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After commit 12e9e3cd03b5 ("simpe_xattr: use per-sb cache"),
kernfs_xattr_set() and kernfs_xattr_get() compute the cache via
kernfs_root(kn) before any other check. kernfs_root(kn) walks
kn->__parent first and falls back to kn->dir.root, both of which are
NULL on a freshly kmem_cache_zalloc()'d kn. kn->__parent was being set
in kernfs_new_node() after __kernfs_new_node() returned, and kn->dir.root
is set even later by kernfs_create_dir_ns() / kernfs_create_empty_dir().
The LSM kernfs_init_security hook is invoked from inside
__kernfs_new_node(), before either field has been initialized.
selinux_kernfs_init_security() ends with kernfs_xattr_set(kn,
XATTR_NAME_SELINUX, ...). kernfs_root(kn) then returns NULL, and
&((struct kernfs_root *)NULL)->xa_cache evaluates to
offsetof(struct kernfs_root, xa_cache) which faults:
BUG: kernel NULL pointer dereference, address: 00000000000000e0
RIP: 0010:simple_xattr_set+0x27/0x8b0
Call Trace:
kernfs_xattr_set+0x63/0xb0
selinux_kernfs_init_security+0x13b/0x270
security_kernfs_init_security+0x36/0xc0
__kernfs_new_node+0x182/0x290
kernfs_new_node+0x80/0xc0
kernfs_create_dir_ns+0x2b/0xa0
cgroup_create+0x116/0x380
cgroup_mkdir+0x7c/0x1a0
Reproduces deterministically at PID 1 (systemd) on an SELinux-enabled
distro. The first cgroup mkdir under /sys/fs/cgroup with a labelled
parent panics the kernel.
The LSM hook's contract is that the kn_dir argument is the parent of
the new kn, so kn->__parent should already point at kn_dir when the
hook runs. Move kernfs_get(parent) and rcu_assign_pointer of
kn->__parent from kernfs_new_node() into __kernfs_new_node() right
before the security hook, and unwind the parent reference on the
err_out4 path. kernfs_root(kn) then takes its parent branch during
the hook and returns parent->dir.root, which is the correct root.
This also closes the same-shape latent bug in kernfs_xattr_get() (which
today is hidden only by kernfs_iattrs_noalloc() returning NULL on a
fresh kn).
Fixes: 12e9e3cd03b5 ("simpe_xattr: use per-sb cache")
Reported-by: Calum Mackay <calum.mackay@oracle.com>
Closes: https://lore.kernel.org/all/5386153f-9112-4971-98fc-de90d7aae2c6@oracle.com/
Link: https://patch.msgid.link/20260526-ablief-demut-wehen-aef8446ef5c9@brauner
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
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| MAINTAINERS | ||
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| README | ||
Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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