bpf: Lock bpf_trace_printk's tmp buf before it is written to

bpf_trace_printk uses a shared static buffer to hold strings before they
are printed. A recent refactoring moved the locking of that buffer after
it gets filled by mistake.

Fixes: d9c9e4db186a ("bpf: Factorize bpf_trace_printk and bpf_seq_printf")
Reported-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Florent Revest <revest@chromium.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210427112958.773132-1-revest@chromium.org

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 2a8bcdc..0e67d12 100644 (file)
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -391,13 +391,13 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
        if (ret < 0)
                return ret;
 
+       raw_spin_lock_irqsave(&trace_printk_lock, flags);
        ret = snprintf(buf, sizeof(buf), fmt, BPF_CAST_FMT_ARG(0, args, mod),
                BPF_CAST_FMT_ARG(1, args, mod), BPF_CAST_FMT_ARG(2, args, mod));
        /* snprintf() will not append null for zero-length strings */
        if (ret == 0)
                buf[0] = '\0';
 
-       raw_spin_lock_irqsave(&trace_printk_lock, flags);
        trace_bpf_trace_printk(buf);
        raw_spin_unlock_irqrestore(&trace_printk_lock, flags);