Re: [PATCH v3 3/7] kprobes: validate the symbol name length

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Excerpts from Michael Ellerman's message of April 20, 2017 11:38:
> "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> writes:
> 
>> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
>> index 6a128f3a7ed1..bb86681c8a10 100644
>> --- a/kernel/kprobes.c
>> +++ b/kernel/kprobes.c
>> @@ -1382,6 +1382,28 @@ bool within_kprobe_blacklist(unsigned long addr)
>>  	return false;
>>  }
>>  
>> +bool is_valid_kprobe_symbol_name(const char *name)
>> +{
>> +	size_t sym_len;
>> +	char *s;
>> +
>> +	s = strchr(name, ':');
>> +	if (s) {
>> +		sym_len = strnlen(s+1, KSYM_NAME_LEN);
>> +		if (sym_len <= 0 || sym_len >= KSYM_NAME_LEN)
>> +			return false;
>> +		sym_len = (size_t)(s - name);
>> +		if (sym_len <= 0 || sym_len >= MODULE_NAME_LEN)
>> +			return false;
>> +	} else {
>> +		sym_len = strnlen(name, MODULE_NAME_LEN);
>> +		if (sym_len <= 0 || sym_len >= MODULE_NAME_LEN)
>> +			return false;
>> +	}
> 
> I think this is probably more elaborate than it needs to be.
> 
> Why not just check the string is <= (MODULE_NAME_LEN + KSYM_NAME_LEN) ?

Yes, that would be sufficient for now.

It's probably just me being paranoid, but I felt it's good to have 
stricter checks for user-provided strings, to guard against future 
changes to how we process this.

- Naveen