The uapi POSIX ACL struct passed through the value argument during
setxattr() contains {g,u}id values encoded via ACL_{GROUP,USER} entries
that should actually be stored in the form of k{g,u}id_t (See [1] for a
long explanation of the issue.).
In 0c5fd887d2bb ("acl: move idmapped mount fixup into vfs_{g,s}etxattr()")
we took the mount's idmapping into account in order to let overlayfs
handle POSIX ACLs on idmapped layers correctly. The fixup is currently
performed directly in vfs_setxattr() which piles on top of the earlier
hackiness by handling the mount's idmapping and stuff the vfs{g,u}id_t
values into the uapi struct as well. While that is all correct and works
fine it's just ugly.
Now that we have introduced vfs_make_posix_acl() earlier move handling
idmapped mounts out of vfs_setxattr() and into the POSIX ACL handler
where it belongs.
Note that we also need to call vfs_make_posix_acl() for EVM which
interpretes POSIX ACLs during security_inode_setxattr(). Leave them a
longer comment for future reference.
All filesystems that support idmapped mounts via FS_ALLOW_IDMAP use the
standard POSIX ACL xattr handlers and are covered by this change. This
includes overlayfs which simply calls vfs_{g,s}etxattr().
The following filesystems use custom POSIX ACL xattr handlers: 9p, cifs,
ecryptfs, and ntfs3 (and overlayfs but we've covered that in the paragraph
above) and none of them support idmapped mounts yet.
Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@xxxxxxxxxx/ [1]
Signed-off-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>
---
fs/posix_acl.c | 52 +++++++------------------------
fs/xattr.c | 3 --
security/integrity/evm/evm_main.c | 17 ++++++++--
3 files changed, 25 insertions(+), 47 deletions(-)
diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index 31eac28e6582..c759b8eef62e 100644
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c
@@ -771,46 +771,6 @@ void posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
}
}
-void posix_acl_setxattr_idmapped_mnt(struct user_namespace *mnt_userns,
- const struct inode *inode,
- void *value, size_t size)
-{
- struct posix_acl_xattr_header *header = value;
- struct posix_acl_xattr_entry *entry = (void *)(header + 1), *end;
- struct user_namespace *fs_userns = i_user_ns(inode);
- int count;
- vfsuid_t vfsuid;
- vfsgid_t vfsgid;
- kuid_t uid;
- kgid_t gid;
-
- if (no_idmapping(mnt_userns, i_user_ns(inode)))
- return;
-
- count = posix_acl_fix_xattr_common(value, size);
- if (count <= 0)
- return;
-
- for (end = entry + count; entry != end; entry++) {
- switch (le16_to_cpu(entry->e_tag)) {
- case ACL_USER:
- uid = make_kuid(&init_user_ns, le32_to_cpu(entry->e_id));
- vfsuid = VFSUIDT_INIT(uid);
- uid = from_vfsuid(mnt_userns, fs_userns, vfsuid);
- entry->e_id = cpu_to_le32(from_kuid(&init_user_ns, uid));
- break;
- case ACL_GROUP:
- gid = make_kgid(&init_user_ns, le32_to_cpu(entry->e_id));
- vfsgid = VFSGIDT_INIT(gid);
- gid = from_vfsgid(mnt_userns, fs_userns, vfsgid);
- entry->e_id = cpu_to_le32(from_kgid(&init_user_ns, gid));
- break;
- default:
- break;
- }
- }
-}
-
static void posix_acl_fix_xattr_userns(
struct user_namespace *to, struct user_namespace *from,
void *value, size_t size)
@@ -1211,7 +1171,17 @@ posix_acl_xattr_set(const struct xattr_handler *handler,
int ret;
if (value) {
- acl = posix_acl_from_xattr(&init_user_ns, value, size);
+ /*
+ * By the time we end up here the {g,u}ids stored in
+ * ACL_{GROUP,USER} have already been mapped according to the
+ * caller's idmapping. The vfs_set_acl_prepare() helper will
+ * recover them and take idmapped mounts into account. The
+ * filesystem will receive the POSIX ACLs in in the correct
+ * format ready to be cached or written to the backing store
+ * taking the filesystem idmapping into account.
+ */
+ acl = vfs_set_acl_prepare(mnt_userns, i_user_ns(inode),
+ value, size);
if (IS_ERR(acl))
return PTR_ERR(acl);
}
diff --git a/fs/xattr.c b/fs/xattr.c
index a1f4998bc6be..3ac68ec0c023 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -305,9 +305,6 @@ vfs_setxattr(struct user_namespace *mnt_userns, struct dentry *dentry,
size = error;
}
- if (size && is_posix_acl_xattr(name))
- posix_acl_setxattr_idmapped_mnt(mnt_userns, inode, value, size);
-
retry_deleg:
inode_lock(inode);
error = __vfs_setxattr_locked(mnt_userns, dentry, name, value, size,
diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 2e6fb6e2ffd2..23d484e05e6f 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -457,10 +457,21 @@ static int evm_xattr_acl_change(struct user_namespace *mnt_userns,
int rc;
/*
- * user_ns is not relevant here, ACL_USER/ACL_GROUP don't have impact
- * on the inode mode (see posix_acl_equiv_mode()).
+ * An earlier comment here mentioned that the idmappings for
+ * ACL_{GROUP,USER} don't matter since EVM is only interested in the
+ * mode stored as part of POSIX ACLs. Nonetheless, if it must translate
+ * from the uapi POSIX ACL representation to the VFS internal POSIX ACL
+ * representation it should do so correctly. There's no guarantee that
+ * we won't change POSIX ACLs in a way that ACL_{GROUP,USER} matters
+ * for the mode at some point and it's difficult to keep track of all
+ * the LSM and integrity modules and what they do to POSIX ACLs.
+ *
+ * Frankly, EVM shouldn't try to interpret the uapi struct for POSIX
+ * ACLs it received. It requires knowledge that only the VFS is
+ * guaranteed to have.
*/
- acl = posix_acl_from_xattr(&init_user_ns, xattr_value, xattr_value_len);
+ acl = vfs_set_acl_prepare(mnt_userns, i_user_ns(inode),
+ xattr_value, xattr_value_len);
if (IS_ERR_OR_NULL(acl))
return 1;
--
2.34.1
