On Tue, Jul 18, 2017 at 6:10 PM, Andy Lutomirski [off-list ref] wrote:

On Tue, Jul 18, 2017 at 3:25 PM, Kees Cook [off-list ref] wrote:

quoted

The commoncap implementation of the bprm_secureexec hook is the only LSM
that depends on the final call to its bprm_set_creds hook (since it may
be called for multiple files, it ignores bprm->called_set_creds). As a
result, it cannot safely _clear_ bprm->secureexec since other LSMs may
have set it.  Instead, remove the bprm_secureexec hook by introducing a
new flag to bprm specific to commoncap: cap_elevated. This is similar to
cap_effective, but that is used for a specific subset of elevated
privileges, and exists solely to track state from bprm_set_creds to
bprm_secureexec. As such, it will be removed in the next patch.

Here, set the new bprm->cap_elevated flag when setuid/setgid has happened
from bprm_fill_uid() or fscapabilities have been prepared. This temporarily
moves the bprm_secureexec hook to a static inline. The helper will be
removed in the next patch; this makes the step easier to review and bisect,
since this does not introduce any changes to inputs nor outputs to the
"elevated privileges" calculation.

The new flag is merged with the bprm->secureexec flag in setup_new_exec()
since this marks the end of any further prepare_binprm() calls.

Reviewed-by: Andy Lutomirski <luto@kernel.org>

with the redundant caveat that...

quoted

--- a/fs/exec.c
+++ b/fs/exec.c

@@ -1330,6 +1330,13 @@ EXPORT_SYMBOL(would_dump);

 void setup_new_exec(struct linux_binprm * bprm)
 {
+       /*
+        * Once here, prepare_binrpm() will not be called any more, so
+        * the final state of setuid/setgid/fscaps can be merged into the
+        * secureexec flag.
+        */
+       bprm->secureexec |= bprm->cap_elevated;
+

...the weird placement of the other assignments to bprm->secureexec
makes this exceedingly confusing.

Any thoughts on how I could improve this? The main take-away is that
commoncap's secureexec is special, and this was the cleanest way I
could find to deal with it...

-Kees

-- 
Kees Cook
Pixel Security
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