Re: [PATCH v6 kspp-next 16/22] livepatch: only match unique symbols when using fgkaslr

[Alexander Lobakin]

From: Miroslav Benes <mbenes@xxxxxxx>
Date: Thu, 9 Sep 2021 13:53:35 +0200 (CEST)

> Hi,

Hi!

> On Tue, 31 Aug 2021, Alexander Lobakin wrote:
> 
> > From: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> > 
> > If any type of function granular randomization is enabled, the sympos
> > algorithm will fail, as it will be impossible to resolve symbols when
> > there are duplicates using the previous symbol position.
> > 
> > Override the value of sympos to always be zero if fgkaslr is enabled for
> > either the core kernel or modules, forcing the algorithm
> > to require that only unique symbols are allowed to be patched.
> > 
> > Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
> > ---
> >  kernel/livepatch/core.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > index 335d988bd811..852bbfa9da7b 100644
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> > @@ -169,6 +169,17 @@ static int klp_find_object_symbol(const char *objname, const char *name,
> >  	else
> >  		kallsyms_on_each_symbol(klp_find_callback, &args);
> >  
> > +	/*
> > +	 * If any type of function granular randomization is enabled, it
> > +	 * will be impossible to resolve symbols when there are duplicates
> > +	 * using the previous symbol position (i.e. sympos != 0). Override
> > +	 * the value of sympos to always be zero in this case. This will
> > +	 * force the algorithm to require that only unique symbols are
> > +	 * allowed to be patched.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_FG_KASLR))
> > +		sympos = 0;
> > +
> 
> I ran the live patching tests and no problem occurred, which is great. We 
> do not have a test for old_sympos, which makes the testing less telling, 
> but at least nothing blows up with the section randomization itself.

Great, thanks!

> However, I want to reiterate what I wrote for the same patch in v5 
> series.
> 
> The above hunk should work, but I wonder if we should make it more 
> explicit. With the change the user will get the error with "unresolvable 
> ambiguity for symbol..." if they specify sympos and the symbol is not 
> unique. It could confuse them.
> 
> So, how about it making it something like
> 
> if (IS_ENABLED(CONFIG_FG_KASLR) || IS_ENABLED(CONFIG_MODULE_FG_KASLR))
>         if (sympos) {
>                 pr_err("fgkaslr is enabled, specifying sympos for symbol '%s' in object '%s' does not work.\n",
>                         name, objname);
>                 *addr = 0;
>                 return -EINVAL;
>         }
> 
> ? (there could be goto to the error out at the end of the function to 
> save copy-pasting).
> 
> In that case, if sympos is not specified, the user will get the message 
> which matches the reality. If the user specifies it, they will get the 
> error in case of fgkaslr (no matter if the symbol is found or not).

Not familiar with livepatching unfortunately, hope Kristen and/or
Kees will comment on this. Looks fine for me anyways.

> What do you think?
> 
> Miroslav

Thanks,
Al