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mm/slab: return NULL early from kmalloc_nolock() in NMI on UP

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 n->list_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
Cc: stable@vger.kernel.org
Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
Link: https://patch.msgid.link/20260427-nolock-api-fix-v2-2-a6b83a92d9a4@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

authored by

Harry Yoo (Oracle) and committed by
Vlastimil Babka (SUSE) 5b31044e 620b46ed

+4
+4
mm/slub.c
··· 5339 5339 if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq())) 5340 5340 return NULL; 5341 5341 5342 + /* On UP, spin_trylock() always succeeds even when it is locked */ 5343 + if (!IS_ENABLED(CONFIG_SMP) && in_nmi()) 5344 + return NULL; 5345 + 5342 5346 retry: 5343 5347 if (unlikely(size > KMALLOC_MAX_CACHE_SIZE)) 5344 5348 return NULL;