From: Sahil Chandna Date: Fri, 14 Nov 2025 06:49:22 +0000 (+0530) Subject: bpf: Prevent nesting overflow in bpf_try_get_buffers X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=c1da3df7191f1b4df9256bcd30d78f78201e1d17;p=thirdparty%2Flinux.git bpf: Prevent nesting overflow in bpf_try_get_buffers 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 Reviewed-by: Sebastian Andrzej Siewior Signed-off-by: Sahil Chandna Link: https://lore.kernel.org/r/20251114064922.11650-1-chandna.sahil@gmail.com Signed-off-by: Alexei Starovoitov --- diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index 865b0dae38d12..81ad01eadcf8c 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -774,9 +774,11 @@ 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]); @@ -789,6 +791,7 @@ 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)