From: Martynas Pumputis <m@lambda.lt>
Date: Fri, 23 Nov 2018 17:43:26 +0100
Subject: bpf: fix check of allowed specifiers in bpf_trace_printk
Patch-mainline: v4.20-rc5
Git-commit: 1efb6ee3edea57f57f9fb05dba8dcb3f7333f61f
References: bsc#1083647

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.

Fixes: 9c959c863f82 ("tracing: Allow BPF programs to call bpf_trace_printk()")
Reported-by: syzbot+1ec5c5ec949c4adaa0c4@syzkaller.appspotmail.com
Signed-off-by: Martynas Pumputis <m@lambda.lt>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Gary Lin <glin@suse.com>
---
 kernel/trace/bpf_trace.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -161,11 +161,13 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt
 			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;