[PATCH] mm/slab: take n->list_lock for the list_add() in __refill_objects_node()

[Hyunwoo Kim]

In __refill_objects_node(), the list_add(&slab->slab_list, &pc.slabs) that
follows a successful __slab_try_return_freelist() is done without
n->list_lock.

__slab_try_return_freelist() only succeeds while slab->freelist is NULL. The
slab we are refilling from is taken off pc.slabs by the list_del() at the
top of the loop, so at that point it is on no list.

If another CPU frees an object of that slab, __slab_free() sees the slab as
full and puts it back on n->partial. If a third CPU then takes that object
in get_from_partial_node(), the freelist becomes NULL again.

A slab that sits on n->partial with a NULL freelist only exists while
get_from_partial_node() holds n->list_lock, between its cmpxchg and its
remove_partial().

A list_add() in that window overwrites slab_list to point into pc.slabs.
The list_del() in remove_partial() then follows the overwritten links, so it
unlinks the slab from pc.slabs and poisons slab_list while leaving the
n->partial side alone. n->partial is left pointing at the poisoned slab.

           CPU0                        CPU1                  CPU2

  __refill_objects_node()
    get_partial_node_bulk()   // n->partial to pc.slabs
    list_del()                // on no list now
    get_freelist_nofreeze()   // freelist = NULL
                              __slab_free()
                                add_partial()
                                // back on n->partial
	                        // freelist is not NULL

                                                    get_from_partial_node()
                                                      lock
                                                      cmpxchg
                                                      // freelist = NULL
    __slab_try_return_freelist()
    list_add(&pc.slabs)       // overwrites slab_list
                                                      remove_partial()
                                                        list_del()
                                                        // off pc.slabs
                                                        // slab_list = POISON

panic log:

  list_add corruption. next->prev should be prev
  (ffff888100000248), but was dead000000000122.
  (next=ffffea000416e410).
  kernel BUG at lib/list_debug.c:29!
  Oops: invalid opcode: 0000 [#1] SMP NOPTI
  CPU: 1 UID: 65534 PID: 144 Comm: poc Not tainted
  7.2.0-16172-gcf72cbb39da8-dirty #1 PREEMPT(lazy)
  RIP: 0010:__list_add_valid_or_report+0x80/0xd0
  ...
  Call Trace:
   alloc_from_new_slab+0x183/0x300
   ___slab_alloc+0x31c/0x890
   __kmalloc_noprof+0x3d4/0x800
   lsm_blob_alloc+0x2d/0x50
   security_msg_msg_alloc+0x26/0x90
   load_msg+0x1aa/0x210
   do_msgsnd+0x91/0x800
   do_syscall_64+0x109/0x5d0
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
  ...
  Kernel panic - not syncing: Fatal exception

Do the list_add() under n->list_lock. Reattaching the freelist stays outside
the lock. Once it succeeds the freelist is no longer NULL, so __slab_free()
cannot put the slab back, and by the time the lock is taken remove_partial()
has finished and the slab is on no list.

The lock is held until the block below that returns the remaining slabs to
the partial list. That block already took the same lock on this path, so no
lock/unlock pair is added.

The unlock is keyed on having taken the lock instead of on pc.slabs being
empty. With CONFIG_DEBUG_LIST or CONFIG_LIST_HARDENED, __list_add() returns
without linking anything if its check fails, which would leave pc.slabs
empty.

Fixes: ba7425312607 ("mm, slab: add an optimistic __slab_try_return_freelist()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Hyunwoo Kim <imv4bel@xxxxxxxxx>
---
 mm/slub.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index f9b56cb439e709..4f6d1a03a8ee46 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7260,6 +7260,7 @@ __refill_objects_node(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int mi
 	struct slab *slab, *slab2;
 	unsigned int refilled = 0;
 	unsigned long flags;
+	bool locked = false;
 	void *object;
 
 	pc.flags = gfp;
@@ -7297,7 +7298,10 @@ __refill_objects_node(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int mi
 			void *tail;
 
 			if (__slab_try_return_freelist(s, slab, head, count)) {
+				/* get_from_partial_node() may be mid-removal of the slab */
+				spin_lock_irqsave(&n->list_lock, flags);
 				list_add(&slab->slab_list, &pc.slabs);
+				locked = true;
 				break;
 			}
 
@@ -7312,9 +7316,12 @@ __refill_objects_node(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int mi
 			break;
 	}
 
-	if (!list_empty(&pc.slabs)) {
+	if (!locked && !list_empty(&pc.slabs)) {
 		spin_lock_irqsave(&n->list_lock, flags);
+		locked = true;
+	}
 
+	if (locked) {
 		list_for_each_entry(slab, &pc.slabs, slab_list)
 			set_node_partial_state(n, slab);
 
-- 
2.43.0