Re: Wrong module .text address in 4.16.0

[Jessica Yu]

+++ Christian Borntraeger [16/04/18 12:53 +0200]:
> Can this be related to
> commit ef0010a30935de4e0211cbc7bdffc30446cdee9b
>    vsprintf: don't use 'restricted_pointer()' when not restricting
> and related commits?
>
> To me it looks like %pk is always printing the hash, but never the real pointer -
> no matter what kernel.kptr_restrict says.

(Added Kees and Tobin to CC)

Can confirm, a git bisect confirms that ef0010a3093 was the first bad commit.

And yeah, the default seems to be to always hash the pointer address now,
regardless of kptr_restrict. See Documentation/sysctl/kernel.txt:

   When kptr_restrict is set to 0 (the default) the address is hashed before
   printing. (This is the equivalent to %p.)

And to quote from the relevant patchset (https://lkml.org/lkml/2017/11/28/1593):

  The added advantage of hashing %p is that security is now opt-out, if
  you _really_ want the address you have to work a little harder and use %px.

So for users of /sys/module/*/sections, we will need to work around
this and possibly use %px for the real address. But perhaps we should
base the usage of %px on kptr_restrict? That is what m_show() in
module.c currently does, which is why you get the correct address when
you look at /proc/modules (it uses %px and either shows 0's or the
real address based on kallsyms_show_value()). module_sect_show() uses
%pK so it's getting hashed. Is there a better way of doing this?

Jessica

> On 04/16/2018 08:23 AM, Christian Borntraeger wrote:
> > FWIW, this breaks at least perf capability to resolve module symbols.
> > Adding some more CCs for perf and module.
> >
> >
> > On 04/16/2018 07:51 AM, Thomas-Mich Richter wrote:
> > > I just installed 4.16.0 and discovered the module .text address is
> > > wrong. It happens on s390 and x86 platforms. I have not tested others.
> > >
> > > Here is the issue, I have used module qeth_l2 on s390 which is the
> > > ethernet device driver:
> > >
> > > root@s35lp76 ~]# lsmod
> > > Module                  Size  Used by
> > > qeth_l2                94208  1
> > > ...
> > >
> > > [root@s35lp76 ~]# cat /proc/modules | egrep '^qeth_l2'
> > > qeth_l2 94208 1 - Live 0x000003ff80401000   <---- This is the correct address in memory
> > > [root@s35lp76 ~]# cat /sys/module/qeth_l2/sections/.text
> > > 0x0000000018ea8363      <---- This is the wrong address
> > > [root@s35lp76 ~]#
> > >
> > > File /sys/module/qeth_l2/sections/.text displays a very strange
> > > address which is definitely wrong. It should be something like
> > > 0x000003ff80401xxx.
> > >
> > > Same on x86.
> > >
> > > I have checked file kernel/module.c function add_sect_attrs()
> > > and it calls module_sect_show() when the sysfs file is read.
> > > And module_sect_show() uses
> > >
> > >   sprintf(buf, "0x%pK\n", (void *)sattr->address);
> > >
> > > and my sysctl setting should be correct:
> > > [root@s35lp76 linux]# sysctl -a | fgrep kernel.kptr_restrict
> > > kernel.kptr_restrict = 0
> > > [root@s35lp76 linux]#
> > >
> > > I wonder if somebody else has seen this issue?
> > > Ideas how to fix this?
> > >
> > > Thanks