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: 48cac3f4a96d ("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>

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index b71e428ad9360fde0fc6aa2c0833a285ae6688dc..ad6df48b540cafb20f350472e522522a179afa31 100644 (file)
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -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++;