linux-kernel - Re: [PATCH] slab: fix kmalloc_nolock() context check for PREEMPT

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Message-ID: <aUVTJyd74OoCtSyN@redhat.com>
Date: Fri, 19 Dec 2025 10:29:11 -0300
From: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Swaraj Gaikwad <swarajgaikwad1925@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...two.org>,
	David Rientjes <rientjes@...gle.com>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Harry Yoo <harry.yoo@...cle.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Alexei Starovoitov <ast@...nel.org>,
	"open list:SLAB ALLOCATOR" <linux-mm@...ck.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:Real-time Linux (PREEMPT_RT):Keyword:PREEMPT_RT" <linux-rt-devel@...ts.linux.dev>,
	skhan@...uxfoundation.org, david.hunter.linux@...il.com,
	syzbot+b1546ad4a95331b2101e@...kaller.appspotmail.com
Subject: Re: [PATCH] slab: fix kmalloc_nolock() context check for PREEMPT_RT

On Fri, Dec 19, 2025 at 10:31:55AM +0100, Vlastimil Babka wrote:
> 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@...kaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=b1546ad4a95331b2101e
> > Signed-off-by: Swaraj Gaikwad <swarajgaikwad1925@...il.com>
> > ---
> > 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() ))
> 
> AFAICS we can just simplify that to preempt_count() then, since in_nmi() and
> in_hardirq() both are a special cases of that.
> 
> Any comment from RT folks please?

Maybe, for the purpose of this change, using in_atomic() or !preemptible()
would be a bit more descriptive, as both macros check preempt_count()?

Luis
 
> > +		/*
> > +		 * 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
> > 
> 
> 
---end quoted text---