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Fri, 19 Dec 2025 15:52:12 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id hqZYEqx0RWl8SgAAD6G6ig (envelope-from ); Fri, 19 Dec 2025 15:52:12 +0000 Message-ID: <5688d766-94b6-41cd-a3c5-909c50f3483a@suse.cz> Date: Fri, 19 Dec 2025 16:52:11 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] slab: fix kmalloc_nolock() context check for PREEMPT_RT Content-Language: en-US To: Swaraj Gaikwad , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , Alexei Starovoitov , "open list:SLAB ALLOCATOR" , open list , "open list:Real-time Linux (PREEMPT_RT):Keyword:PREEMPT_RT" Cc: skhan@linuxfoundation.org, david.hunter.linux@gmail.com, syzbot+b1546ad4a95331b2101e@syzkaller.appspotmail.com References: <20251219085755.139846-1-swarajgaikwad1925@gmail.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 56F768000B X-Stat-Signature: x77ayhaoxpi71c58jzao8g9taybzwwyn X-Rspam-User: X-HE-Tag: 1766159534-565680 X-HE-Meta: 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 ec/iI6XU 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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@syzkaller.appspotmail.com > 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 > --- > 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 >