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

[Song Liu]

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

Acked-by: Song Liu <songliubraving@xxxxxx>

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