Re: [PATCH v2] slab: fix kmalloc_nolock() context check for PREEMPT_RT

[Vlastimil Babka]

On 1/13/26 16:06, 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 checking !preemptible() on PREEMPT_RT, which directly
> expresses the constraint that we cannot take a sleeping lock when
> preemption is disabled. This encompasses the previous checks for NMI
> and hard IRQ contexts while also catching cases where preemption is
> disabled.
> 
> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> Reported-by: syzbot+b1546ad4a95331b2101e@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=b1546ad4a95331b2101e
> Signed-off-by: Swaraj Gaikwad <swarajgaikwad1925@xxxxxxxxx>

Added to slab/for-next-fixes, thanks!

> ---
> Changes in v2:
> - Simplified condition from (in_nmi() || in_hardirq() || preempt_count())
>   to !preemptible() as suggested by Luis Claudio R. Goncalves and agreed
>   by Vlastimil Babka
> - Updated comment to reflect the more descriptive check
> 
> 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..642f4744d5c6 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) && !preemptible())
> +		/*
> +		 * 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
>