bpf: fix check of allowed specifiers in bpf_trace_printk

PATCH Superseded Martynas Pumputis 2 participants 2 messages Nov 23, 2018

Subsystems (6)

Networking Network drivers BPF Tracing +2 more subsystems

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Martynas Pumputis

author

2018-11-23 02:41 UTC #0 lore raw

PATCH 1/1 bpf: fix check of allowed specifiers in bpf_trace_printk

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 [email protected]

Signed-off-by Martynas Pumputis <[email protected]>

---
 kernel/trace/bpf_trace.c | 2 ++
 1 file changed, 2 insertions(+)

kernel/trace/bpf_trace.c +2 -0

index 08fcfe440c63..9ab05736e1a1 100644
@@ -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;
 		}

Signature · 1 line

2.19.1

Daniel Borkmann

maintainer

2018-11-23 08:12 UTC #1 lore raw

Hi Martynas,

Quoted 15 lines · 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 [email protected]
Signed-off-by Martynas Pumputis <[email protected]>
---
 kernel/trace/bpf_trace.c | 2 ++
 1 file changed, 2 insertions(+)
kernel/trace/bpf_trace.c +0 -0
index 08fcfe440c63..9ab05736e1a1 100644

kernel/trace/bpf_trace.c BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1, · line 231

Quoted 7 lines

kernel/trace/bpf_trace.c +2 -0

@@ -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?

kernel/trace/bpf_trace.c +5 -3

index 08fcfe4..ff83b8c 100644
@@ -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