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acl: move idmapping handling into posix_acl_xattr_set() 

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 0c5fd887d2 ("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@kernel.org/ [1]
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
This commit is contained in:
Christian Brauner 2022-08-29 14:38:43 +02:00 committed by Christian Brauner (Microsoft)
parent 6b70fe0601
commit 52edb4080e
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
4 changed files with 25 additions and 56 deletions
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52
fs/posix_acl.c
View File

@ -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);
}

3
fs/xattr.c
View File

@ -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,

9
include/linux/posix_acl_xattr.h
View File

@ -38,9 +38,6 @@ void posix_acl_fix_xattr_to_user(void *value, size_t size);
void posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
const struct inode *inode,
void *value, size_t size);
void posix_acl_setxattr_idmapped_mnt(struct user_namespace *mnt_userns,
const struct inode *inode,
void *value, size_t size);
#else
static inline void posix_acl_fix_xattr_from_user(void *value, size_t size)
{
@ -54,12 +51,6 @@ posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
size_t size)
{
}
static inline void
posix_acl_setxattr_idmapped_mnt(struct user_namespace *mnt_userns,
const struct inode *inode, void *value,
size_t size)
{
}
#endif
struct posix_acl *posix_acl_from_xattr(struct user_namespace *user_ns,

17
security/integrity/evm/evm_main.c
View File

@ -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;