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Fri, 19 Dec 2025 09:31:56 +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 /l3GAowbRWm/AwAAD6G6ig (envelope-from ); Fri, 19 Dec 2025 09:31:56 +0000 Message-ID: <6fcfe0cc-3826-42c2-9c54-c127dc8379e1@suse.cz> Date: Fri, 19 Dec 2025 10:31:55 +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-Action: no action X-Rspamd-Queue-Id: 8D6364000F X-Rspamd-Server: rspam03 X-Stat-Signature: 39czjwsua9jg85r9zubg3ec3hgj9mmr8 X-Rspam-User: X-HE-Tag: 1766136720-812582 X-HE-Meta: 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 zpXCl4a0 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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 > 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() )) 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? > + /* > + * 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 >