linux-stable/fs/attr.c
Christian Brauner 168f912893
fs: account for group membership
When calling setattr_prepare() to determine the validity of the
attributes the ia_{g,u}id fields contain the value that will be written
to inode->i_{g,u}id. This is exactly the same for idmapped and
non-idmapped mounts and allows callers to pass in the values they want
to see written to inode->i_{g,u}id.

When group ownership is changed a caller whose fsuid owns the inode can
change the group of the inode to any group they are a member of. When
searching through the caller's groups we need to use the gid mapped
according to the idmapped mount otherwise we will fail to change
ownership for unprivileged users.

Consider a caller running with fsuid and fsgid 1000 using an idmapped
mount that maps id 65534 to 1000 and 65535 to 1001. Consequently, a file
owned by 65534:65535 in the filesystem will be owned by 1000:1001 in the
idmapped mount.

The caller now requests the gid of the file to be changed to 1000 going
through the idmapped mount. In the vfs we will immediately map the
requested gid to the value that will need to be written to inode->i_gid
and place it in attr->ia_gid. Since this idmapped mount maps 65534 to
1000 we place 65534 in attr->ia_gid.

When we check whether the caller is allowed to change group ownership we
first validate that their fsuid matches the inode's uid. The
inode->i_uid is 65534 which is mapped to uid 1000 in the idmapped mount.
Since the caller's fsuid is 1000 we pass the check.

We now check whether the caller is allowed to change inode->i_gid to the
requested gid by calling in_group_p(). This will compare the passed in
gid to the caller's fsgid and search the caller's additional groups.

Since we're dealing with an idmapped mount we need to pass in the gid
mapped according to the idmapped mount. This is akin to checking whether
a caller is privileged over the future group the inode is owned by. And
that needs to take the idmapped mount into account. Note, all helpers
are nops without idmapped mounts.

New regression test sent to xfstests.

Link: https://github.com/lxc/lxd/issues/10537
Link: https://lore.kernel.org/r/20220613111517.2186646-1-brauner@kernel.org
Fixes: 2f221d6f7b ("attr: handle idmapped mounts")
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org # 5.15+
CC: linux-fsdevel@vger.kernel.org
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
2022-06-14 12:18:47 +02:00

436 lines
14 KiB
C

// SPDX-License-Identifier: GPL-2.0
/*
* linux/fs/attr.c
*
* Copyright (C) 1991, 1992 Linus Torvalds
* changes by Thomas Schoebel-Theuer
*/
#include <linux/export.h>
#include <linux/time.h>
#include <linux/mm.h>
#include <linux/string.h>
#include <linux/sched/signal.h>
#include <linux/capability.h>
#include <linux/fsnotify.h>
#include <linux/fcntl.h>
#include <linux/security.h>
#include <linux/evm.h>
#include <linux/ima.h>
/**
* chown_ok - verify permissions to chown inode
* @mnt_userns: user namespace of the mount @inode was found from
* @inode: inode to check permissions on
* @uid: uid to chown @inode to
*
* If the inode has been found through an idmapped mount the user namespace of
* the vfsmount must be passed through @mnt_userns. This function will then
* take care to map the inode according to @mnt_userns before checking
* permissions. On non-idmapped mounts or if permission checking is to be
* performed on the raw inode simply passs init_user_ns.
*/
static bool chown_ok(struct user_namespace *mnt_userns,
const struct inode *inode,
kuid_t uid)
{
kuid_t kuid = i_uid_into_mnt(mnt_userns, inode);
if (uid_eq(current_fsuid(), kuid) && uid_eq(uid, inode->i_uid))
return true;
if (capable_wrt_inode_uidgid(mnt_userns, inode, CAP_CHOWN))
return true;
if (uid_eq(kuid, INVALID_UID) &&
ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
return true;
return false;
}
/**
* chgrp_ok - verify permissions to chgrp inode
* @mnt_userns: user namespace of the mount @inode was found from
* @inode: inode to check permissions on
* @gid: gid to chown @inode to
*
* If the inode has been found through an idmapped mount the user namespace of
* the vfsmount must be passed through @mnt_userns. This function will then
* take care to map the inode according to @mnt_userns before checking
* permissions. On non-idmapped mounts or if permission checking is to be
* performed on the raw inode simply passs init_user_ns.
*/
static bool chgrp_ok(struct user_namespace *mnt_userns,
const struct inode *inode, kgid_t gid)
{
kgid_t kgid = i_gid_into_mnt(mnt_userns, inode);
if (uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode))) {
kgid_t mapped_gid;
if (gid_eq(gid, inode->i_gid))
return true;
mapped_gid = mapped_kgid_fs(mnt_userns, i_user_ns(inode), gid);
if (in_group_p(mapped_gid))
return true;
}
if (capable_wrt_inode_uidgid(mnt_userns, inode, CAP_CHOWN))
return true;
if (gid_eq(kgid, INVALID_GID) &&
ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
return true;
return false;
}
/**
* setattr_prepare - check if attribute changes to a dentry are allowed
* @mnt_userns: user namespace of the mount the inode was found from
* @dentry: dentry to check
* @attr: attributes to change
*
* Check if we are allowed to change the attributes contained in @attr
* in the given dentry. This includes the normal unix access permission
* checks, as well as checks for rlimits and others. The function also clears
* SGID bit from mode if user is not allowed to set it. Also file capabilities
* and IMA extended attributes are cleared if ATTR_KILL_PRIV is set.
*
* If the inode has been found through an idmapped mount the user namespace of
* the vfsmount must be passed through @mnt_userns. This function will then
* take care to map the inode according to @mnt_userns before checking
* permissions. On non-idmapped mounts or if permission checking is to be
* performed on the raw inode simply passs init_user_ns.
*
* Should be called as the first thing in ->setattr implementations,
* possibly after taking additional locks.
*/
int setattr_prepare(struct user_namespace *mnt_userns, struct dentry *dentry,
struct iattr *attr)
{
struct inode *inode = d_inode(dentry);
unsigned int ia_valid = attr->ia_valid;
/*
* First check size constraints. These can't be overriden using
* ATTR_FORCE.
*/
if (ia_valid & ATTR_SIZE) {
int error = inode_newsize_ok(inode, attr->ia_size);
if (error)
return error;
}
/* If force is set do it anyway. */
if (ia_valid & ATTR_FORCE)
goto kill_priv;
/* Make sure a caller can chown. */
if ((ia_valid & ATTR_UID) && !chown_ok(mnt_userns, inode, attr->ia_uid))
return -EPERM;
/* Make sure caller can chgrp. */
if ((ia_valid & ATTR_GID) && !chgrp_ok(mnt_userns, inode, attr->ia_gid))
return -EPERM;
/* Make sure a caller can chmod. */
if (ia_valid & ATTR_MODE) {
kgid_t mapped_gid;
if (!inode_owner_or_capable(mnt_userns, inode))
return -EPERM;
if (ia_valid & ATTR_GID)
mapped_gid = mapped_kgid_fs(mnt_userns,
i_user_ns(inode), attr->ia_gid);
else
mapped_gid = i_gid_into_mnt(mnt_userns, inode);
/* Also check the setgid bit! */
if (!in_group_p(mapped_gid) &&
!capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
attr->ia_mode &= ~S_ISGID;
}
/* Check for setting the inode time. */
if (ia_valid & (ATTR_MTIME_SET | ATTR_ATIME_SET | ATTR_TIMES_SET)) {
if (!inode_owner_or_capable(mnt_userns, inode))
return -EPERM;
}
kill_priv:
/* User has permission for the change */
if (ia_valid & ATTR_KILL_PRIV) {
int error;
error = security_inode_killpriv(mnt_userns, dentry);
if (error)
return error;
}
return 0;
}
EXPORT_SYMBOL(setattr_prepare);
/**
* inode_newsize_ok - may this inode be truncated to a given size
* @inode: the inode to be truncated
* @offset: the new size to assign to the inode
*
* inode_newsize_ok must be called with i_mutex held.
*
* inode_newsize_ok will check filesystem limits and ulimits to check that the
* new inode size is within limits. inode_newsize_ok will also send SIGXFSZ
* when necessary. Caller must not proceed with inode size change if failure is
* returned. @inode must be a file (not directory), with appropriate
* permissions to allow truncate (inode_newsize_ok does NOT check these
* conditions).
*
* Return: 0 on success, -ve errno on failure
*/
int inode_newsize_ok(const struct inode *inode, loff_t offset)
{
if (inode->i_size < offset) {
unsigned long limit;
limit = rlimit(RLIMIT_FSIZE);
if (limit != RLIM_INFINITY && offset > limit)
goto out_sig;
if (offset > inode->i_sb->s_maxbytes)
goto out_big;
} else {
/*
* truncation of in-use swapfiles is disallowed - it would
* cause subsequent swapout to scribble on the now-freed
* blocks.
*/
if (IS_SWAPFILE(inode))
return -ETXTBSY;
}
return 0;
out_sig:
send_sig(SIGXFSZ, current, 0);
out_big:
return -EFBIG;
}
EXPORT_SYMBOL(inode_newsize_ok);
/**
* setattr_copy - copy simple metadata updates into the generic inode
* @mnt_userns: user namespace of the mount the inode was found from
* @inode: the inode to be updated
* @attr: the new attributes
*
* setattr_copy must be called with i_mutex held.
*
* setattr_copy updates the inode's metadata with that specified
* in attr on idmapped mounts. If file ownership is changed setattr_copy
* doesn't map ia_uid and ia_gid. It will asssume the caller has already
* provided the intended values. Necessary permission checks to determine
* whether or not the S_ISGID property needs to be removed are performed with
* the correct idmapped mount permission helpers.
* Noticeably missing is inode size update, which is more complex
* as it requires pagecache updates.
*
* If the inode has been found through an idmapped mount the user namespace of
* the vfsmount must be passed through @mnt_userns. This function will then
* take care to map the inode according to @mnt_userns before checking
* permissions. On non-idmapped mounts or if permission checking is to be
* performed on the raw inode simply passs init_user_ns.
*
* The inode is not marked as dirty after this operation. The rationale is
* that for "simple" filesystems, the struct inode is the inode storage.
* The caller is free to mark the inode dirty afterwards if needed.
*/
void setattr_copy(struct user_namespace *mnt_userns, struct inode *inode,
const struct iattr *attr)
{
unsigned int ia_valid = attr->ia_valid;
if (ia_valid & ATTR_UID)
inode->i_uid = attr->ia_uid;
if (ia_valid & ATTR_GID)
inode->i_gid = attr->ia_gid;
if (ia_valid & ATTR_ATIME)
inode->i_atime = attr->ia_atime;
if (ia_valid & ATTR_MTIME)
inode->i_mtime = attr->ia_mtime;
if (ia_valid & ATTR_CTIME)
inode->i_ctime = attr->ia_ctime;
if (ia_valid & ATTR_MODE) {
umode_t mode = attr->ia_mode;
kgid_t kgid = i_gid_into_mnt(mnt_userns, inode);
if (!in_group_p(kgid) &&
!capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
mode &= ~S_ISGID;
inode->i_mode = mode;
}
}
EXPORT_SYMBOL(setattr_copy);
int may_setattr(struct user_namespace *mnt_userns, struct inode *inode,
unsigned int ia_valid)
{
int error;
if (ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID | ATTR_TIMES_SET)) {
if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
return -EPERM;
}
/*
* If utimes(2) and friends are called with times == NULL (or both
* times are UTIME_NOW), then we need to check for write permission
*/
if (ia_valid & ATTR_TOUCH) {
if (IS_IMMUTABLE(inode))
return -EPERM;
if (!inode_owner_or_capable(mnt_userns, inode)) {
error = inode_permission(mnt_userns, inode, MAY_WRITE);
if (error)
return error;
}
}
return 0;
}
EXPORT_SYMBOL(may_setattr);
/**
* notify_change - modify attributes of a filesytem object
* @mnt_userns: user namespace of the mount the inode was found from
* @dentry: object affected
* @attr: new attributes
* @delegated_inode: returns inode, if the inode is delegated
*
* The caller must hold the i_mutex on the affected object.
*
* If notify_change discovers a delegation in need of breaking,
* it will return -EWOULDBLOCK and return a reference to the inode in
* delegated_inode. The caller should then break the delegation and
* retry. Because breaking a delegation may take a long time, the
* caller should drop the i_mutex before doing so.
*
* If file ownership is changed notify_change() doesn't map ia_uid and
* ia_gid. It will asssume the caller has already provided the intended values.
*
* Alternatively, a caller may pass NULL for delegated_inode. This may
* be appropriate for callers that expect the underlying filesystem not
* to be NFS exported. Also, passing NULL is fine for callers holding
* the file open for write, as there can be no conflicting delegation in
* that case.
*
* If the inode has been found through an idmapped mount the user namespace of
* the vfsmount must be passed through @mnt_userns. This function will then
* take care to map the inode according to @mnt_userns before checking
* permissions. On non-idmapped mounts or if permission checking is to be
* performed on the raw inode simply passs init_user_ns.
*/
int notify_change(struct user_namespace *mnt_userns, struct dentry *dentry,
struct iattr *attr, struct inode **delegated_inode)
{
struct inode *inode = dentry->d_inode;
umode_t mode = inode->i_mode;
int error;
struct timespec64 now;
unsigned int ia_valid = attr->ia_valid;
WARN_ON_ONCE(!inode_is_locked(inode));
error = may_setattr(mnt_userns, inode, ia_valid);
if (error)
return error;
if ((ia_valid & ATTR_MODE)) {
umode_t amode = attr->ia_mode;
/* Flag setting protected by i_mutex */
if (is_sxid(amode))
inode->i_flags &= ~S_NOSEC;
}
now = current_time(inode);
attr->ia_ctime = now;
if (!(ia_valid & ATTR_ATIME_SET))
attr->ia_atime = now;
else
attr->ia_atime = timestamp_truncate(attr->ia_atime, inode);
if (!(ia_valid & ATTR_MTIME_SET))
attr->ia_mtime = now;
else
attr->ia_mtime = timestamp_truncate(attr->ia_mtime, inode);
if (ia_valid & ATTR_KILL_PRIV) {
error = security_inode_need_killpriv(dentry);
if (error < 0)
return error;
if (error == 0)
ia_valid = attr->ia_valid &= ~ATTR_KILL_PRIV;
}
/*
* We now pass ATTR_KILL_S*ID to the lower level setattr function so
* that the function has the ability to reinterpret a mode change
* that's due to these bits. This adds an implicit restriction that
* no function will ever call notify_change with both ATTR_MODE and
* ATTR_KILL_S*ID set.
*/
if ((ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID)) &&
(ia_valid & ATTR_MODE))
BUG();
if (ia_valid & ATTR_KILL_SUID) {
if (mode & S_ISUID) {
ia_valid = attr->ia_valid |= ATTR_MODE;
attr->ia_mode = (inode->i_mode & ~S_ISUID);
}
}
if (ia_valid & ATTR_KILL_SGID) {
if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
if (!(ia_valid & ATTR_MODE)) {
ia_valid = attr->ia_valid |= ATTR_MODE;
attr->ia_mode = inode->i_mode;
}
attr->ia_mode &= ~S_ISGID;
}
}
if (!(attr->ia_valid & ~(ATTR_KILL_SUID | ATTR_KILL_SGID)))
return 0;
/*
* Verify that uid/gid changes are valid in the target
* namespace of the superblock.
*/
if (ia_valid & ATTR_UID &&
!kuid_has_mapping(inode->i_sb->s_user_ns, attr->ia_uid))
return -EOVERFLOW;
if (ia_valid & ATTR_GID &&
!kgid_has_mapping(inode->i_sb->s_user_ns, attr->ia_gid))
return -EOVERFLOW;
/* Don't allow modifications of files with invalid uids or
* gids unless those uids & gids are being made valid.
*/
if (!(ia_valid & ATTR_UID) &&
!uid_valid(i_uid_into_mnt(mnt_userns, inode)))
return -EOVERFLOW;
if (!(ia_valid & ATTR_GID) &&
!gid_valid(i_gid_into_mnt(mnt_userns, inode)))
return -EOVERFLOW;
error = security_inode_setattr(dentry, attr);
if (error)
return error;
error = try_break_deleg(inode, delegated_inode);
if (error)
return error;
if (inode->i_op->setattr)
error = inode->i_op->setattr(mnt_userns, dentry, attr);
else
error = simple_setattr(mnt_userns, dentry, attr);
if (!error) {
fsnotify_change(dentry, ia_valid);
ima_inode_post_setattr(mnt_userns, dentry);
evm_inode_post_setattr(dentry, ia_valid);
}
return error;
}
EXPORT_SYMBOL(notify_change);