[kernel-hardening] Re: [PATCH v3a] vsprintf: Check real user/group id for %pK

[Djalal Harouni <tixxdz@opendz.org>]

On Fri, Oct 11, 2013 at 02:19:14PM +1100, Ryan Mallon wrote:
> On 11/10/13 13:20, Eric W. Biederman wrote:
> > Joe Perches <joe@perches.com> writes:
> > 
> >> Some setuid binaries will allow reading of files which have read
> >> permission by the real user id. This is problematic with files which
> >> use %pK because the file access permission is checked at open() time,
> >> but the kptr_restrict setting is checked at read() time. If a setuid
> >> binary opens a %pK file as an unprivileged user, and then elevates
> >> permissions before reading the file, then kernel pointer values may be
> >> leaked.
> >>
> >> This happens for example with the setuid pppd application on Ubuntu
> >> 12.04:
> >>
> >>   $ head -1 /proc/kallsyms
> >>   00000000 T startup_32
> >>
> >>   $ pppd file /proc/kallsyms
> >>   pppd: In file /proc/kallsyms: unrecognized option 'c1000000'
> >>
> >> This will only leak the pointer value from the first line, but other
> >> setuid binaries may leak more information.
> >>
> >> Fix this by adding a check that in addition to the current process
> >> having CAP_SYSLOG, that effective user and group ids are equal to the
> >> real ids. If a setuid binary reads the contents of a file which uses
> >> %pK then the pointer values will be printed as NULL if the real user
> >> is unprivileged.
> >>
> >> Update the sysctl documentation to reflect the changes, and also
> >> correct the documentation to state the kptr_restrict=0 is the default.
> > 
> > Sigh.  This is all wrong.  The only correct thing to test is
> > file->f_cred.  Aka the capabilities of the program that opened the
> > file.
> > 
> > Which means that the interface to %pK in the case of kptr_restrict is
> > broken as it has no way to be passed the information it needs to make
> > a sensible decision.
> 
> Would it make sense to add a struct file * to struct seq_file and set
> that in seq_open? Then the capability check can be done against seq->file.
For the "add a struct file * to struct seq_file" and set it during
seq_open(), It was proposed by Linus, but Al Viro didn't like it:
https://lkml.org/lkml/2013/9/25/765

I'm not sure if this will work for you: you can make seq_file->private
cache some data, by calling single_open()... at ->open(), later check it
during read()...


As noted by Eric, I'll also go for the capability check at ->open(), if it
does not break some userspace. BTW the CAP_SYSLOG check should do the job

Checks during read() are not sufficient, since the design allows passing
file descriptors and dup() stdin/stdout of suid-execve.


IMO: unprivileged code should not get that file descriptor, so ->open()
should fail.
If this will break userspace then allow open() and cache result for read()


Can you emulate the behaviour of kptr_restrict=1 ? If so:
1) perform check during open() and cache data
2) during read() check kptr_restrict==1
   check the cached value and if opener had CAP_SYSLOG  if so:
   print something like this: 00000000 T startup_32

All this without modifying vsprintf, I mean just do the checks outside
vsprintf() inside your ->read()

Just my 2 cents


Please cc:me, Thanks

-- 
Djalal Harouni
http://opendz.org