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authorLinus Torvalds <torvalds@linux-foundation.org>2022-01-11 14:26:55 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2022-01-11 14:26:55 -0800
commit5dfbfe71e32406f08480185d396d94cf7fc7a7d6 (patch)
treef42e43b71a9f7ee740796a10cfaeda41a98ad5fa /fs/namespace.c
parent84bfcc0b6994057905cf98d2c5cedef48b3322b5 (diff)
parentbd303368b776eead1c29e6cdda82bde7128b82a7 (diff)
Merge tag 'fs.idmapped.v5.17' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux
Pull fs idmapping updates from Christian Brauner: "This contains the work to enable the idmapping infrastructure to support idmapped mounts of filesystems mounted with an idmapping. In addition this contains various cleanups that avoid repeated open-coding of the same functionality and simplify the code in quite a few places. We also finish the renaming of the mapping helpers we started a few kernel releases back and move them to a dedicated header to not continue polluting the fs header needlessly with low-level idmapping helpers. With this series the fs header only contains idmapping helpers that interact with fs objects. Currently we only support idmapped mounts for filesystems mounted without an idmapping themselves. This was a conscious decision mentioned in multiple places (cf. [1]). As explained at length in [3] it is perfectly fine to extend support for idmapped mounts to filesystem's mounted with an idmapping should the need arise. The need has been there for some time now (cf. [2]). Before we can port any filesystem that is mountable with an idmapping to support idmapped mounts in the coming cycles, we need to first extend the mapping helpers to account for the filesystem's idmapping. This again, is explained at length in our documentation at [3] and also in the individual commit messages so here's an overview. Currently, the low-level mapping helpers implement the remapping algorithms described in [3] in a simplified manner as we could rely on the fact that all filesystems supporting idmapped mounts are mounted without an idmapping. In contrast, filesystems mounted with an idmapping are very likely to not use an identity mapping and will instead use a non-identity mapping. So the translation step from or into the filesystem's idmapping in the remapping algorithm cannot be skipped for such filesystems. Non-idmapped filesystems and filesystems not supporting idmapped mounts are unaffected by this change as the remapping algorithms can take the same shortcut as before. If the low-level helpers detect that they are dealing with an idmapped mount but the underlying filesystem is mounted without an idmapping we can rely on the previous shortcut and can continue to skip the translation step from or into the filesystem's idmapping. And of course, if the low-level helpers detect that they are not dealing with an idmapped mount they can simply return the relevant id unchanged; no remapping needs to be performed at all. These checks guarantee that only the minimal amount of work is performed. As before, if idmapped mounts aren't used the low-level helpers are idempotent and no work is performed at all" Link: 2ca4dcc4909d ("fs/mount_setattr: tighten permission checks") [1] Link: https://github.com/containers/podman/issues/10374 [2] Link: Documentations/filesystems/idmappings.rst [3] Link: a65e58e791a1 ("fs: document and rename fsid helpers") [4] * tag 'fs.idmapped.v5.17' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux: fs: support mapped mounts of mapped filesystems fs: add i_user_ns() helper fs: port higher-level mapping helpers fs: remove unused low-level mapping helpers fs: use low-level mapping helpers docs: update mapping documentation fs: account for filesystem mappings fs: tweak fsuidgid_has_mapping() fs: move mapping helpers fs: add is_idmapped_mnt() helper
Diffstat (limited to 'fs/namespace.c')
-rw-r--r--fs/namespace.c53
1 files changed, 39 insertions, 14 deletions
diff --git a/fs/namespace.c b/fs/namespace.c
index b696543adab8..dc31ad6b370f 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -31,6 +31,7 @@
#include <uapi/linux/mount.h>
#include <linux/fs_context.h>
#include <linux/shmem_fs.h>
+#include <linux/mnt_idmapping.h>
#include "pnode.h"
#include "internal.h"
@@ -561,7 +562,7 @@ static void free_vfsmnt(struct mount *mnt)
struct user_namespace *mnt_userns;
mnt_userns = mnt_user_ns(&mnt->mnt);
- if (mnt_userns != &init_user_ns)
+ if (!initial_idmapping(mnt_userns))
put_user_ns(mnt_userns);
kfree_const(mnt->mnt_devname);
#ifdef CONFIG_SMP
@@ -965,6 +966,7 @@ static struct mount *skip_mnt_tree(struct mount *p)
struct vfsmount *vfs_create_mount(struct fs_context *fc)
{
struct mount *mnt;
+ struct user_namespace *fs_userns;
if (!fc->root)
return ERR_PTR(-EINVAL);
@@ -982,6 +984,10 @@ struct vfsmount *vfs_create_mount(struct fs_context *fc)
mnt->mnt_mountpoint = mnt->mnt.mnt_root;
mnt->mnt_parent = mnt;
+ fs_userns = mnt->mnt.mnt_sb->s_user_ns;
+ if (!initial_idmapping(fs_userns))
+ mnt->mnt.mnt_userns = get_user_ns(fs_userns);
+
lock_mount_hash();
list_add_tail(&mnt->mnt_instance, &mnt->mnt.mnt_sb->s_mounts);
unlock_mount_hash();
@@ -1072,7 +1078,7 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
atomic_inc(&sb->s_active);
mnt->mnt.mnt_userns = mnt_user_ns(&old->mnt);
- if (mnt->mnt.mnt_userns != &init_user_ns)
+ if (!initial_idmapping(mnt->mnt.mnt_userns))
mnt->mnt.mnt_userns = get_user_ns(mnt->mnt.mnt_userns);
mnt->mnt.mnt_sb = sb;
mnt->mnt.mnt_root = dget(root);
@@ -3927,28 +3933,32 @@ static unsigned int recalc_flags(struct mount_kattr *kattr, struct mount *mnt)
static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
{
struct vfsmount *m = &mnt->mnt;
+ struct user_namespace *fs_userns = m->mnt_sb->s_user_ns;
if (!kattr->mnt_userns)
return 0;
/*
+ * Creating an idmapped mount with the filesystem wide idmapping
+ * doesn't make sense so block that. We don't allow mushy semantics.
+ */
+ if (kattr->mnt_userns == fs_userns)
+ return -EINVAL;
+
+ /*
* Once a mount has been idmapped we don't allow it to change its
* mapping. It makes things simpler and callers can just create
* another bind-mount they can idmap if they want to.
*/
- if (mnt_user_ns(m) != &init_user_ns)
+ if (is_idmapped_mnt(m))
return -EPERM;
/* The underlying filesystem doesn't support idmapped mounts yet. */
if (!(m->mnt_sb->s_type->fs_flags & FS_ALLOW_IDMAP))
return -EINVAL;
- /* Don't yet support filesystem mountable in user namespaces. */
- if (m->mnt_sb->s_user_ns != &init_user_ns)
- return -EINVAL;
-
/* We're not controlling the superblock. */
- if (!capable(CAP_SYS_ADMIN))
+ if (!ns_capable(fs_userns, CAP_SYS_ADMIN))
return -EPERM;
/* Mount has already been visible in the filesystem hierarchy. */
@@ -4002,14 +4012,27 @@ out:
static void do_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
{
- struct user_namespace *mnt_userns;
+ struct user_namespace *mnt_userns, *old_mnt_userns;
if (!kattr->mnt_userns)
return;
+ /*
+ * We're the only ones able to change the mount's idmapping. So
+ * mnt->mnt.mnt_userns is stable and we can retrieve it directly.
+ */
+ old_mnt_userns = mnt->mnt.mnt_userns;
+
mnt_userns = get_user_ns(kattr->mnt_userns);
/* Pairs with smp_load_acquire() in mnt_user_ns(). */
smp_store_release(&mnt->mnt.mnt_userns, mnt_userns);
+
+ /*
+ * If this is an idmapped filesystem drop the reference we've taken
+ * in vfs_create_mount() before.
+ */
+ if (!initial_idmapping(old_mnt_userns))
+ put_user_ns(old_mnt_userns);
}
static void mount_setattr_commit(struct mount_kattr *kattr,
@@ -4133,13 +4156,15 @@ static int build_mount_idmapped(const struct mount_attr *attr, size_t usize,
}
/*
- * The init_user_ns is used to indicate that a vfsmount is not idmapped.
- * This is simpler than just having to treat NULL as unmapped. Users
- * wanting to idmap a mount to init_user_ns can just use a namespace
- * with an identity mapping.
+ * The initial idmapping cannot be used to create an idmapped
+ * mount. We use the initial idmapping as an indicator of a mount
+ * that is not idmapped. It can simply be passed into helpers that
+ * are aware of idmapped mounts as a convenient shortcut. A user
+ * can just create a dedicated identity mapping to achieve the same
+ * result.
*/
mnt_userns = container_of(ns, struct user_namespace, ns);
- if (mnt_userns == &init_user_ns) {
+ if (initial_idmapping(mnt_userns)) {
err = -EPERM;
goto out_fput;
}