Re: [RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too

On 11/22/19 8:37 AM, Stephen Smalley wrote:

On 11/21/19 7:30 PM, Paul Moore wrote:

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On Thu, Nov 21, 2019 at 7:12 PM Paul Moore [off-list ref] wrote:

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On Thu, Nov 21, 2019 at 9:52 AM Stephen Smalley [off-list ref] wrote:

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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 <redacted>
---
  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@kernel.org/T/#t (local)

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.

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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.

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For the LSM folks just joining, the full patchset can be found here:
* https://lore.kernel.org/selinux/20191121145245.8637-1-sds@tycho.nsa.gov/T/#t (local)