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Tue, 13 Jan 2026 07:07:20 -0800 (PST) From: Swaraj Gaikwad To: Vlastimil Babka , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , Alexei Starovoitov , linux-mm@kvack.org (open list:SLAB ALLOCATOR), linux-kernel@vger.kernel.org (open list), linux-rt-devel@lists.linux.dev (open list:Real-time Linux (PREEMPT_RT):Keyword:PREEMPT_RT) Cc: skhan@linuxfoundation.org, david.hunter.linux@gmail.com, Swaraj Gaikwad , syzbot+b1546ad4a95331b2101e@syzkaller.appspotmail.com Subject: [PATCH v2] slab: fix kmalloc_nolock() context check for PREEMPT_RT Date: Tue, 13 Jan 2026 20:36:39 +0530 Message-ID: <20260113150639.48407-1-swarajgaikwad1925@gmail.com> X-Mailer: git-send-email 2.52.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: D604F80007 X-Rspam-User: X-Stat-Signature: 3k3ip8drojwo3kshqmwx9jkapp5egswc X-HE-Tag: 1768316841-661267 X-HE-Meta: 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 FPYfFkdL 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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 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@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=b1546ad4a95331b2101e Signed-off-by: Swaraj Gaikwad --- 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