Re: [PATCH v4 2/3] LSM: module hierarchy in /proc/.../attr

[Kees Cook]

On Fri, Jun 24, 2016 at 1:05 PM, Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
> On 6/24/2016 12:11 PM, Paul Moore wrote:
>> On Thu, Jun 23, 2016 at 5:11 PM, Casey Schaufler <casey@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxx>
>>>
>>> ---
>>>  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