From: Daniel Borkmann <daniel@iogearbox.net>
To: Daniel Xu <dxu@dxuuu.xyz>,
bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
ast@kernel.org
Cc: kernel-team@fb.com
Subject: Re: [PATCH bpf-next] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator
Date: Wed, 4 Nov 2020 17:24:52 +0100 [thread overview]
Message-ID: <7831c092-5ab4-033e-8fb3-ad9702332d79@iogearbox.net> (raw)
In-Reply-To: <eb78270e61e4d2e8ece047430d8397e000ef8569.1604456921.git.dxu@dxuuu.xyz>
On 11/4/20 3:29 AM, Daniel Xu 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.
>
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
It looks like this is a regression from the recent refactoring of the mem probing
util functions? Could we add a Fixes tag and then we'd also need to target the fix
against bpf tree instead of bpf-next, no?
Moreover, a BPF kselftest would help to make sure it doesn't regress in future again.
Thanks,
Daniel
next prev parent reply other threads:[~2020-11-04 16:26 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-04 2:29 [PATCH bpf-next] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator Daniel Xu
2020-11-04 16:24 ` Daniel Borkmann [this message]
2020-11-04 20:18 ` Daniel Xu
2020-11-04 22:36 ` Daniel Borkmann
2020-11-05 2:21 ` Daniel Xu
[not found] ` <20201106020930.GA18349@xsang-OptiPlex-9020>
2020-11-06 4:32 ` [lib/strncpy_from_user.c] 00a4ef91e8: BUG:KASAN:slab-out-of-bounds_in_s Alexei Starovoitov
2020-11-06 18:54 ` Daniel Xu
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