Re: [PATCH bpf v2 1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator

[Daniel Xu]

On Thu Nov 5, 2020 at 10:16 AM PST, Song Liu wrote:
>
>
> > On Nov 4, 2020, at 6:25 PM, Daniel Xu <dxu@xxxxxxxxx> wrote:
> > 
> > do_strncpy_from_user() may copy some extra bytes after the NUL
>
> We have multiple use of "NUL" here, should be "NULL"?
>
> > 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>
> > ---
> > lib/strncpy_from_user.c | 9 +++++++--
> > 1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
> > index e6d5fcc2cdf3..d084189eb05c 100644
> > --- a/lib/strncpy_from_user.c
> > +++ b/lib/strncpy_from_user.c
> > @@ -35,17 +35,22 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src,
> > 		goto byte_at_a_time;
> > 
> > 	while (max >= sizeof(unsigned long)) {
> > -		unsigned long c, data;
> > +		unsigned long c, data, mask, *out;
> > 
> > 		/* 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);
> > 
> > -		*(unsigned long *)(dst+res) = c;
> > 		if (has_zero(c, &data, &constants)) {
> > 			data = prep_zero_mask(c, data, &constants);
> > 			data = create_zero_mask(data);
> > +			mask = zero_bytemask(data);
> > +			out = (unsigned long *)(dst+res);
> > +			*out = (*out & ~mask) | (c & mask);
> > 			return res + find_zero(data);
> > +		} else  {
>
> This else clause is not needed, as we return in the if clause.

Thanks, will change in v3.

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