Messages in this thread | | | Subject | Re: [PATCH bpf-next] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator | From | Daniel Borkmann <> | Date | Wed, 4 Nov 2020 17:24:52 +0100 |
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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
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