On 2/1/2024 3:52 PM, Paul Moore wrote:
> On Tue, Jan 30, 2024 at 9:19 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>> I'll come back to this tomorrow with some fresh eyes.
> My apologies, "tomorrow" turned into "the day after tomorrow" (as it
> often does) ...
>
> I've been struggling with the idea that there are individual LSMs
> still calling into the capability hooks instead of leveraging the LSM
> stacking infrastructure, and the "magic" involved to make it all work.
> While your patch looks like it should restore proper behavior - that's
> good! - I keep thinking that we can, and should, do better.
Apology for attaching a patch rather than inlining it.
I've attached patch #38 from the current stacking set.
It addresses the issue.
>
> The only thing that I coming up with is to create two new LSM hooks,
> in addition to the existing 'inode_setxattr' hook. The new LSM hooks
> would be 'inode_setxattr_owned' and 'inode_setxattr_cap'. The _owned
> hook would simply check the xattr name and return a positive value if
> the LSM "owned" the xattr, e.g. XATTR_NAME_SELINUX for SELinux, and
> zero otherwise. The _cap hook would only be used by the capabilities
> code (or something similar), and would match up with
> cap_inode_setxattr(). With these two new hooks I think we could do
> something like this:
>
> int security_inode_setxattr(...)
> {
> owned = false
> hook_loop(inode_setxattr_owned) {
> trc = hook->inode_setxattr_owned(name);
> if (trc > 0) {
> owned = true;
> break;
> }
> }
> if (owned) {
> hook_loop(inode_setxattr) {
> /* run the existing inode_setxattr hooks, e.g. SELinux and Smack */
> }
> } else {
> hook_loop(inode_setxattr_cap) {
> /* run the capability setxattr hooks, e.g. commoncap.c */
> }
> }
> }
>
> .. with security_inode_removexattr() following a similar pattern.
>
> I will admit that there is some duplication in having to check the
> xattr twice (once in _owned, again in inode_setxattr), and the
> multiple hook approach is less than ideal, but this seems much less
> fragile to me.
>
> Thoughts?
>From 644ac239cbbdee3d4fc3ba0173c85b34382670c6 Mon Sep 17 00:00:00 2001
From: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
Date: Thu, 26 Oct 2023 12:52:55 -0700
Subject: [PATCH v39 38/42] LSM: Correct handling of ENOSYS in inode_setxattr
The usual "bail on fail" behavior of LSM hooks doesn't
work for security_inode_setxattr(). Modules are allowed
to return -ENOSYS if the attribute specified isn't one
they manage. Fix the code to accommodate this unusal case.
This requires changes to the hooks in SELinux and Smack.
Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
---
security/security.c | 29 +++++++++++++++--------------
security/selinux/hooks.c | 7 ++-----
security/smack/smack_lsm.c | 10 +++++-----
3 files changed, 22 insertions(+), 24 deletions(-)
diff --git a/security/security.c b/security/security.c
index 64cdf0e09832..b1a849e8589c 100644
--- a/security/security.c
+++ b/security/security.c
@@ -2346,24 +2346,25 @@ int security_inode_setxattr(struct mnt_idmap *idmap,
struct dentry *dentry, const char *name,
const void *value, size_t size, int flags)
{
- int ret;
+ struct security_hook_list *hp;
+ int rc = -ENOSYS;
if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
return 0;
- /*
- * SELinux and Smack integrate the cap call,
- * so assume that all LSMs supplying this call do so.
- */
- ret = call_int_hook(inode_setxattr, 1, idmap, dentry, name, value,
- size, flags);
- if (ret == 1)
- ret = cap_inode_setxattr(dentry, name, value, size, flags);
- if (ret)
- return ret;
- ret = ima_inode_setxattr(dentry, name, value, size);
- if (ret)
- return ret;
+ hlist_for_each_entry(hp, &security_hook_heads.inode_setxattr, list) {
+ rc = hp->hook.inode_setxattr(idmap, dentry, name, value, size,
+ flags);
+ if (rc != -ENOSYS)
+ break;
+ }
+ if (rc == -ENOSYS)
+ rc = cap_inode_setxattr(dentry, name, value, size, flags);
+ if (rc)
+ return rc;
+ rc = ima_inode_setxattr(dentry, name, value, size);
+ if (rc)
+ return rc;
return evm_inode_setxattr(idmap, dentry, name, value, size);
}
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 46dee63eec12..4ac4b536c568 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3207,13 +3207,10 @@ static int selinux_inode_setxattr(struct mnt_idmap *idmap,
int rc = 0;
if (strcmp(name, XATTR_NAME_SELINUX)) {
- rc = cap_inode_setxattr(dentry, name, value, size, flags);
- if (rc)
- return rc;
-
/* Not an attribute we recognize, so just check the
ordinary setattr permission. */
- return dentry_has_perm(current_cred(), dentry, FILE__SETATTR);
+ rc = dentry_has_perm(current_cred(), dentry, FILE__SETATTR);
+ return rc ? rc : -ENOSYS;
}
if (!selinux_initialized())
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 61bd3f626e7d..02b9aa200ad4 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -1340,7 +1340,7 @@ static int smack_inode_setxattr(struct mnt_idmap *idmap,
strncmp(value, TRANS_TRUE, TRANS_TRUE_SIZE) != 0)
rc = -EINVAL;
} else
- rc = cap_inode_setxattr(dentry, name, value, size, flags);
+ rc = -ENOSYS;
if (check_priv && !smack_privileged(CAP_MAC_ADMIN))
rc = -EPERM;
@@ -1354,11 +1354,11 @@ static int smack_inode_setxattr(struct mnt_idmap *idmap,
rc = -EINVAL;
}
- smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_DENTRY);
- smk_ad_setfield_u_fs_path_dentry(&ad, dentry);
-
if (rc == 0) {
- rc = smk_curacc(smk_of_inode(d_backing_inode(dentry)), MAY_WRITE, &ad);
+ smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_DENTRY);
+ smk_ad_setfield_u_fs_path_dentry(&ad, dentry);
+ rc = smk_curacc(smk_of_inode(d_backing_inode(dentry)),
+ MAY_WRITE, &ad);
rc = smk_bu_inode(d_backing_inode(dentry), MAY_WRITE, rc);
}
--
2.41.0
