linux-hardening - Re: [PATCH v2] kdb: replace deprecated strncpy

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Date: Tue, 9 Apr 2024 13:48:38 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Daniel Thompson <daniel.thompson@...aro.org>
Cc: Jason Wessel <jason.wessel@...driver.com>, Douglas Anderson <dianders@...omium.org>, 
	kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] kdb: replace deprecated strncpy

Hi,

On Tue, Apr 9, 2024 at 11:36 AM Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> On Mon, Apr 08, 2024 at 05:46:42PM -0700, Justin Stitt wrote:
> > On Fri, Apr 5, 2024 at 2:51 AM Daniel Thompson
> > <daniel.thompson@...aro.org> wrote:
> > >
> > > >                       len_tmp = strlen(p_tmp);
> > > > -                     strncpy(cp, p_tmp+len, len_tmp-len + 1);
> > > > +                     strscpy(cp, p_tmp+len, len_tmp-len + 1);
> > >
> > > Again, I really don't think the third argument provides the number of
> > > characters in the destination buffer.
> > >
> >
> > Right, the third argument is the length of the "remaining" characters
> > from the completion point.
>
> Which is not how strscpy() is designed to be used.
>
>
> > if you type "tes" and press tab then kallsyms_symbol_complete() will
> > populate p_tmp with "test". Prior to rendering to the user, @cp points
> > to "s", we need to catch the user up and print the rest of the symbol
> > name since they've already typed "tes" we only need to print out "t".
>
> I'm more concerned about the case where you fill the buffer entirely
> then move the cursor left until you get to the tes and then press Tab.
> I think at the point we write too many bytes to cp.
>
>
> > len_tmp is the length of the entire symbol part as returned by
> > kallsyms_symbol_complete() and len is the length of only the
> > user-typed symbol. Therefore, the amount of remaining characters to
> > print is given by len_tmp-len (and +1 for a NUL-byte).
> >
> > So, yeah, you're right. This isn't the length of the destination but I
> > don't see why we can't use memcpy() (or strscpy()) and have this not
> > be considered "broken". The pointer arithmetic checks out.
>
> The problem with substituting strncpy() with memcpy() is that is *not*
> obviously wrong... but it could be subtly wrong.
>
> We can see that the person who originally wrote this code made a pretty
> serious mistake with strncpy() and the third argument if garbage. It is
> therefore important to figure out what the *correct* value for argument
> #3 should have been *before* we attempt to replace strncpy() with
> anything.
>
> Transforming something we know to be broken without fixing it first
> means it is impossible to know if the transformation is correct or not.
> Hence the original question, how do we know there is enough space
> after cp to store the string?

Gotcha, I will find time to seriously refactor/rewrite this function
(or at the very least the tab handling part of it).

At the end of the day, though, I just want this strncpy() gone.

>
>
> Daniel.

Thanks
Justin