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

[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+1ec5c5ec949c4adaa0c4@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

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

[Daniel Borkmann]

Hi Martynas,

On 11/22/2018 05:00 PM, Martynas Pumputis wrote:
> 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+1ec5c5ec949c4adaa0c4@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;
>  		}

Thanks for the fix! Could we simplify the logic a bit to avoid having to
navigate i back and forth which got us in trouble in the first place? Like
below (untested) perhaps?

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 08fcfe4..ff83b8c 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;

Thanks,
Daniel