Subject: Re: [PATCH V11 4/5] vsprintf: add printk specifier %px

From: "Tobin C. Harding" <[email-masked]>
Date: Thu, 30 Nov 2017 15:18:24 +1100

On Wed, Nov 29, 2017 at 07:58:26PM -0800, Joe Perches wrote:
> On Thu, 2017-11-30 at 10:26 +1100, Tobin C. Harding wrote:
> > On Wed, Nov 29, 2017 at 03:20:58PM -0800, Andrew Morton wrote:
> > > On Wed, 29 Nov 2017 13:05:04 +1100 "Tobin C. Harding" <[email-masked]> wrote:
> > > 
> > > > printk specifier %p now hashes all addresses before printing. Sometimes
> > > > we need to see the actual unmodified address. This can be achieved using
> > > > %lx but then we face the risk that if in future we want to change the
> > > > way the Kernel handles printing of pointers we will have to grep through
> > > > the already existent 50 000 %lx call sites. Let's add specifier %px as a
> > > > clear, opt-in, way to print a pointer and maintain some level of
> > > > isolation from all the other hex integer output within the Kernel.
> > > > 
> > > > Add printk specifier %px to print the actual unmodified address.
> > > > 
> > > > ...
> > > > 
> > > > +Unmodified Addresses
> > > > +====================
> > > > +
> > > > +::
> > > > +
> > > > +	%px	01234567 or 0123456789abcdef
> > > > +
> > > > +For printing pointers when you _really_ want to print the address. Please
> > > > +consider whether or not you are leaking sensitive information about the
> > > > +Kernel layout in memory before printing pointers with %px. %px is
> > > > +functionally equivalent to %lx. %px is preferred to %lx because it is more
> > > > +uniquely grep'able. If, in the future, we need to modify the way the Kernel
> > > > +handles printing pointers it will be nice to be able to find the call
> > > > +sites.
> > > > +
> > > 
> > > You might want to add a checkpatch rule which emits a stern
> > > do-you-really-want-to-do-this warning when someone uses %px.
> > > 
> > 
> > Oh, nice idea. It has to be a CHECK but right?
> 
> No, it has to be something that's not --strict
> so a WARN would probably be best.
> 
> > By stern, you mean use stern language?
> 
> I hope he doesn't mean tweet.

/me says tweet tweet (like a bird)

> Something like:
> ---
>  scripts/checkpatch.pl | 31 +++++++++++++++++++++++++------
>  1 file changed, 25 insertions(+), 6 deletions(-)
> 
> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
> index 0ce249f157a1..9d789cbe7df5 100755
> --- a/scripts/checkpatch.pl
> +++ b/scripts/checkpatch.pl
> @@ -5758,21 +5758,40 @@ sub process {
>  		    defined $stat &&
>  		    $stat =~ /^\+(?![^\{]*\{\s*).*\b(\w+)\s*\(.*$String\s*,/s &&
>  		    $1 !~ /^_*volatile_*$/) {
> +			my $complete_extension = "";
> +			my $extension = "";
>  			my $bad_extension = "";
>  			my $lc = $stat =~ tr@\n@@;
>  			$lc = $lc + $linenr;
> +			my $stat_real;
>  		        for (my $count = $linenr; $count <= $lc; $count++) {
>  				my $fmt = get_quoted_string($lines[$count - 1], raw_line($count, 0));
>  				$fmt =~ s/%%//g;
> -				if ($fmt =~ /(\%[\*\d\.]*p(?![\WFfSsBKRraEhMmIiUDdgVCbGNO]).)/) {
> -					$bad_extension = $1;
> -					last;
> +				while ($fmt =~ /(\%[\*\d\.]*p(\w))/g) {
> +					$complete_extension = $1;
> +					$extension = $2;
> +					if ($extension !~ /[FfSsBKRraEhMmIiUDdgVCbGNOx]/) {
> +						$bad_extension = $complete_extension;
> +						last;
> +					}
> +					if ($extension eq "x") {
> +						if (!defined($stat_real)) {
> +							$stat_real = raw_line($linenr, 0);
> +							for (my $count = $linenr + 1; $count <= $lc; $count++) {
> +								$stat_real = $stat_real . "\n" . raw_line($count, 0);
> +							}
> +						}
> +						WARN("VSPRINTF_POINTER_PX",
> +						     "Using vsprintf pointer extension '$complete_extension' exposes kernel address for possible hacking\n" . "$here\n$stat_real\n");
> +					}
>  				}
>  			}
>  			if ($bad_extension ne "") {
> -				my $stat_real = raw_line($linenr, 0);
> -				for (my $count = $linenr + 1; $count <= $lc; $count++) {
> -					$stat_real = $stat_real . "\n" . raw_line($count, 0);
> +				if (!defined($stat_real)) {
> +					$stat_real = raw_line($linenr, 0);
> +					for (my $count = $linenr + 1; $count <= $lc; $count++) {
> +						$stat_real = $stat_real . "\n" . raw_line($count, 0);
> +					}
>  				}
>  				WARN("VSPRINTF_POINTER_EXTENSION",
>  				     "Invalid vsprintf pointer extension '$bad_extension'\n" . "$here\n$stat_real\n");
> 

Awesome. So moving forward, I should apply this code. Test it, commit it
with a log message stating you wrote it and I just tested it then submit
the patch, right?

thanks,
Tobin.