Commit f8242745 authored Jul 01, 2025 by Paul Chaignon Committed by Alexei Starovoitov Jul 01, 2025

bpf: Reject %p% format string in bprintf-like helpers



static const char fmt[] = "%p%";
    bpf_trace_printk(fmt, sizeof(fmt));

The above BPF program isn't rejected and causes a kernel warning at
runtime:

    Please remove unsupported %\x00 in format string
    WARNING: CPU: 1 PID: 7244 at lib/vsprintf.c:2680 format_decode+0x49c/0x5d0

This happens because bpf_bprintf_prepare skips over the second %,
detected as punctuation, while processing %p. This patch fixes it by
not skipping over punctuation. %\x00 is then processed in the next
iteration and rejected.

Reported-by:  <syzbot+e2c932aec5c8a6e1d31c@syzkaller.appspotmail.com>
Fixes: 48cac3f4 ("bpf: Implement formatted output helpers with bstr_printf")
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/a0e06cc479faec9e802ae51ba5d66420523251ee.1751395489.git.paul.chaignon@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>

parent 66701750

kernel/bpf/helpers.c

+8 −3

Original line number	Diff line number	Diff line
		@@ -884,6 +884,13 @@ int bpf_bprintf_prepare(char fmt, u32 fmt_size, const u64 raw_args,
		if (fmt[i] == 'p') {
		sizeof_cur_arg = sizeof(long);

		if (fmt[i + 1] == 0 \|\| isspace(fmt[i + 1]) \|\|
		ispunct(fmt[i + 1])) {
		if (tmp_buf)
		cur_arg = raw_args[num_spec];
		goto nocopy_fmt;
		}

		if ((fmt[i + 1] == 'k' \|\| fmt[i + 1] == 'u') &&
		fmt[i + 2] == 's') {
		fmt_ptype = fmt[i + 1];
		@@ -891,11 +898,9 @@ int bpf_bprintf_prepare(char fmt, u32 fmt_size, const u64 raw_args,
		goto fmt_str;
		}

		if (fmt[i + 1] == 0 \|\| isspace(fmt[i + 1]) \|\|
		ispunct(fmt[i + 1]) \|\| fmt[i + 1] == 'K' \|\|
		if (fmt[i + 1] == 'K' \|\|
		fmt[i + 1] == 'x' \|\| fmt[i + 1] == 's' \|\|
		fmt[i + 1] == 'S') {
		/* just kernel pointers */
		if (tmp_buf)
		cur_arg = raw_args[num_spec];
		i++;