From: Song Liu <songliubraving@fb.com>
To: Daniel Xu <dxu@dxuuu.xyz>
Cc: "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"ast@kernel.org" <ast@kernel.org>,
"daniel@iogearbox.net" <daniel@iogearbox.net>,
"andrii.nakryiko@gmail.com" <andrii.nakryiko@gmail.com>,
Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH bpf v4 1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator
Date: Fri, 6 Nov 2020 00:15:57 +0000 [thread overview]
Message-ID: <472C44CE-2DB2-48B1-8F58-0541F877AEA7@fb.com> (raw)
In-Reply-To: <4ff12d0c19de63e7172d25922adfb83ae7c8691f.1604620776.git.dxu@dxuuu.xyz>
> On Nov 5, 2020, at 4:06 PM, Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> do_strncpy_from_user() may copy some extra bytes after the NUL
> terminator into the destination buffer. This usually does not matter for
> normal string operations. However, when BPF programs key BPF maps with
> strings, this matters a lot.
>
> A BPF program may read strings from user memory by calling the
> bpf_probe_read_user_str() helper which eventually calls
> do_strncpy_from_user(). The program can then key a map with the
> resulting string. BPF map keys are fixed-width and string-agnostic,
> meaning that map keys are treated as a set of bytes.
>
> The issue is when do_strncpy_from_user() overcopies bytes after the NUL
> terminator, it can result in seemingly identical strings occupying
> multiple slots in a BPF map. This behavior is subtle and totally
> unexpected by the user.
>
> This commit uses the proper word-at-a-time APIs to avoid overcopying.
>
> Fixes: 6ae08ae3dea2 ("bpf: Add probe_read_{user, kernel} and probe_read_{user, kernel}_str helpers")
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Acked-by: Song Liu <songliubraving@fb.com>
[...]
next prev parent reply other threads:[~2020-11-06 0:16 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-06 0:06 [PATCH bpf v4 0/2] Fix bpf_probe_read_user_str() overcopying Daniel Xu
2020-11-06 0:06 ` [PATCH bpf v4 1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator Daniel Xu
2020-11-06 0:15 ` Song Liu [this message]
2020-11-09 16:30 ` patchwork-bot+netdevbpf
2020-11-06 0:06 ` [PATCH bpf v4 2/2] selftest/bpf: Test bpf_probe_read_user_str() strips trailing bytes after NUL Daniel Xu
2020-11-06 0:19 ` Song Liu
2020-11-06 3:53 ` Andrii Nakryiko
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