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

[Naveen N. Rao]

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().

> [...]
> > 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() ;-)

> > +{
> > +	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?

> > +		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>