Fuzzing reports a warning in format_decode()
Please remove unsupported %� in format string
WARNING: CPU: 0 PID: 5091 at lib/vsprintf.c:2680 format_decode+0x1193/0x1bb0 lib/vsprintf.c:2680
Modules linked in:
CPU: 0 PID: 5091 Comm: syz-executor879 Not tainted 6.10.0-rc1-syzkaller-00021-ge0cce98fe279 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/02/2024
RIP: 0010:format_decode+0x1193/0x1bb0 lib/vsprintf.c:2680
Call Trace:
<TASK>
bstr_printf+0x137/0x1210 lib/vsprintf.c:3253
____bpf_trace_printk kernel/trace/bpf_trace.c:390 [inline]
bpf_trace_printk+0x1a1/0x230 kernel/trace/bpf_trace.c:375
bpf_prog_21da1b68f62e1237+0x36/0x41
bpf_dispatcher_nop_func include/linux/bpf.h:1243 [inline]
__bpf_prog_run include/linux/filter.h:691 [inline]
bpf_prog_run include/linux/filter.h:698 [inline]
bpf_test_run+0x40b/0x910 net/bpf/test_run.c:425
bpf_prog_test_run_skb+0xafa/0x13a0 net/bpf/test_run.c:1066
bpf_prog_test_run+0x33c/0x3b0 kernel/bpf/syscall.c:4291
__sys_bpf+0x48d/0x810 kernel/bpf/syscall.c:5705
__do_sys_bpf kernel/bpf/syscall.c:5794 [inline]
__se_sys_bpf kernel/bpf/syscall.c:5792 [inline]
__x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5792
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
The problem occurs when trying to pass %p% at the end of format string,
which would result in skipping last % and passing invalid format string
down to format_decode() that would cause warning because of invalid
character after %.
Fix issue by advancing pointer only if next char is format modifier.
If next char is null/space/punct, then just accept formatting as is,
without advancing the pointer.
Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
Reported-by: syzbot+e2c932aec5c8a6e1d31c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e2c932aec5c8a6e1d31c
Fixes: 48cac3f ("bpf: Implement formatted output helpers with bstr_printf")
Co-developed-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Ilya Shchipletsov <rabbelkin@mail.ru>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Florent Revest <revest@chromium.org>