[PATCH bpf] bpf: Don't WARN_ON_ONCE in bpf_bprintf_prepare

[Florent Revest]

The bpf_seq_printf, bpf_trace_printk and bpf_snprintf helpers share one
per-cpu buffer that they use to store temporary data (arguments to
bprintf). They "get" that buffer with try_get_fmt_tmp_buf and "put" it
by the end of their scope with bpf_bprintf_cleanup.

If one of these helpers gets called within the scope of one of these
helpers, for example: a first bpf program gets called, uses
bpf_trace_printk which calls raw_spin_lock_irqsave which is traced by
another bpf program that calls bpf_trace_printk again, then the second
"get" fails. Essentially, these helpers are not re-entrant, and it's not
that bad because they would simply return -EBUSY and recover gracefully.

However, when this happens, the code hits a WARN_ON_ONCE. The guidelines
in include/asm-generic/bug.h say "Do not use these macros [...] on
transient conditions like ENOMEM or EAGAIN."

This condition qualifies as transient, for example, the next
raw_spin_lock_irqsave probe is likely to succeed, so it does not deserve
a WARN_ON_ONCE.

The guidelines also say "Do not use these macros when checking for
invalid external inputs (e.g. invalid system call arguments" and, in a
way, this can be seen as an invalid input because syzkaller triggered
it.

Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
Reported-by: syzbot <syzbot@xxxxxxxxxxxxxxxxxxxxxxxxx>
Fixes: d9c9e4db186a ("bpf: Factorize bpf_trace_printk and bpf_seq_printf")
---
 kernel/bpf/helpers.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 544773970dbc..007fa26eb3f5 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -709,7 +709,7 @@ static int try_get_fmt_tmp_buf(char **tmp_buf)
 
 	preempt_disable();
 	used = this_cpu_inc_return(bpf_printf_buf_used);
-	if (WARN_ON_ONCE(used > 1)) {
+	if (used > 1) {
 		this_cpu_dec(bpf_printf_buf_used);
 		preempt_enable();
 		return -EBUSY;
-- 
2.31.1.527.g47e6f16901-goog