linux-kernel - Re: [PATCH v4 3/7] kprobes: validate the symbol name provided during probe registration

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Date:   Sun, 23 Apr 2017 17:41:11 +0000
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4 3/7] kprobes: validate the symbol name provided during
 probe registration

Excerpts from Michael Ellerman's message of April 22, 2017 11:25:
> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> writes:
> 
>> 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.
> 
> What are we actually trying to protect against here?
> 
> If you ignore powerpc for a moment, kprobe_lookup_name() is just
> kallsyms_lookup_name().
> 
> All kallsyms_lookup_name() does with name is strcmp() it against a
> legitimate symbol name which is at most KSYM_NAME_LEN.
> 
> So I don't think any of this validation helps in that case?

It does:
https://patchwork.kernel.org/patch/9695139/

It is far too easy to cause a OOPS due to the above, though this is 
root-only (for modules).

As I stated earlier, I think it is always a good practice to validate 
inputs right on entry, rather than later. Code gets refactored, 
different arch support gets added, and so on.

Doing this validation here ensures we don't have to worry about how we 
process this later, or if another arch has to over-ride 
kprobe_lookup_name().

> 
> In the powerpc version of kprobe_lookup_name() we do need to do some
> string juggling, for which it helps to know the input is sane. But I
> think we should just make that code more robust by checking the input
> before we do anything with it.

Ok, I will fold those tests in with the powerpc implementation for now 
and consider a patch against kallsyms_lookup_name(), like Masami 
recommends.


- Naveen