[PATCH] selinux: Perform both commoncap and selinux xattr checks

[Eric W. Biederman]

Paul Moore <paul at paul-moore.com> writes:

> On Mon, Oct 2, 2017 at 10:38 AM, Eric W. Biederman
> <ebiederm at xmission.com> wrote:
>>
>> When selinux is loaded the relax permission checks for writing
>> security.capable are not honored.  Which keeps file capabilities
>> from being used in user namespaces.
>>
>> Stephen Smalley <sds at tycho.nsa.gov> writes:
>>> Originally SELinux called the cap functions directly since there was no
>>> stacking support in the infrastructure and one had to manually stack a
>>> secondary module internally.  inode_setxattr and inode_removexattr
>>> however were special cases because the cap functions would check
>>> CAP_SYS_ADMIN for any non-capability attributes in the security.*
>>> namespace, and we don't want to impose that requirement on setting
>>> security.selinux.  Thus, we inlined the capabilities logic into the
>>> selinux hook functions and adapted it appropriately.
>>
>> Now that the permission checks in commoncap have evolved this
>> inlining of their contents has become a problem.  So restructure
>> selinux_inode_removexattr, and selinux_inode_setxattr to call
>> both the corresponding cap_inode_ function and dentry_has_perm
>> when the attribute is not a selinux security xattr.   This ensures
>> the policies of both commoncap and selinux are enforced.
>>
>> This results in smack and selinux having the same basic structure
>> for setxattr and removexattr.  Performing their own special permission
>> checks when it is their modules xattr being written to, and deferring
>> to commoncap when that is not the case.  Then finally performing their
>> generic module policy on all xattr writes.
>>
>> This structure is fine when you only consider stacking with the
>> commoncap lsm, but it becomes a problem if two lsms that don't want
>> the commoncap security checks on their own attributes need to be
>> stack.  This means there will need to be updates in the future as lsm
>> stacking is improved, but at least now the structure between smack and
>> selinux is common making the code easier to refactor.
>>
>> This change also has the effect that selinux_linux_setotherxattr becomes
>> unnecessary so it is removed.
>>
>> Fixes: 8db6c34f1dbc ("Introduce v3 namespaced file capabilities")
>> Fixes: 7bbf0e052b76 ("[PATCH] selinux merge")
>> Historical Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
>> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
>> ---
>>
>> While this fixes some things this isn't a regression so it should be
>> able to wait until the next merge window to be merged.   Would you like
>> to take this through the selinux tree?  Or shall I take it through mine?
>>
>> security/selinux/hooks.c | 43 ++++++++++++++++++-------------------------
>>  1 file changed, 18 insertions(+), 25 deletions(-)
>
> This patch looks sane to me and I believe it addresses Stephen's
> concerns, but I'll wait on him to comment on-list.

He has alredy acked this publicly.

I may have skipped Cc'ing the selinux list as the discussion was
originally more general and the selinux list is reported to be
subscribers (not me) only.

> As far as how this gets merged, I'd much prefer to take this via the
> SELinux tree given that all of the changes are internal to SELinux.

Sounds good.  I just care that it get's picked up.

Eric


>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>> index f5d304736852..c78dbec627f6 100644
>> --- a/security/selinux/hooks.c
>> +++ b/security/selinux/hooks.c
>> @@ -3124,27 +3124,6 @@ static int selinux_inode_getattr(const struct path *path)
>>         return path_has_perm(current_cred(), path, FILE__GETATTR);
>>  }
>>
>> -static int selinux_inode_setotherxattr(struct dentry *dentry, const char *name)
>> -{
>> -       const struct cred *cred = current_cred();
>> -
>> -       if (!strncmp(name, XATTR_SECURITY_PREFIX,
>> -                    sizeof XATTR_SECURITY_PREFIX - 1)) {
>> -               if (!strcmp(name, XATTR_NAME_CAPS)) {
>> -                       if (!capable(CAP_SETFCAP))
>> -                               return -EPERM;
>> -               } else if (!capable(CAP_SYS_ADMIN)) {
>> -                       /* A different attribute in the security namespace.
>> -                          Restrict to administrator. */
>> -                       return -EPERM;
>> -               }
>> -       }
>> -
>> -       /* Not an attribute we recognize, so just check the
>> -          ordinary setattr permission. */
>> -       return dentry_has_perm(cred, dentry, FILE__SETATTR);
>> -}
>> -
>>  static bool has_cap_mac_admin(bool audit)
>>  {
>>         const struct cred *cred = current_cred();
>> @@ -3167,8 +3146,15 @@ static int selinux_inode_setxattr(struct dentry *dentry, const char *name,
>>         u32 newsid, sid = current_sid();
>>         int rc = 0;
>>
>> -       if (strcmp(name, XATTR_NAME_SELINUX))
>> -               return selinux_inode_setotherxattr(dentry, name);
>> +       if (strcmp(name, XATTR_NAME_SELINUX)) {
>> +               rc = cap_inode_setxattr(dentry, name, value, size, flags);
>> +               if (rc)
>> +                       return rc;
>> +
>> +               /* Not an attribute we recognize, so just check the
>> +                  ordinary setattr permission. */
>> +               return dentry_has_perm(current_cred(), dentry, FILE__SETATTR);
>> +       }
>>
>>         sbsec = inode->i_sb->s_security;
>>         if (!(sbsec->flags & SBLABEL_MNT))
>> @@ -3282,8 +3268,15 @@ static int selinux_inode_listxattr(struct dentry *dentry)
>>
>>  static int selinux_inode_removexattr(struct dentry *dentry, const char *name)
>>  {
>> -       if (strcmp(name, XATTR_NAME_SELINUX))
>> -               return selinux_inode_setotherxattr(dentry, name);
>> +       if (strcmp(name, XATTR_NAME_SELINUX)) {
>> +               int rc = cap_inode_removexattr(dentry, name);
>> +               if (rc)
>> +                       return rc;
>> +
>> +               /* Not an attribute we recognize, so just check the
>> +                  ordinary setattr permission. */
>> +               return dentry_has_perm(current_cred(), dentry, FILE__SETATTR);
>> +       }
>>
>>         /* No one is allowed to remove a SELinux security label.
>>            You can change the label, but all data must be labeled. */
>> --
>> 2.14.1
>>
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