[kernel-hardening] Re: [PATCH V11 4/5] vsprintf: add printk specifier %px

[Joe Perches <joe@perches.com>]

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" <me@tobin.cc> 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.

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");