linux-kernel - Re: [PATCH bpf v4 1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator

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Message-ID: <472C44CE-2DB2-48B1-8F58-0541F877AEA7@fb.com>
Date:   Fri, 6 Nov 2020 00:15:57 +0000
From:   Song Liu <songliubraving@...com>
To:     Daniel Xu <dxu@...uu.xyz>
CC:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf v4 1/2] lib/strncpy_from_user.c: Don't overcopy bytes
 after NUL terminator



> On Nov 5, 2020, at 4:06 PM, Daniel Xu <dxu@...uu.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@...uu.xyz>

Acked-by: Song Liu <songliubraving@...com>

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