bpf: use preempt_disable/enable() to protect bpf_bprintf_buffers nesting

The bpf_bprintf_prepare() and related helpers (bpf_try_get_buffers() /
bpf_put_buffers()) rely on a per-CPU counter bpf_bprintf_nest_level to
manage nested buffer usage. However, when invoked from different contexts
(process, softirq, NMI), the nesting counter can become inconsistent if
task  migration occurs between CPUs during these operations. This can
result in warnings such as:

WARNING: CPU: 1 PID: 6145 at kernel/bpf/helpers.c:781 bpf_try_get_buffers kernel/bpf/helpers.c:781 [inline]
WARNING: CPU: 1 PID: 6145 at kernel/bpf/helpers.c:781 bpf_bprintf_prepare+0x12cf/0x13a0 kernel/bpf/helpers.c:834

Having only migrate_disable is insufficient here to prevent nesting,
hence add preempt_disable()/enable() around buffer acquisition and release.

Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/
Fixes: 4223bf8 ("bpf: Remove preempt_disable in bpf_try_get_buffers")
Suggested-by: Yonghong Song <[email protected]>
Signed-off-by: Sahil Chandna <[email protected]>

Author: Schandna-gitinit

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)