Re: [PATCH] slab: fix kmalloc_nolock() context check for PREEMPT_RT

[Vlastimil Babka]

On 12/19/25 09:57, Swaraj Gaikwad wrote:
> On PREEMPT_RT kernels, local_lock becomes a sleeping lock. The current
> check in kmalloc_nolock() only verifies we're not in NMI or hard IRQ
> context, but misses the case where preemption is disabled.
> 
> When a BPF program runs from a tracepoint with preemption disabled
> (preempt_count > 0), kmalloc_nolock() proceeds to call
> local_lock_irqsave() which attempts to acquire a sleeping lock,
> triggering:
> 
>   BUG: sleeping function called from invalid context
>   in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 6128
>   preempt_count: 2, expected: 0
> 
> Fix this by also checking preempt_count() on PREEMPT_RT, ensuring
> kmalloc_nolock() returns NULL early when called from any
> non-preemptible context.
> 
> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> Reported-by: syzbot+b1546ad4a95331b2101e@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=b1546ad4a95331b2101e

As a side-note, I'm puzzled why that report says it's bisected to commit
0db4941d9dae ("bpf: Use rcu_read_lock_dont_migrate in bpf_sk_storage.c")

I'll reply to the original report though.

> Signed-off-by: Swaraj Gaikwad <swarajgaikwad1925@xxxxxxxxx>
> ---
> Tested by building with syz config and running the syzbot
> reproducer - kernel no longer crashes.
> 
>  mm/slub.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 2acce22590f8..1dd8a25664c5 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5689,8 +5689,12 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node)
>  	if (unlikely(!size))
>  		return ZERO_SIZE_PTR;
> 
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq()))
> -		/* kmalloc_nolock() in PREEMPT_RT is not supported from irq */
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq() || preempt_count() ))
> +		/*
> +		 * kmalloc_nolock() in PREEMPT_RT is not supported from
> +		 * non-preemptible context because local_lock becomes a
> +		 * sleeping lock on RT.
> +		 */
>  		return NULL;
>  retry:
>  	if (unlikely(size > KMALLOC_MAX_CACHE_SIZE))
> 
> base-commit: 559e608c46553c107dbba19dae0854af7b219400
> --
> 2.52.0
>