linux-kernel - Re: [PATCH bpf v2 1/2] bpf: fix %p% runtime check in bpf_bprintf

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Message-ID: <f4c533a8-0d5e-476a-96cb-e76b67a4d62c@linux.dev>
Date: Mon, 28 Oct 2024 23:18:40 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Ilya Shchipletsov <rabbelkin@...l.ru>, bpf@...r.kernel.org
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Florent Revest <revest@...omium.org>,
 Nikita Marushkin <hfggklm@...il.com>, lvc-project@...uxtesting.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf v2 1/2] bpf: fix %p% runtime check in
 bpf_bprintf_prepare


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@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=e2c932aec5c8a6e1d31c
> Fixes: 48cac3f4a96d ("bpf: Implement formatted output helpers with bstr_printf")
> Co-developed-by: Nikita Marushkin <hfggklm@...il.com>
> Signed-off-by: Nikita Marushkin <hfggklm@...il.com>
> Signed-off-by: Ilya Shchipletsov <rabbelkin@...l.ru>

Acked-by: Yonghong Song <yonghong.song@...ux.dev>