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

From: "Tobin C. Harding" <[email-masked]>
Date: Thu, 30 Nov 2017 16:00:34 +1100

On Wed, Nov 29, 2017 at 08:41:36PM -0800, Joe Perches wrote:
> On Thu, 2017-11-30 at 15:18 +1100, Tobin C. Harding wrote:
> > 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$stat_real\n");
> > > 
> > 
> > Awesome. So moving forward, I should apply this code. Test it,
> 
> I didn't sign it and just trivially tested it.
> 
> So test it locally, see if it doesn't work
> and check if the wording could be improved.
> 
> One possible negative is that if the format
> contains multiple %px uses, then each use is
> warned.
> 
> Maybe it should be
> 				if ($extension eq "x" && !defined($stat_real)) {
> 					...
> 					WARN("VSPRINTF_POINTER_PX", ...)
> 				}
> so that only the first %px is warned.

Ok, will do as suggested.

> If/when the %px series is applied, then this
> can go in via whatever tree.

The %px series is in Linus' mainline now. I'll get this stuff to you and
Andy for ack'ing (and LKML) soon as its done.

thanks,
Tobin.