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Message-ID: <CAGXu5jJNOw=5RX0YqxRxKrU6_0hsdEQ+bfr1aEFtcZGAyWNSiw@mail.gmail.com>
Date: Fri, 24 Jun 2016 13:08:48 -0700
From: Kees Cook <keescook@...omium.org>
To: Casey Schaufler <casey@...aufler-ca.com>
Cc: Paul Moore <paul@...l-moore.com>,
LSM <linux-security-module@...r.kernel.org>,
James Morris <jmorris@...ei.org>,
John Johansen <john.johansen@...onical.com>,
Stephen Smalley <sds@...ho.nsa.gov>,
Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
LKLM <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/3] LSM: module hierarchy in /proc/.../attr
On Fri, Jun 24, 2016 at 1:05 PM, Casey Schaufler <casey@...aufler-ca.com> wrote:
> On 6/24/2016 12:11 PM, Paul Moore wrote:
>> On Thu, Jun 23, 2016 at 5:11 PM, Casey Schaufler <casey@...aufler-ca.com> wrote:
>>> Subject: [PATCH v4 2/3] LSM: module hierarchy in /proc/.../attr
>>>
>>> Back in 2007 I made what turned out to be a rather serious
>>> mistake in the implementation of the Smack security module.
>>> The SELinux module used an interface in /proc to manipulate
>>> the security context on processes. Rather than use a similar
>>> interface, I used the same interface. The AppArmor team did
>>> likewise. Now /proc/.../attr/current will tell you the
>>> security "context" of the process, but it will be different
>>> depending on the security module you're using. That hasn't
>>> been a problem to date, as you can only have one module
>>> that supports process attributes at a time. We are coming
>>> up on a change to that, where multiple modules with process
>>> attributes can be supported. (Not included here)
>>>
>>> This patch provides a subdirectory in /proc/.../attr for
>>> each of the security modules that use the LSM hooks
>>> getprocattr() and setprocattr(). Each of the interfaces
>>> used by a module are presented in the subdirectory. The
>>> old interfaces remain and work the same as before.
>>> User space code can begin migrating to the subdirectory
>>> interfaces in anticipation of the time when what comes
>>> from /proc/self/attr/current might not be what a runtime
>>> wants.
>>>
>>> The original implementation is by Kees Cook. The code
>>> has been changed a bit to reflect changes in the direction
>>> of the multiple concurrent module work, to be independent
>>> of it, and to bring it up to date with the current tree.
>>>
>>> Signed-off-by: Casey Schaufler <casey@...aufler-ca.com>
>>>
>>> ---
>>> Documentation/security/LSM.txt | 26 +++++++++---
>>> fs/proc/base.c | 91 +++++++++++++++++++++++++++++++++++++-----
>>> fs/proc/internal.h | 1 +
>>> include/linux/security.h | 15 ++++---
>>> security/security.c | 31 ++++++++++++--
>>> 5 files changed, 140 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/Documentation/security/LSM.txt b/Documentation/security/LSM.txt
>>> index 3db7e67..125c489 100644
>>> --- a/Documentation/security/LSM.txt
>>> +++ b/Documentation/security/LSM.txt
>>> @@ -16,11 +16,25 @@ MAC extensions, other extensions can be built using the LSM to provide
>>> specific changes to system operation when these tweaks are not available
>>> in the core functionality of Linux itself.
>>>
>>> -Without a specific LSM built into the kernel, the default LSM will be the
>>> -Linux capabilities system. Most LSMs choose to extend the capabilities
>>> -system, building their checks on top of the defined capability hooks.
>>> -For more details on capabilities, see capabilities(7) in the Linux
>>> -man-pages project.
>>> +The Linux capabilities modules will always be included. For more details
>>> +on capabilities, see capabilities(7) in the Linux man-pages project.
>>> +This may be followed by any number of "minor" modules and at most one
>>> +"major" module.
>>> +
>>> +A list of the active security modules can be found by reading
>>> +/sys/kernel/security/lsm. This is a comma separated list, and
>>> +will always include the capability module. The list reflects the
>>> +order in which checks are made. The capability module will always
>>> +be first, followed by any "minor" modules (e.g. Yama) and then
>>> +the one "major" module (e.g. SELinux) if there is one configured.
>> I wouldn't respin it just for this, but it seems like the paragraph
>> above should really be part of patch 1/3, yes?
>
> Yes. I can fix that for v5.
>
>>> +Process attributes associated with "ma
>>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>>> index a11eb71..182bc28 100644
>>> --- a/fs/proc/base.c
>>> +++ b/fs/proc/base.c
>>> @@ -131,9 +131,13 @@ struct pid_entry {
>>> #define REG(NAME, MODE, fops) \
>>> NOD(NAME, (S_IFREG|(MODE)), NULL, &fops, {})
>>> #define ONE(NAME, MODE, show) \
>>> - NOD(NAME, (S_IFREG|(MODE)), \
>>> + NOD(NAME, (S_IFREG|(MODE)), \
>>> NULL, &proc_single_file_operations, \
>>> { .proc_show = show } )
>>> +#define ATTR(LSM, NAME, MODE) \
>>> + NOD(NAME, (S_IFREG|(MODE)), \
>>> + NULL, &proc_pid_attr_operations, \
>>> + { .lsm = LSM })
>>>
>>> /*
>>> * Count the number of hardlinks for the pid_entry table, excluding the .
>>> @@ -2433,7 +2437,7 @@ static ssize_t proc_pid_attr_read(struct file * file, char __user * buf,
>>> if (!task)
>>> return -ESRCH;
>>>
>>> - length = security_getprocattr(task,
>>> + length = security_getprocattr(task, PROC_I(inode)->op.lsm,
>>> (char*)file->f_path.dentry->d_name.name,
>>> &p);
>>> put_task_struct(task);
>>> @@ -2473,7 +2477,7 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
>>> if (length < 0)
>>> goto out_free;
>>>
>>> - length = security_setprocattr(task,
>>> + length = security_setprocattr(task, PROC_I(inode)->op.lsm,
>>> (char*)file->f_path.dentry->d_name.name,
>>> page, count);
>>> mutex_unlock(&task->signal->cred_guard_mutex);
>>> @@ -2491,13 +2495,82 @@ static const struct file_operations proc_pid_attr_operations = {
>>> .llseek = generic_file_llseek,
>>> };
>>>
>>> +#define LSM_DIR_OPS(LSM) \
>>> +static int proc_##LSM##_attr_dir_iterate(struct file *filp, \
>>> + struct dir_context *ctx) \
>>> +{ \
>>> + return proc_pident_readdir(filp, ctx, \
>>> + LSM##_attr_dir_stuff, \
>>> + ARRAY_SIZE(LSM##_attr_dir_stuff)); \
>>> +} \
>>> +\
>>> +static const struct file_operations proc_##LSM##_attr_dir_ops = { \
>>> + .read = generic_read_dir, \
>>> + .iterate = proc_##LSM##_attr_dir_iterate, \
>>> + .llseek = default_llseek, \
>>> +}; \
>>> +\
>>> +static struct dentry *proc_##LSM##_attr_dir_lookup(struct inode *dir, \
>>> + struct dentry *dentry, unsigned int flags) \
>>> +{ \
>>> + return proc_pident_lookup(dir, dentry, \
>>> + LSM##_attr_dir_stuff, \
>>> + ARRAY_SIZE(LSM##_attr_dir_stuff)); \
>>> +} \
>>> +\
>>> +static const struct inode_operations proc_##LSM##_attr_dir_inode_ops = { \
>>> + .lookup = proc_##LSM##_attr_dir_lookup, \
>>> + .getattr = pid_getattr, \
>>> + .setattr = proc_setattr, \
>>> +}
>>> +
>>> +#ifdef CONFIG_SECURITY_SELINUX
>>> +static const struct pid_entry selinux_attr_dir_stuff[] = {
>>> + ATTR("selinux", "current", S_IRUGO|S_IWUGO),
>>> + ATTR("selinux", "prev", S_IRUGO),
>>> + ATTR("selinux", "exec", S_IRUGO|S_IWUGO),
>>> + ATTR("selinux", "fscreate", S_IRUGO|S_IWUGO),
>>> + ATTR("selinux", "keycreate", S_IRUGO|S_IWUGO),
>>> + ATTR("selinux", "sockcreate", S_IRUGO|S_IWUGO),
>>> +};
>>> +LSM_DIR_OPS(selinux);
>>> +#endif
>>> +
>>> +#ifdef CONFIG_SECURITY_SMACK
>>> +static const struct pid_entry smack_attr_dir_stuff[] = {
>>> + ATTR("smack", "current", S_IRUGO|S_IWUGO),
>>> +};
>>> +LSM_DIR_OPS(smack);
>>> +#endif
>>> +
>>> +#ifdef CONFIG_SECURITY_APPARMOR
>>> +static const struct pid_entry apparmor_attr_dir_stuff[] = {
>>> + ATTR("apparmor", "current", S_IRUGO|S_IWUGO),
>>> + ATTR("apparmor", "prev", S_IRUGO),
>>> + ATTR("apparmor", "exec", S_IRUGO|S_IWUGO),
>>> +};
>>> +LSM_DIR_OPS(apparmor);
>>> +#endif
>>> +
>>> static const struct pid_entry attr_dir_stuff[] = {
>>> - REG("current", S_IRUGO|S_IWUGO, proc_pid_attr_operations),
>>> - REG("prev", S_IRUGO, proc_pid_attr_operations),
>>> - REG("exec", S_IRUGO|S_IWUGO, proc_pid_attr_operations),
>>> - REG("fscreate", S_IRUGO|S_IWUGO, proc_pid_attr_operations),
>>> - REG("keycreate", S_IRUGO|S_IWUGO, proc_pid_attr_operations),
>>> - REG("sockcreate", S_IRUGO|S_IWUGO, proc_pid_attr_operations),
>>> + ATTR(NULL, "current", S_IRUGO|S_IWUGO),
>>> + ATTR(NULL, "prev", S_IRUGO),
>>> + ATTR(NULL, "exec", S_IRUGO|S_IWUGO),
>>> + ATTR(NULL, "fscreate", S_IRUGO|S_IWUGO),
>>> + ATTR(NULL, "keycreate", S_IRUGO|S_IWUGO),
>>> + ATTR(NULL, "sockcreate", S_IRUGO|S_IWUGO),
>>> +#ifdef CONFIG_SECURITY_SELINUX
>>> + DIR("selinux", S_IRUGO|S_IXUGO,
>>> + proc_selinux_attr_dir_inode_ops, proc_selinux_attr_dir_ops),
>>> +#endif
>>> +#ifdef CONFIG_SECURITY_SMACK
>>> + DIR("smack", S_IRUGO|S_IXUGO,
>>> + proc_smack_attr_dir_inode_ops, proc_smack_attr_dir_ops),
>>> +#endif
>>> +#ifdef CONFIG_SECURITY_APPARMOR
>>> + DIR("apparmor", S_IRUGO|S_IXUGO,
>>> + proc_apparmor_attr_dir_inode_ops, proc_apparmor_attr_dir_ops),
>>> +#endif
>>> };
>> With the number of LSMs set to grow, it seems like it might be a lot
>> cleaner, and easier to maintain, if we moved the various LSM pid_entry
>> definitions into the LSMs themselves. Granted, I say this without
>> seriously looking at how one would do that, I'm just mentioning it
>> here; it may prove to be more hassle than it is worth.
I had the same suggestion, and when I looked at what it would take, I
decided this was just fine. ;)
> I have looked into doing it that way, but have yet to
> come up with anything that would work. It seems like a
> wonderful challenge for a young, nimble brain. Or maybe
> an old wise one. In neither case, mine.
I think it would require creating a number of new APIs to the proc
interface, and none of it looked fun.
-Kees
--
Kees Cook
Chrome OS & Brillo Security
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