linux-kernel - Re: [PATCH v2 2/9] procfs: add proc_allow_access() to check if file's opener may access task

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Date:	Wed, 2 Oct 2013 17:44:17 +0100
From:	Andy Lutomirski <luto@...capital.net>
To:	Djalal Harouni <tixxdz@...ndz.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Kees Cook <keescook@...omium.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	David Rientjes <rientjes@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	kernel-hardening@...ts.openwall.com, tixxdz@...il.com
Subject: Re: [PATCH v2 2/9] procfs: add proc_allow_access() to check if file's
 opener may access task

On Wed, Oct 2, 2013 at 3:55 PM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> On Tue, Oct 01, 2013 at 06:36:34PM -0700, Andy Lutomirski wrote:
>> On 10/01/2013 01:26 PM, Djalal Harouni wrote:
>> > Since /proc entries varies at runtime, permission checks need to happen
>> > during each system call.
>> >
>> > However even with that /proc file descriptors can be passed to a more
>> > privileged process (e.g. a suid-exec) which will pass the classic
>> > ptrace_may_access() permission check. The open() call will be issued in
>> > general by an unprivileged process while the disclosure of sensitive
>> > /proc information will happen using a more privileged process at
>> > read(),write()...
>> >
>> > Therfore we need a more sophisticated check to detect if the cred of the
>> > process have changed, and if the cred of the original opener that are
>> > stored in the file->f_cred have enough permission to access the task's
>> > /proc entries during read(), write()...
>> >
>> > Add the proc_allow_access() function that will receive the file->f_cred
>> > as an argument, and tries to check if the opener had enough permission
>> > to access the task's /proc entries.
>> >
>> > This function should be used with the ptrace_may_access() check.
>> >
>> > Cc: Kees Cook <keescook@...omium.org>
>> > Suggested-by: Eric W. Biederman <ebiederm@...ssion.com>
>> > Signed-off-by: Djalal Harouni <tixxdz@...ndz.org>
>> > ---
>> >  fs/proc/base.c     | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>> >  fs/proc/internal.h |  2 ++
>> >  2 files changed, 58 insertions(+)
>> >
>> > diff --git a/fs/proc/base.c b/fs/proc/base.c
>> > index e834946..c29eeae 100644
>> > --- a/fs/proc/base.c
>> > +++ b/fs/proc/base.c
>> > @@ -168,6 +168,62 @@ int proc_same_open_cred(const struct cred *fcred)
>> >             cap_issubset(cred->cap_permitted, fcred->cap_permitted));
>> >  }
>> >
>> > +/* Returns 0 on success, -errno on denial. */
>> > +static int __proc_allow_access(const struct cred *cred,
>> > +                          struct task_struct *task, unsigned int mode)
>> > +{
>> > +   int ret = 0;
>> > +   const struct cred *tcred;
>> > +   const struct cred *fcred = cred;
>> > +
>> > +   rcu_read_lock();
>> > +   tcred = __task_cred(task);
>> > +   if (uid_eq(fcred->uid, tcred->euid) &&
>> > +       uid_eq(fcred->uid, tcred->suid) &&
>> > +       uid_eq(fcred->uid, tcred->uid)  &&
>> > +       gid_eq(fcred->gid, tcred->egid) &&
>> > +       gid_eq(fcred->gid, tcred->sgid) &&
>> > +       gid_eq(fcred->gid, tcred->gid))
>> > +           goto out;
>> > +
>>
>> What's this for?  Is it supposed to be an optimization?  If so, it looks
>> potentially exploitable, although I don't really understand what you're
>> trying to do.
> This function should be used in addition to the ptrace_may_access() one.

Sorry, I was unclear.  I meant: what are the uid and gid checks for?
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