[PATCH] bpf: fix check of allowed specifiers in bpf_trace_printk

[Martynas Pumputis]

A format string consisting of "%p" or "%s" followed by an invalid
specifier (e.g. "%p%\n" or "%s%") could pass the check which
would make format_decode (lib/vsprintf.c) to warn.

Reported-by: syzbot+1ec5c5ec949c4adaa...@syzkaller.appspotmail.com
Signed-off-by: Martynas Pumputis <m...@lambda.lt>
---
 kernel/trace/bpf_trace.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 08fcfe440c63..9ab05736e1a1 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -225,6 +225,8 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, 
u64, arg1,
                                                    (void *) (long) unsafe_addr,
                                                    sizeof(buf));
                        }
+                       if (fmt[i] == '%')
+                               i--;
                        continue;
                }
 
-- 
2.19.1