Re: [PATCH security v1] apparmor: fix a use-after-free in aa_lookupn_ns()

[John Johansen <john.johansen@canonical.com>]

On 9/22/26 22:46, Binbin Deng wrote:
> aa_lookupn_ns() takes a reference on the namespace it finds with
> aa_get_ns(),which is kref_get() on the reference count that the
> namespace is released through, without checking whether the namespace
> is still in use.  A concurrent removal of a ":"-prefixed namespace
> looks the same namespace up and only then takes the parent lock to
> remove it, so the last reference can be dropped between the lookup and
> the aa_get_ns() call: the reference count then reports the increment on
> a zero count and the kernel classifies the following use of the object
> as a use-after-free.  A user with CAP_MAC_ADMIN can trigger this by
> writing a ":<name>" removal and a ":<name>/<profile>" removal to
> /sys/kernel/security/apparmor/.remove concurrently.
> 
> refcount_t: addition on 0; use-after-free.
> WARNING: lib/refcount.c:25 at refcount_warn_saturate+0xc0/0xe0, CPU#6
> Call Trace:
>   <TASK>
>   aa_lookupn_ns+0xa9/0xc0
>   aa_remove_profiles+0x4fb/0x730
>   ? __kvmalloc_node_noprof+0x219/0x5c0
>   ? __pfx_aa_remove_profiles+0x10/0x10
>   ? aa_loaddata_alloc+0x65/0x1f0
>   ? _copy_from_user+0x2d/0x80
>   ? aa_loaddata_alloc+0x111/0x1f0
>   profile_remove+0x2ee/0x4a0
>   vfs_write+0x21e/0xcf0
>   ? rcu_core+0x29c/0x1740
>   ? timerqueue_linked_add+0x1f4/0x3d0
>   ? __pfx_vfs_write+0x10/0x10
>   ? mutex_lock+0x81/0xe0
>   ? __pfx_mutex_lock+0x10/0x10
>   ? __pfx_rcu_core+0x10/0x10
>   ? __pfx_hrtimer_update_next_event+0x10/0x10
>   ? mutex_unlock+0x7b/0xd0
>   ? fdget_pos+0x24d/0x4b0
>   ksys_write+0xf7/0x1c0
>   ? __pfx_ksys_write+0x10/0x10
>   ? restore_fpregs_from_fpstate+0x37/0xc0
>   do_syscall_64+0xf9/0x540
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
>   </TASK>
> 
> Fix by rejecting a namespace whose reference count.
> 
> Fixes: 3664268f19ea0 ("apparmor: add namespace lookup fns()")
> Signed-off-by: Binbin Deng <18983559317@163.com>

I have pulled this into the apparmor tree

> ---
>   security/apparmor/include/policy_ns.h | 16 ++++++++++++++++
>   security/apparmor/policy_ns.c         |  2 +-
>   2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/security/apparmor/include/policy_ns.h b/security/apparmor/include/policy_ns.h
> index cc6e84151812..4f8359875476 100644
> --- a/security/apparmor/include/policy_ns.h
> +++ b/security/apparmor/include/policy_ns.h
> @@ -117,6 +117,22 @@ static inline struct aa_ns *aa_get_ns(struct aa_ns *ns)
>   	return ns;
>   }
>   
> +/**
> + * aa_get_ns_not0 - increment references count on @ns found via lookup
> + * @ns: namespace to increment reference count of (MAYBE NULL)
> + *
> + * Returns: pointer to @ns, if @ns is NULL returns NULL,
> + *          NULL if @ns is being freed
> + * Requires: @ns must be held with valid refcount when called
> + */
> +static inline struct aa_ns *aa_get_ns_not0(struct aa_ns *ns)
> +{
> +	if (ns && aa_get_profile_not0(ns->unconfined))
> +		return ns;
> +
> +	return NULL;
> +}
> +
>   /**
>    * aa_put_ns - decrement refcount on @ns
>    * @ns: namespace to put reference of
> diff --git a/security/apparmor/policy_ns.c b/security/apparmor/policy_ns.c
> index 5a907a875d8f..ae9168c1a85e 100644
> --- a/security/apparmor/policy_ns.c
> +++ b/security/apparmor/policy_ns.c
> @@ -207,7 +207,7 @@ struct aa_ns *aa_lookupn_ns(struct aa_ns *view, const char *name, size_t n)
>   	struct aa_ns *ns = NULL;
>   
>   	rcu_read_lock();
> -	ns = aa_get_ns(__aa_lookupn_ns(view, name, n));
> +	ns = aa_get_ns_not0(__aa_lookupn_ns(view, name, n));
>   	rcu_read_unlock();
>   
>   	return ns;