffff8801359da280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
    >ffff8801359da300: fb fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc
			     ^
     ffff8801359da380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
     ffff8801359da400: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
    ==================================================================
    Disabling lock debugging due to kernel taint

which looks like the slab containing the radixtree iter was freed as we
traversed the tree, taking the rcu read lock across the loop should
prevent that (deferring all the frees until the end).

Reported-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Fixes: 96d776345277 ("drm/i915: Use a radixtree for random access to the object's backing storage")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171026130032.10677-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -2267,8 +2267,10 @@ static void __i915_gem_object_reset_page_iter(struct drm_i915_gem_object *obj)
 	struct radix_tree_iter iter;
 	void __rcu **slot;
 
+	rcu_read_lock();
 	radix_tree_for_each_slot(slot, &obj->mm.get_page.radix, &iter, 0)
 		radix_tree_delete(&obj->mm.get_page.radix, iter.index);
+	rcu_read_unlock();
 }
 
 void __i915_gem_object_put_pages(struct drm_i915_gem_object *obj,