On 10/28/24 12:53 PM, Ilya Shchipletsov wrote:
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@xxxxxxxxxxxxxxxxxxxxxxxxx Closes: https://syzkaller.appspot.com/bug?extid=e2c932aec5c8a6e1d31c Fixes: 48cac3f4a96d ("bpf: Implement formatted output helpers with bstr_printf") Co-developed-by: Nikita Marushkin <hfggklm@xxxxxxxxx> Signed-off-by: Nikita Marushkin <hfggklm@xxxxxxxxx> Signed-off-by: Ilya Shchipletsov <rabbelkin@xxxxxxx>
Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
