smack: fix bug: setting task label silently ignores input garbage

This command:
    # echo foo/bar >/proc/$$/attr/smack/current

gives the task a label 'foo' w/o indication
that label does not match input.
Setting the label with lsm_set_self_attr() syscall
behaves identically.

This occures because:

1) smk_parse_smack() is used to convert input to a label
2) smk_parse_smack() takes only that part from the
   beginning of the input that looks like a label.
3) `/' is prohibited in labels, so only "foo" is taken.

(2) is by design, because smk_parse_smack() is used
for parsing strings which are more than just a label.

Silent failure is not a good thing, and there are two
indicators that this was not done intentionally:

    (size >= SMK_LONGLABEL) ~> invalid

clause at the beginning of the do_setattr() and the
"Returns the length of the smack label" claim
in the do_setattr() description.

So I fixed this by adding one tiny check:
the taken label length == input length.

Since input length is now strictly controlled,
I changed the two ways of setting label

   smack_setselfattr(): lsm_set_self_attr() syscall
   smack_setprocattr(): > /proc/.../current

to accommodate the divergence in
what they understand by "input length":

  smack_setselfattr counts mandatory \0 into input length,
  smack_setprocattr does not.

  smack_setprocattr allows various trailers after label

Related changes:

* fixed description for smk_parse_smack

* allow unprivileged tasks validate label syntax.

* extract smk_parse_label_len() from smk_parse_smack()
  so parsing may be done w/o string allocation.

* extract smk_import_valid_label() from smk_import_entry()
  to avoid repeated parsing.

* smk_parse_smack(): scan null-terminated strings
  for no more than SMK_LONGLABEL(256) characters

* smack_setselfattr(): require struct lsm_ctx . flags == 0
  to reserve them for future.

Fixes: e114e473771c ("Smack: Simplified Mandatory Access Control Kernel")
Signed-off-by: Konstantin Andreev <andreev@swemel.ru>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

1 files changed, 8 insertions, 3 deletions

diff --git a/Documentation/admin-guide/LSM/Smack.rst b/Documentation/admin-guide/LSM/Smack.rst
index 1b554b5bf98e..c5ed775f2d10 100644
--- a/Documentation/admin-guide/LSM/Smack.rst
+++ b/Documentation/admin-guide/LSM/Smack.rst
@@ -601,10 +601,15 @@ specification.
 Task Attribute
 ~~~~~~~~~~~~~~
 
-The Smack label of a process can be read from /proc/<pid>/attr/current. A
-process can read its own Smack label from /proc/self/attr/current. A
+The Smack label of a process can be read from ``/proc/<pid>/attr/current``. A
+process can read its own Smack label from ``/proc/self/attr/current``. A
 privileged process can change its own Smack label by writing to
-/proc/self/attr/current but not the label of another process.
+``/proc/self/attr/current`` but not the label of another process.
+
+Format of writing is : only the label or the label followed by one of the
+3 trailers: ``\n`` (by common agreement for ``/proc/...`` interfaces),
+``\0`` (because some applications incorrectly include it),
+``\n\0`` (because we think some applications may incorrectly include it).
 
 File Attribute
 ~~~~~~~~~~~~~~