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Tue, 29 Sep 2026 01:16:27 -0700 (PDT) From: =?UTF-8?q?=C3=96mer=20Mete=20Kaya?= To: ast@kernel.org, daniel@iogearbox.net Cc: andrii@kernel.org, eddyz87@gmail.com, memxor@gmail.com, martin.lau@linux.dev, song@kernel.org, yonghong.song@linux.dev, jolsa@kernel.org, emil@etsalapatis.com, ihor.solodrai@linux.dev, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, =?UTF-8?q?=C3=96mer=20Mete=20Kaya?= , syzbot+fd7e415d891073b83e1f@syzkaller.appspotmail.com Subject: [PATCH bpf v2] bpf: Fix missing migration protection in __rhtab_map_lookup_and_delete_batch() Date: Tue, 29 Sep 2026 11:10:34 +0300 Message-ID: <20260929081609.557899-1-omermetekaya0@gmail.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit bpf_mem_cache_free_rcu() uses this_cpu_ptr() which requires migration to be disabled. All callers of rhtab_delete_elem() disable migration except __rhtab_map_lookup_and_delete_batch(), which calls it under rcu_read_lock() only. On CONFIG_PREEMPT_RCU, rcu_read_lock() does not disable preemption or migration, so the task can migrate between CPUs during the delete loop, causing this_cpu_ptr() to trigger: BUG: using smp_processor_id() in preemptible [00000000] code Fix by wrapping the delete loop in migrate_disable()/migrate_enable() in __rhtab_map_lookup_and_delete_batch(), matching the migration protection that the other callers already provide. Fixes: 818e00848227 ("bpf: Implement iteration ops for resizable hashtab") Reported-by: syzbot+fd7e415d891073b83e1f@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=fd7e415d891073b83e1f Suggested-by: Alexei Starovoitov Signed-off-by: Ömer Mete Kaya --- Changes in v2: - Fix root cause in __rhtab_map_lookup_and_delete_batch() with migrate_disable()/migrate_enable() instead of stretching bpf_disable_instrumentation() in rhtab_delete_elem(). - Fix Fixes: tag to 818e00848227. - Fix commit message: migration disabled, not preemption. Tested with syzkaller: - Before the fix: the reported warning was reproduced reliably. - After the fix: the warning was no longer reproducible. kernel/bpf/hashtab.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c index f744a42bb813..2bb9ac7fc73e 100644 --- a/kernel/bpf/hashtab.c +++ b/kernel/bpf/hashtab.c @@ -3369,8 +3369,10 @@ static int __rhtab_map_lookup_and_delete_batch(struct bpf_map *map, } if (do_delete) { + migrate_disable(); for (i = 0; i < total; i++) rhtab_delete_elem(rhtab, del_elems[i], NULL, 0); + migrate_enable(); } rcu_read_unlock(); -- 2.55.0