6.18-stable review patch. If anyone has any objections, please let me know.
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From: Sahil Chandna chandna.sahil@gmail.com
[ Upstream commit c1da3df7191f1b4df9256bcd30d78f78201e1d17 ]
bpf_try_get_buffers() returns one of multiple per-CPU buffers based on a per-CPU nesting counter. This mechanism expects that buffers are not endlessly acquired before being returned. migrate_disable() ensures that a task remains on the same CPU, but it does not prevent the task from being preempted by another task on that CPU.
Without disabled preemption, a task may be preempted while holding a buffer, allowing another task to run on same CPU and acquire an additional buffer. Several such preemptions can cause the per-CPU nest counter to exceed MAX_BPRINTF_NEST_LEVEL and trigger the warning in bpf_try_get_buffers(). Adding preempt_disable()/preempt_enable() around buffer acquisition and release prevents this task preemption and preserves the intended bounded nesting behavior.
Reported-by: syzbot+b0cff308140f79a9c4cb@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/68f6a4c8.050a0220.1be48.0011.GAE@google.com/ Fixes: 4223bf833c849 ("bpf: Remove preempt_disable in bpf_try_get_buffers") Suggested-by: Yonghong Song yonghong.song@linux.dev Reviewed-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Signed-off-by: Sahil Chandna chandna.sahil@gmail.com Link: https://lore.kernel.org/r/20251114064922.11650-1-chandna.sahil@gmail.com Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/helpers.c | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index e4007fea49091..81ef159ef89bd 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -777,9 +777,11 @@ int bpf_try_get_buffers(struct bpf_bprintf_buffers **bufs) { int nest_level;
+ preempt_disable(); nest_level = this_cpu_inc_return(bpf_bprintf_nest_level); if (WARN_ON_ONCE(nest_level > MAX_BPRINTF_NEST_LEVEL)) { this_cpu_dec(bpf_bprintf_nest_level); + preempt_enable(); return -EBUSY; } *bufs = this_cpu_ptr(&bpf_bprintf_bufs[nest_level - 1]); @@ -792,6 +794,7 @@ void bpf_put_buffers(void) if (WARN_ON_ONCE(this_cpu_read(bpf_bprintf_nest_level) == 0)) return; this_cpu_dec(bpf_bprintf_nest_level); + preempt_enable(); }
void bpf_bprintf_cleanup(struct bpf_bprintf_data *data)