linux-kernel - Re: [PATCH bpf-next] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator

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Date:   Wed, 4 Nov 2020 17:24:52 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Daniel Xu <dxu@...uu.xyz>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, ast@...nel.org
Cc:     kernel-team@...com
Subject: Re: [PATCH bpf-next] lib/strncpy_from_user.c: Don't overcopy bytes
 after NUL terminator

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@...uu.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