linux-kernel - Re: [PATCH] bpf: Format string can't be empty

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Message-ID: <CAADnVQ+oMVuUjZi0MtGf52P3Xg9p4RBFarwZ_PiLWMAu+hU=rg@mail.gmail.com>
Date: Wed, 7 Jan 2026 19:02:37 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Edward Adam Davis <eadavis@...com>
Cc: syzbot+2c29addf92581b410079@...kaller.appspotmail.com, 
	Andrii Nakryiko <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Eduard <eddyz87@...il.com>, Hao Luo <haoluo@...gle.com>, 
	John Fastabend <john.fastabend@...il.com>, Jiri Olsa <jolsa@...nel.org>, 
	KP Singh <kpsingh@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Network Development <netdev@...r.kernel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Song Liu <song@...nel.org>, 
	syzkaller-bugs <syzkaller-bugs@...glegroups.com>, Yonghong Song <yonghong.song@...ux.dev>
Subject: Re: [PATCH] bpf: Format string can't be empty

On Wed, Jan 7, 2026 at 1:39 AM Edward Adam Davis <eadavis@...com> wrote:
>
> The user constructed a BPF program containing a bpf_snprintf() call.
> The fmt parameter passed to bpf_snprintf() was not assigned a value;
> it only executed the BPF_MAP_FREEZE command to freeze the fmt string.
> Furthermore, when bpf_check() executed check_reg_const_str() and
> check_bpf_snprintf_call() to check the fmt input parameter of the
> user-constructed BPF program's bpf_snprintf() call, strnchr() only
> checked if fmt was a null-terminated string. This led the BPF verifier
> to incorrectly assume the constant format string was valid.
> When the BPF program was actually executed, the out-of-bounds (OOB)
> issue reported by syzbot occurred [1].
>
> This issue is strongly related to bpf_snprintf(), therefore adding a
> check for an empty format string in check_bpf_snprintf_call() would
> be beneficial. Since it calls bpf_bprintf_prepare(), only adding a
> check on the result of strnchr() is needed to prevent the case where
> the format string is empty.
>
> [1]
> BUG: KASAN: slab-out-of-bounds in strnchr+0x5e/0x80 lib/string.c:405
> Read of size 1 at addr ffff888029e093b0 by task ksoftirqd/1/23
> Call Trace:
>  strnchr+0x5e/0x80 lib/string.c:405
>  bpf_bprintf_prepare+0x167/0x13d0 kernel/bpf/helpers.c:829
>  ____bpf_snprintf kernel/bpf/helpers.c:1065 [inline]
>  bpf_snprintf+0xd3/0x1b0 kernel/bpf/helpers.c:1049
>
> Allocated by task 6022:
>  __bpf_map_area_alloc kernel/bpf/syscall.c:395 [inline]
>  bpf_map_area_alloc+0x64/0x180 kernel/bpf/syscall.c:408
>  insn_array_alloc+0x52/0x140 kernel/bpf/bpf_insn_array.c:49
>  map_create+0xafd/0x16a0 kernel/bpf/syscall.c:1514
>
> The buggy address is located 0 bytes to the right of
>  allocated 944-byte region [ffff888029e09000, ffff888029e093b0)
>
> Fixes: d9c9e4db186a ("bpf: Factorize bpf_trace_printk and bpf_seq_printf")
> Reported-by: syzbot+2c29addf92581b410079@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=2c29addf92581b410079
> Tested-by: syzbot+2c29addf92581b410079@...kaller.appspotmail.com
> Signed-off-by: Edward Adam Davis <eadavis@...com>
> ---
>  kernel/bpf/helpers.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index db72b96f9c8c..88da2d0e634c 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -827,7 +827,7 @@ int bpf_bprintf_prepare(const char *fmt, u32 fmt_size, const u64 *raw_args,
>         char fmt_ptype, cur_ip[16], ip_spec[] = "%pXX";
>
>         fmt_end = strnchr(fmt, fmt_size, 0);
> -       if (!fmt_end)
> +       if (!fmt_end || fmt_end == fmt)
>                 return -EINVAL;

I don't think you root caused it correctly.
The better fix and analysis:
https://patchwork.kernel.org/project/netdevbpf/patch/20260107021037.289644-1-kartikey406@gmail.com/

pw-bot: cr