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+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;
>               }

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