Re: [PATCH 26/26] x86, pkeys: Documentation

[Ingo Molnar <mingo@kernel.org>]

* Dave Hansen <dave@sr71.net> wrote:

> On 10/01/2015 01:39 PM, Kees Cook wrote:
> > On Thu, Oct 1, 2015 at 4:17 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >> So could we try to add an (opt-in) kernel option that enables this transparently
> >> and automatically for all PROT_EXEC && !PROT_WRITE mappings, without any
> >> user-space changes and syscalls necessary?
> > 
> > I would like this very much. :)
> 
> Here it is in a quite fugly form (well, it's not opt-in).  Init crashes
> if I boot with this, though.
> 
> I'll see if I can turn it in to a bit more of an opt-in and see what's
> actually going wrong.

So the reality of modern Linux distros is that, according to some limited 
strace-ing around, pure PROT_EXEC usage does not seem to exist: 99% of executable 
mappings are mapped via PROT_EXEC|PROT_READ.

So the most usable kernel testing approach would be to enable these types of pkeys 
for a child task via some mechanism and inherit it to all children (including 
inheriting it over non-suid exec) - but not to any other task.

You could hijack a new personality bit just for debug purposes - see the (totally 
untested) patch below.

Depending on user-space's assumptions it might not end up being anything usable we 
can apply, but it would be a great testing tool if it worked to a certain degree.

I.e. allow the system to boot in without pkeys set for any task, then set the 
personality of a shell process to PER_LINUX_PKEYS and see which binaries (if any!) 
will start up without segfaulting.

This way you don't have to debug SystemD, which is extremely fragile and 
passive-aggressive towards kernels that don't behave in precisely the fashion 
under which SystemD is being developed.

Thanks,

	Ingo

========>

Absolutely-Not-Signed-off-by: Ingo Molnar <mingo@kernel.org>

 include/uapi/linux/personality.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/uapi/linux/personality.h b/include/uapi/linux/personality.h
index aa169c4339d2..bead47213419 100644
--- a/include/uapi/linux/personality.h
+++ b/include/uapi/linux/personality.h
@@ -8,6 +8,7 @@
  * These occupy the top three bytes.
  */
 enum {
+	PROT_READ_EXEC_HACK =	0x0010000,	/* PROT_READ|PROT_EXEC == PROT_EXEC hack */
 	UNAME26	=               0x0020000,
 	ADDR_NO_RANDOMIZE = 	0x0040000,	/* disable randomization of VA space */
 	FDPIC_FUNCPTRS =	0x0080000,	/* userspace function ptrs point to descriptors
@@ -41,6 +42,7 @@ enum {
 enum {
 	PER_LINUX =		0x0000,
 	PER_LINUX_32BIT =	0x0000 | ADDR_LIMIT_32BIT,
+	PER_LINUX_PKEYS =	0x0000 | PROT_READ_EXEC_HACK,
 	PER_LINUX_FDPIC =	0x0000 | FDPIC_FUNCPTRS,
 	PER_SVR4 =		0x0001 | STICKY_TIMEOUTS | MMAP_PAGE_ZERO,
 	PER_SVR3 =		0x0002 | STICKY_TIMEOUTS | SHORT_INODE,

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