On 11/22/19 8:37 AM, Stephen Smalley wrote:
On 11/21/19 7:30 PM, Paul Moore wrote:
On Thu, Nov 21, 2019 at 7:12 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
On Thu, Nov 21, 2019 at 9:52 AM Stephen Smalley <sds@xxxxxxxxxxxxx>
wrote:
commit bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
passed down the rcu flag to the SELinux AVC, but failed to adjust the
test in slow_avc_audit() to also return -ECHILD on
LSM_AUDIT_DATA_DENTRY.
Previously, we only returned -ECHILD if generating an audit record with
LSM_AUDIT_DATA_INODE since this was only relevant from
inode_permission.
Return -ECHILD on either LSM_AUDIT_DATA_INODE or LSM_AUDIT_DATA_DENTRY.
LSM_AUDIT_DATA_INODE only requires this handling due to the fact
that dump_common_audit_data() calls d_find_alias() and collects the
dname from the result if any.
Other cases that might require similar treatment in the future are
LSM_AUDIT_DATA_PATH and LSM_AUDIT_DATA_FILE if any hook that takes
a path or file is called under RCU-walk.
Fixes: bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
Signed-off-by: Stephen Smalley <sds@xxxxxxxxxxxxx>
---
security/selinux/avc.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/security/selinux/avc.c b/security/selinux/avc.c
index 74c43ebe34bb..f1fa1072230c 100644
--- a/security/selinux/avc.c
+++ b/security/selinux/avc.c
@@ -779,7 +779,8 @@ noinline int slow_avc_audit(struct selinux_state
*state,
* during retry. However this is logically just as if the
operation
* happened a little later.
*/
- if ((a->type == LSM_AUDIT_DATA_INODE) &&
+ if ((a->type == LSM_AUDIT_DATA_INODE ||
+ a->type == LSM_AUDIT_DATA_DENTRY) &&
(flags & MAY_NOT_BLOCK))
return -ECHILD;
With LSM_AUDIT_DATA_INODE we eventually end up calling d_find_alias()
in dump_common_audit_data() which could block, which is bad, that I
understand. However, looking at LSM_AUDIT_DATA_DENTRY I'm less clear
on why that is bad? It makes a few audit_log*() calls and one call to
d_backing_inode() which is non-blocking and trivial.
What am I missing?
For those who haven't, you may wish to also read the earlier thread here
that led to this one:
https://lore.kernel.org/selinux/20191119184057.14961-1-will@xxxxxxxxxx/T/#t
AFAIK, neither the LSM_AUDIT_DATA_INODE nor the LSM_AUDIT_DATA_DENTRY
case truly block (d_find_alias does not block AFAICT, nor should
audit_log* as long as we use audit_log_start with GFP_ATOMIC or
GFP_NOWAIT). My impression from the comment in slow_avc_audit() is that
the issue is not really about blocking but rather about the inability to
safely dereference the dentry->d_name during RCU walk, which is
something that can occur under LSM_AUDIT_DATA_INODE or _DENTRY (or _PATH
or _FILE, but neither of the latter two are currently used from the two
hooks that are called during RCU walk, inode_permission and
inode_follow_link). Originally _PATH, _DENTRY, and _INODE were all
under a single _FS type and the original test in slow_avc_audit() was
against LSM_AUDIT_DATA_FS before the split.
Added the LSM list as I'm beginning to wonder if we should push this
logic down into common_lsm_audit(), this problem around blocking
shouldn't be SELinux specific.
That would require passing down the MAY_NOT_BLOCK flag or a rcu bool to
common_lsm_audit() just so that it could immediately return if that is
set and a->type is _INODE or _DENTRY (or _PATH or _FILE). And the
individual security module still needs to have its own handling of
MAY_NOT_BLOCK/rcu for its own processing, so it won't free the security
module authors from thinking about it. This is only relevant for
modules implementing the inode_permission and/or inode_follow_link
hooks, so it only currently affects SELinux and Smack, and Smack only
presently implements inode_permission and always returns -ECHILD if
MAY_NOT_BLOCK (aside from a couple trivial cases), so it will never
reach common_lsm_audit() in that case.
This would also require changing common_lsm_audit() to be able to return
errors so that it can return -ECHILD and updating all callers to handle
that.
For the LSM folks just joining, the full patchset can be found here:
*
https://lore.kernel.org/selinux/20191121145245.8637-1-sds@xxxxxxxxxxxxx/T/#t
