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Fri, 19 Dec 2025 00:58:12 -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] slab: fix kmalloc_nolock() context check for PREEMPT_RT Date: Fri, 19 Dec 2025 14:27:54 +0530 Message-ID: <20251219085755.139846-1-swarajgaikwad1925@gmail.com> X-Mailer: git-send-email 2.52.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: A05CC80009 X-Stat-Signature: e8kfx7rgeyfn6pqi39apftt7pas87fhq X-Rspam-User: X-HE-Tag: 1766134694-830688 X-HE-Meta: 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 hTU+Yinu 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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 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 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