[PATCH mm-hotfixes 2/2] mm/slub: return NULL early from kmalloc_nolock() in NMI on UP

[Harry Yoo (Oracle)]

On UP kernels (!CONFIG_SMP), spin_trylock() is a no-op that
unconditionally succeeds even when the lock is already held. As a
result, kmalloc_nolock() called from NMI context can re-enter the slab
allocator and acquire a lock that the interrupted context is already
holding, corrupting slab state.

With CONFIG_DEBUG_SPINLOCK on UP, the following BUG is triggered with
the slub_kunit test module:

  BUG: spinlock trylock failure on UP on CPU#0, kunit_try_catch/243
  [...]
  Call Trace:
   <NMI>
   dump_stack_lvl+0x3f/0x60
   do_raw_spin_trylock+0x41/0x50
   _raw_spin_trylock+0x24/0x50
   get_from_partial_node+0x120/0x4d0
   ___slab_alloc+0x8a/0x4c0
   kmalloc_nolock_noprof+0x164/0x310
   [...]
   </NMI>

Fix this by returning NULL early when invoked from NMI on a UP kernel.

Link: https://lore.kernel.org/linux-mm/ad_cqe51pvr1WaDg@hyeyoo
Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Signed-off-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
---
 mm/slub.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index 92362eeb13e5..b4ec15df92f6 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5339,6 +5339,10 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node)
 	if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq()))
 		return NULL;
 
+	/* On UP, spin_trylock() always succeeds even when it is locked */
+	if (!IS_ENABLED(CONFIG_SMP) && in_nmi())
+		return NULL;
+
 retry:
 	if (unlikely(size > KMALLOC_MAX_CACHE_SIZE))
 		return NULL;
-- 
2.43.0