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

[Masami Hiramatsu]

On Wed, 19 Apr 2017 16:38:22 +0000
"Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:

> Excerpts from Masami Hiramatsu's message of April 19, 2017 20:07:
> > On Wed, 19 Apr 2017 18:21:02 +0530
> > "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:
> > 
> >> When a kprobe is being registered, we use the symbol_name field to
> >> lookup the address where the probe should be placed. Since this is a
> >> user-provided field, let's ensure that the length of the string is
> >> within expected limits.
> > 
> > Would we really need this? Of course it may filter out longer
> > strings... anyway such name should be rejected by kallsyms.
> 
> I felt this would be good to have generically, as kallsyms does many 
> string operations on the symbol name, including an unbounded strchr().

OK, so this is actually for performance reason.

> 
> > 
> > [...]
> >> 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)
> > 
> > This just check the length of symbol_name buffer, and can contain
> > some invalid chars.
> 
> Yes, I kept the function name generic incase we would like to do more 
> validation in future, plus it's shorter than 
> is_valid_kprobe_symbol_name_len() ;-)

OK, if this is enough general, we'd better define this in
kernel/kallsyms.c or in kallsyms.h. Of course the function
should be called is_valid_symbol_name(). :-)

> >> +{
> >> +	size_t sym_len;
> >> +	char *s;
> >> +
> >> +	s = strchr(name, ':');
> 
> Hmm.. this should be strnchr(). I re-factored the code that moved the 
> strnlen() above this below. I'll fix this.
> 
> >> +	if (s) {
> >> +		sym_len = strnlen(s+1, KSYM_NAME_LEN);
> > 
> > If you use strnlen() here, you just need to ensure sym_len < KSYM_NAME_LEN.
> 
> Hmm.. not sure I follow. Are you saying the check for sym_len <= 0 is 
> not needed?

You can check sym_len != 0, but anyway, here we concern about
"longer" string (for performance reason), we can focus on
such case.
(BTW, could you also check the name != NULL at first?)

So, what I think it can be;

if (strnlen(s+1, KSYM_NAME_LEN) == KSYM_NAME_LEN ||
    (size_t)(s - name) >= MODULE_NAME_LEN)
	return false;

Thanks,

> >> +		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)
> > 
> > Would you mean KSYM_NAME_LEN here?
> 
> Oops... nice catch, Thanks!
> 
> - Naveen
> 
> > 
> >> +			return false;
> >> +	}
> >> +
> >> +	return true;
> >> +}
> >> +
> >>  /*
> >>   * If we have a symbol_name argument, look it up and add the offset field
> >>   * to it. This way, we can specify a relative address to a symbol.
> >> @@ -1397,6 +1419,8 @@ static kprobe_opcode_t *kprobe_addr(struct kprobe *p)
> >>  		goto invalid;
> >>  
> >>  	if (p->symbol_name) {
> >> +		if (!is_valid_kprobe_symbol_name(p->symbol_name))
> >> +			return ERR_PTR(-EINVAL);
> >>  		addr = kprobe_lookup_name(p->symbol_name, p->offset);
> >>  		if (!addr)
> >>  			return ERR_PTR(-ENOENT);
> >> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> >> index 5f688cc724f0..bf73e5f31128 100644
> >> --- a/kernel/trace/trace_kprobe.c
> >> +++ b/kernel/trace/trace_kprobe.c
> >> @@ -704,6 +704,10 @@ static int create_trace_kprobe(int argc, char **argv)
> >>  			pr_info("Return probe must be used without offset.\n");
> >>  			return -EINVAL;
> >>  		}
> >> +		if (!is_valid_kprobe_symbol_name(symbol)) {
> >> +			pr_info("Symbol name is too long.\n");
> >> +			return -EINVAL;
> >> +		}
> >>  	}
> >>  	argc -= 2; argv += 2;
> >>  
> >> -- 
> >> 2.12.1
> >> 
> > 
> > Thanks,
> > 
> > -- 
> > Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > 
> > 
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>