Re: [PATCH bpf v5 1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator

[Alexei Starovoitov]

On Wed, Nov 11, 2020 at 02:45:54PM -0800, 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 has strncpy start copying a byte at a time if a NUL is
> spotted.
> 
> Fixes: 6ae08ae3dea2 ("bpf: Add probe_read_{user, kernel} and probe_read_{user, kernel}_str helpers")
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
>  lib/strncpy_from_user.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
> index e6d5fcc2cdf3..83180742e729 100644
> --- a/lib/strncpy_from_user.c
> +++ b/lib/strncpy_from_user.c
> @@ -40,12 +40,11 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src,
>  		/* Fall back to byte-at-a-time if we get a page fault */
>  		unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
>  
> +		if (has_zero(c, &data, &constants))
> +			goto byte_at_a_time;
> +
>  		*(unsigned long *)(dst+res) = c;
> -		if (has_zero(c, &data, &constants)) {
> -			data = prep_zero_mask(c, data, &constants);
> -			data = create_zero_mask(data);
> -			return res + find_zero(data);
> -		}
> +
>  		res += sizeof(unsigned long);
>  		max -= sizeof(unsigned long);
>  	}

The fix looks good to me. It's indeed better than v4 approach.

Linus,
I think you might have an opinion about it.
Please see commit log for the reason we need this fix.