In the Linux kernel, the following vulnerability has been resolved:
bpf: Reject %p% format string in bprintf-like helpers
static const char fmt[] = %p%; bpf_trace_printk(fmt, sizeof(fmt));
The above BPF program isnt 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.