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acl: handle idmapped mounts for idmapped filesystems

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Ensure that POSIX ACLs checking, getting, and setting works correctly
for filesystems mountable with a filesystem idmapping ("fs_idmapping")
that want to support idmapped mounts ("mnt_idmapping").

Note that no filesystems mountable with an fs_idmapping do yet support
idmapped mounts. This is required infrastructure work to unblock this.

As we explained in detail in [1] the fs_idmapping is irrelevant for
getxattr() and setxattr() when mapping the ACL_{GROUP,USER} {g,u}ids
stored in the uapi struct posix_acl_xattr_entry in
posix_acl_fix_xattr_{from,to}_user().

But for acl_permission_check() and posix_acl_{g,s}etxattr_idmapped_mnt()
the fs_idmapping matters.

acl_permission_check():
  During lookup POSIX ACLs are retrieved directly via i_op->get_acl() and
  are returned via the kernel internal struct posix_acl which contains
  e_{g,u}id members of type k{g,u}id_t that already take the
  fs_idmapping into acccount.

  For example, a POSIX ACL stored with u4 on the backing store is mapped
  to k10000004 in the fs_idmapping. The mnt_idmapping remaps the POSIX ACL
  to k20000004. In order to do that the fs_idmapping needs to be taken
  into account but that doesn't happen yet (Again, this is a
  counterfactual currently as fuse doesn't support idmapped mounts
  currently. It's just used as a convenient example.):

  fs_idmapping:  u0:k10000000:r65536
  mnt_idmapping: u0:v20000000:r65536
  ACL_USER:      k10000004

  acl_permission_check()
  -> check_acl()
     -> get_acl()
        -> i_op->get_acl() == fuse_get_acl()
           -> posix_acl_from_xattr(u0:k10000000:r65536 /* fs_idmapping */, ...)
              {
                      k10000004 = make_kuid(u0:k10000000:r65536 /* fs_idmapping */,
                                            u4 /* ACL_USER */);
              }
     -> posix_acl_permission()
        {
                -1 = make_vfsuid(u0:v20000000:r65536 /* mnt_idmapping */,
                                 &init_user_ns,
                                 k10000004);
                vfsuid_eq_kuid(-1, k10000004 /* caller_fsuid */)
        }

  In order to correctly map from the fs_idmapping into mnt_idmapping we
  require the relevant fs_idmaping to be passed:

  acl_permission_check()
  -> check_acl()
     -> get_acl()
        -> i_op->get_acl() == fuse_get_acl()
           -> posix_acl_from_xattr(u0:k10000000:r65536 /* fs_idmapping */, ...)
              {
                      k10000004 = make_kuid(u0:k10000000:r65536 /* fs_idmapping */,
                                            u4 /* ACL_USER */);
              }
     -> posix_acl_permission()
        {
                v20000004 = make_vfsuid(u0:v20000000:r65536 /* mnt_idmapping */,
                                        u0:k10000000:r65536 /* fs_idmapping */,
                                        k10000004);
                vfsuid_eq_kuid(v20000004, k10000004 /* caller_fsuid */)
        }

  The initial_idmapping is only correct for the current situation because
  all filesystems that currently support idmapped mounts do not support
  being mounted with an fs_idmapping.

  Note that ovl_get_acl() is used to retrieve the POSIX ACLs from the
  relevant lower layer and the lower layer's mnt_idmapping needs to be
  taken into account and so does the fs_idmapping. See 0c5fd887d2 ("acl:
  move idmapped mount fixup into vfs_{g,s}etxattr()") for more details.

For posix_acl_{g,s}etxattr_idmapped_mnt() it is not as obvious why the
fs_idmapping matters as it is for acl_permission_check(). Especially
because it doesn't matter for posix_acl_fix_xattr_{from,to}_user() (See
[1] for more context.).

Because posix_acl_{g,s}etxattr_idmapped_mnt() operate on the uapi
struct posix_acl_xattr_entry which contains {g,u}id_t values and thus
give the impression that the fs_idmapping is irrelevant as at this point
appropriate {g,u}id_t values have seemlingly been generated.

As we've stated multiple times this assumption is wrong and in fact the
uapi struct posix_acl_xattr_entry is taking idmappings into account
depending at what place it is operated on.

posix_acl_getxattr_idmapped_mnt()
  When posix_acl_getxattr_idmapped_mnt() is called the values stored in
  the uapi struct posix_acl_xattr_entry are mapped according to the
  fs_idmapping. This happened when they were read from the backing store
  and then translated from struct posix_acl into the uapi
  struct posix_acl_xattr_entry during posix_acl_to_xattr().

  In other words, the fs_idmapping matters as the values stored as
  {g,u}id_t in the uapi struct posix_acl_xattr_entry have been generated
  by it.

  So we need to take the fs_idmapping into account during make_vfsuid()
  in posix_acl_getxattr_idmapped_mnt().

posix_acl_setxattr_idmapped_mnt()
  When posix_acl_setxattr_idmapped_mnt() is called the values stored as
  {g,u}id_t in uapi struct posix_acl_xattr_entry are intended to be the
  values that ultimately get turned back into a k{g,u}id_t in
  posix_acl_from_xattr() (which turns the uapi
  struct posix_acl_xattr_entry into the kernel internal struct posix_acl).

  In other words, the fs_idmapping matters as the values stored as
  {g,u}id_t in the uapi struct posix_acl_xattr_entry are intended to be
  the values that will be undone in the fs_idmapping when writing to the
  backing store.

  So we need to take the fs_idmapping into account during from_vfsuid()
  in posix_acl_setxattr_idmapped_mnt().

Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org [1]
Fixes: 0c5fd887d2 ("acl: move idmapped mount fixup into vfs_{g,s}etxattr()")
Cc: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Link: https://lore.kernel.org/r/20220816113514.43304-1-brauner@kernel.org
This commit is contained in:
Christian Brauner 2022-08-16 13:35:13 +02:00 • committed by Christian Brauner (Microsoft)
parent 568035b01c
commit abfcf55d8b
No known key found for this signature in database
GPG key ID: 91C61BC06578DCA2
2 changed files with 16 additions and 10 deletions
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11
fs/overlayfs/inode.c
View file

@ -460,9 +460,12 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
* of the POSIX ACLs retrieved from the lower layer to this function to not
* alter the POSIX ACLs for the underlying filesystem.
*/
static void ovl_idmap_posix_acl(struct user_namespace *mnt_userns,
static void ovl_idmap_posix_acl(struct inode *realinode,
struct user_namespace *mnt_userns,
struct posix_acl *acl)
{
struct user_namespace *fs_userns = i_user_ns(realinode);
for (unsigned int i = 0; i < acl->a_count; i++) {
vfsuid_t vfsuid;
vfsgid_t vfsgid;
@ -470,11 +473,11 @@ static void ovl_idmap_posix_acl(struct user_namespace *mnt_userns,
struct posix_acl_entry *e = &acl->a_entries[i];
switch (e->e_tag) {
case ACL_USER:
vfsuid = make_vfsuid(mnt_userns, &init_user_ns, e->e_uid);
vfsuid = make_vfsuid(mnt_userns, fs_userns, e->e_uid);
e->e_uid = vfsuid_into_kuid(vfsuid);
break;
case ACL_GROUP:
vfsgid = make_vfsgid(mnt_userns, &init_user_ns, e->e_gid);
vfsgid = make_vfsgid(mnt_userns, fs_userns, e->e_gid);
e->e_gid = vfsgid_into_kgid(vfsgid);
break;
}
@ -536,7 +539,7 @@ struct posix_acl *ovl_get_acl(struct inode *inode, int type, bool rcu)
if (!clone)
clone = ERR_PTR(-ENOMEM);
else
ovl_idmap_posix_acl(mnt_user_ns(realpath.mnt), clone);
ovl_idmap_posix_acl(realinode, mnt_user_ns(realpath.mnt), clone);
/*
* Since we're not in RCU path walk we always need to release the
* original ACLs.

15
fs/posix_acl.c
View file

@ -361,6 +361,7 @@ posix_acl_permission(struct user_namespace *mnt_userns, struct inode *inode,
const struct posix_acl *acl, int want)
{
const struct posix_acl_entry *pa, *pe, *mask_obj;
struct user_namespace *fs_userns = i_user_ns(inode);
int found = 0;
vfsuid_t vfsuid;
vfsgid_t vfsgid;
@ -376,7 +377,7 @@ posix_acl_permission(struct user_namespace *mnt_userns, struct inode *inode,
goto check_perm;
break;
case ACL_USER:
vfsuid = make_vfsuid(mnt_userns, &init_user_ns,
vfsuid = make_vfsuid(mnt_userns, fs_userns,
pa->e_uid);
if (vfsuid_eq_kuid(vfsuid, current_fsuid()))
goto mask;
@ -390,7 +391,7 @@ posix_acl_permission(struct user_namespace *mnt_userns, struct inode *inode,
}
break;
case ACL_GROUP:
vfsgid = make_vfsgid(mnt_userns, &init_user_ns,
vfsgid = make_vfsgid(mnt_userns, fs_userns,
pa->e_gid);
if (vfsgid_in_group_p(vfsgid)) {
found = 1;
@ -736,6 +737,7 @@ void posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
{
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;
@ -753,13 +755,13 @@ void posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
switch (le16_to_cpu(entry->e_tag)) {
case ACL_USER:
uid = make_kuid(&init_user_ns, le32_to_cpu(entry->e_id));
vfsuid = make_vfsuid(mnt_userns, &init_user_ns, uid);
vfsuid = make_vfsuid(mnt_userns, fs_userns, uid);
entry->e_id = cpu_to_le32(from_kuid(&init_user_ns,
vfsuid_into_kuid(vfsuid)));
break;
case ACL_GROUP:
gid = make_kgid(&init_user_ns, le32_to_cpu(entry->e_id));
vfsgid = make_vfsgid(mnt_userns, &init_user_ns, gid);
vfsgid = make_vfsgid(mnt_userns, fs_userns, gid);
entry->e_id = cpu_to_le32(from_kgid(&init_user_ns,
vfsgid_into_kgid(vfsgid)));
break;
@ -775,6 +777,7 @@ void posix_acl_setxattr_idmapped_mnt(struct user_namespace *mnt_userns,
{
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;
@ -793,13 +796,13 @@ void posix_acl_setxattr_idmapped_mnt(struct user_namespace *mnt_userns,
case ACL_USER:
uid = make_kuid(&init_user_ns, le32_to_cpu(entry->e_id));
vfsuid = VFSUIDT_INIT(uid);
uid = from_vfsuid(mnt_userns, &init_user_ns, vfsuid);
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, &init_user_ns, vfsgid);
gid = from_vfsgid(mnt_userns, fs_userns, vfsgid);
entry->e_id = cpu_to_le32(from_kgid(&init_user_ns, gid));
break;
default: