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

[Song Liu <songliubraving@fb.com>]

> On Nov 5, 2020, at 10:16 AM, Song Liu <songliubraving@fb.com> wrote:
> 
> 
> 
>> On Nov 4, 2020, at 6:25 PM, Daniel Xu <dxu@dxuuu.xyz> wrote:
>> 
>> do_strncpy_from_user() may copy some extra bytes after the NUL
> 
> We have multiple use of "NUL" here, should be "NULL"?

Just realized strncpy_from_user.c uses "NUL", so nevermind...

> 
>> 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@dxuuu.xyz>
>> ---
>> 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. 
> 
>> +			*(unsigned long *)(dst+res) = c;
>> 		}
>> +
>> 		res += sizeof(unsigned long);
>> 		max -= sizeof(unsigned long);
>> 	}
>> -- 
>> 2.28.0