[PATCH v2] 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 | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 08fcfe440c63..9864a35c8bb5 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -196,11 +196,13 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, 
u64, arg1,
                        i++;
                } else if (fmt[i] == 'p' || fmt[i] == 's') {
                        mod[fmt_cnt]++;
-                       i++;
-                       if (!isspace(fmt[i]) && !ispunct(fmt[i]) && fmt[i] != 0)
+                       /* disallow any further format extensions */
+                       if (fmt[i + 1] != 0 &&
+                           !isspace(fmt[i + 1]) &&
+                           !ispunct(fmt[i + 1]))
                                return -EINVAL;
                        fmt_cnt++;
-                       if (fmt[i - 1] == 's') {
+                       if (fmt[i] == 's') {
                                if (str_seen)
                                        /* allow only one '%s' per fmt string */
                                        return -EINVAL;
-- 
2.19.1