On 11/20/19 8:13 AM, Will Deacon wrote:
Hi Stephen,
Thanks for the quick review.
On Tue, Nov 19, 2019 at 01:46:37PM -0500, Stephen Smalley wrote:
On 11/19/19 1:40 PM, Will Deacon wrote:
'selinux_inode_follow_link()' can be called as part of an RCU path walk,
and is passed a 'bool rcu' parameter to indicate whether or not it is
being called from within an RCU read-side critical section.
Unfortunately, this knowledge is not propagated further and, instead,
'avc_has_perm()' unconditionally passes a flags argument of '0' to both
'avc_has_perm_noaudit()' and 'avc_audit()' which may block.
Introduce 'avc_has_perm_flags()' which can be used safely from within an
RCU read-side critical section.
Please see e46e01eebbbcf2ff6d28ee7cae9f117e9d1572c8 ("selinux: stop passing
MAY_NOT_BLOCK to the AVC upon follow_link").
Ha, not sure how I missed that -- my patch is almost a direct revert,
including the name 'avs_has_perm_flags()'! My only concern is that the
commit message for e46e01eebbbc asserts that the only use of MAY_NOT_BLOCK
is in slow_avc_audit(), but AVC_NONBLOCKING is used more widely than that.
For example:
selinux_inode_follow_link()
-> avc_has_perm()
-> avc_has_perm_noaudit()
-> avc_denied()
-> avc_update_node()
where we return early if AVC_NONBLOCKING is set, except flags are always
zero on this path.
That was introduced by 3a28cff3bd4bf43f02be0c4e7933aebf3dc8197e
("selinux: avoid silent denials in permissive mode under RCU walk") and
is only needed if we have to pass MAY_NOT_BLOCK to slow_avc_audit(),
which is only presently needed in the selinux_inode_permission() case
AFAICT. Both AVC_NONBLOCKING and MAY_NOT_BLOCK are misnomers wrt the
AVC since it should never block regardless; the issue IIUC was rather
the inability to safely collect the dentry name in an audit message
during RCU walk per commit 0dc1ba24f7fff659725eecbba2c9ad679a0954cd ("
SELINUX: Make selinux cache VFS RCU walks safe").
I'm not 100% certain about this; it is possible that the test in
slow_avc_audit() is wrong and we ought to be doing this for any of
LSM_AUDIT_DATA_PATH, _DENTRY, or _INODE (these were split out of
LSM_AUDIT_DATA_FS). In that case, we should revert my earlier commit
for follow_link and fix the test inside of slow_avc_audit() instead.
I cc'd some additional folks who may have insight. Al, tell us if we
got it wrong please!
