Re: [PATCH 1/3] capabilities: user namespace capabilities

From: Serge E. Hallyn
Date: Sun May 19 2024 - 23:31:22 EST

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On Thu, May 16, 2024 at 02:22:03AM -0700, Jonathan Calmels wrote:
> Attackers often rely on user namespaces to get elevated (yet confined)
> privileges in order to target specific subsystems (e.g. [1]). Distributions
> have been pretty adamant that they need a way to configure these, most of
> them carry out-of-tree patches to do so, or plainly refuse to enable them.
> As a result, there have been multiple efforts over the years to introduce
> various knobs to control and/or disable user namespaces (e.g. [2][3][4]).
>
> While we acknowledge that there are already ways to control the creation of
> such namespaces (the most recent being a LSM hook), there are inherent
> issues with these approaches. Preventing the user namespace creation is not
> fine-grained enough, and in some cases, incompatible with various userspace
> expectations (e.g. container runtimes, browser sandboxing, service
> isolation)
>
> This patch addresses these limitations by introducing an additional
> capability set used to restrict the permissions granted when creating user
> namespaces. This way, processes can apply the principle of least privilege
> by configuring only the capabilities they need for their namespaces.
>
> For compatibility reasons, processes always start with a full userns
> capability set.
>
> On namespace creation, the userns capability set (pU) is assigned to the
> new effective (pE), permitted (pP) and bounding set (X) of the task:
>
> pU = pE = pP = X
>
> The userns capability set obeys the invariant that no bit can ever be set
> if it is not already part of the task’s bounding set. This ensures that no
> namespace can ever gain more privileges than its predecessors.
> Additionally, if a task is not privileged over CAP_SETPCAP, setting any bit
> in the userns set requires its corresponding bit to be set in the permitted
> set. This effectively mimics the inheritable set rules and means that, by
> default, only root in the initial user namespace can gain userns
> capabilities:
>
> p’U = (pE & CAP_SETPCAP) ? X : (X & pP)
>
> Note that since userns capabilities are strictly hierarchical, policies can
> be enforced at various levels (e.g. init, pam_cap) and inherited by every
> child namespace.
>
> Here is a sample program that can be used to verify the functionality:
>
> /*
> * Test program that drops CAP_SYS_RAWIO from subsequent user namespaces.
> *
> * ./cap_userns_test unshare -r grep Cap /proc/self/status
> * CapInh: 0000000000000000
> * CapPrm: 000001fffffdffff
> * CapEff: 000001fffffdffff
> * CapBnd: 000001fffffdffff
> * CapAmb: 0000000000000000
> * CapUNs: 000001fffffdffff
> */
>
> int main(int argc, char *argv[])
> {
> if (prctl(PR_CAP_USERNS, PR_CAP_USERNS_LOWER, CAP_SYS_RAWIO, 0, 0) < 0)
> err(1, "cannot drop userns cap");
>
> execvp(argv[1], argv + 1);
> err(1, "cannot exec");
> }
>
> Link: https://security.googleblog.com/2023/06/learnings-from-kctf-vrps-42-linux.html
> Link: https://lore.kernel.org/lkml/1453502345-30416-1-git-send-email-keescook@xxxxxxxxxxxx
> Link: https://lore.kernel.org/lkml/20220815162028.926858-1-fred@xxxxxxxxxxxxxx
> Link: https://lore.kernel.org/containers/168547265011.24337.4306067683997517082-0@xxxxxxxxx
>
> Signed-off-by: Jonathan Calmels <jcalmels@xxxxxxxx>

Thanks! Of course we'llnsee how the conversations fall out, but

Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>

> ---
> fs/proc/array.c | 9 ++++++
> include/linux/cred.h | 3 ++
> include/uapi/linux/prctl.h | 7 +++++
> kernel/cred.c | 3 ++
> kernel/umh.c | 16 ++++++++++
> kernel/user_namespace.c | 12 +++-----
> security/commoncap.c | 59 ++++++++++++++++++++++++++++++++++++
> security/keys/process_keys.c | 3 ++
> 8 files changed, 105 insertions(+), 7 deletions(-)
>
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index 34a47fb0c57f..364e8bb19f9d 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -313,6 +313,9 @@ static inline void task_cap(struct seq_file *m, struct task_struct *p)
> const struct cred *cred;
> kernel_cap_t cap_inheritable, cap_permitted, cap_effective,
> cap_bset, cap_ambient;
> +#ifdef CONFIG_USER_NS
> + kernel_cap_t cap_userns;
> +#endif
>
> rcu_read_lock();
> cred = __task_cred(p);
> @@ -321,6 +324,9 @@ static inline void task_cap(struct seq_file *m, struct task_struct *p)
> cap_effective = cred->cap_effective;
> cap_bset = cred->cap_bset;
> cap_ambient = cred->cap_ambient;
> +#ifdef CONFIG_USER_NS
> + cap_userns = cred->cap_userns;
> +#endif
> rcu_read_unlock();
>
> render_cap_t(m, "CapInh:\t", &cap_inheritable);
> @@ -328,6 +334,9 @@ static inline void task_cap(struct seq_file *m, struct task_struct *p)
> render_cap_t(m, "CapEff:\t", &cap_effective);
> render_cap_t(m, "CapBnd:\t", &cap_bset);
> render_cap_t(m, "CapAmb:\t", &cap_ambient);
> +#ifdef CONFIG_USER_NS
> + render_cap_t(m, "CapUNs:\t", &cap_userns);
> +#endif
> }
>
> static inline void task_seccomp(struct seq_file *m, struct task_struct *p)
> diff --git a/include/linux/cred.h b/include/linux/cred.h
> index 2976f534a7a3..adab0031443e 100644
> --- a/include/linux/cred.h
> +++ b/include/linux/cred.h
> @@ -124,6 +124,9 @@ struct cred {
> kernel_cap_t cap_effective; /* caps we can actually use */
> kernel_cap_t cap_bset; /* capability bounding set */
> kernel_cap_t cap_ambient; /* Ambient capability set */
> +#ifdef CONFIG_USER_NS
> + kernel_cap_t cap_userns; /* User namespace capability set */
> +#endif
> #ifdef CONFIG_KEYS
> unsigned char jit_keyring; /* default keyring to attach requested
> * keys to */
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 370ed14b1ae0..e09475171f62 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -198,6 +198,13 @@ struct prctl_mm_map {
> # define PR_CAP_AMBIENT_LOWER 3
> # define PR_CAP_AMBIENT_CLEAR_ALL 4
>
> +/* Control the userns capability set */
> +#define PR_CAP_USERNS 48
> +# define PR_CAP_USERNS_IS_SET 1
> +# define PR_CAP_USERNS_RAISE 2
> +# define PR_CAP_USERNS_LOWER 3
> +# define PR_CAP_USERNS_CLEAR_ALL 4
> +
> /* arm64 Scalable Vector Extension controls */
> /* Flag values must be kept in sync with ptrace NT_ARM_SVE interface */
> #define PR_SVE_SET_VL 50 /* set task vector length */
> diff --git a/kernel/cred.c b/kernel/cred.c
> index 075cfa7c896f..9912c6f3bc6b 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -56,6 +56,9 @@ struct cred init_cred = {
> .cap_permitted = CAP_FULL_SET,
> .cap_effective = CAP_FULL_SET,
> .cap_bset = CAP_FULL_SET,
> +#ifdef CONFIG_USER_NS
> + .cap_userns = CAP_FULL_SET,
> +#endif
> .user = INIT_USER,
> .user_ns = &init_user_ns,
> .group_info = &init_groups,
> diff --git a/kernel/umh.c b/kernel/umh.c
> index 1b13c5d34624..51f1e1d25d49 100644
> --- a/kernel/umh.c
> +++ b/kernel/umh.c
> @@ -32,6 +32,9 @@
>
> #include <trace/events/module.h>
>
> +#ifdef CONFIG_USER_NS
> +static kernel_cap_t usermodehelper_userns = CAP_FULL_SET;
> +#endif
> static kernel_cap_t usermodehelper_bset = CAP_FULL_SET;
> static kernel_cap_t usermodehelper_inheritable = CAP_FULL_SET;
> static DEFINE_SPINLOCK(umh_sysctl_lock);
> @@ -94,6 +97,10 @@ static int call_usermodehelper_exec_async(void *data)
> new->cap_bset = cap_intersect(usermodehelper_bset, new->cap_bset);
> new->cap_inheritable = cap_intersect(usermodehelper_inheritable,
> new->cap_inheritable);
> +#ifdef CONFIG_USER_NS
> + new->cap_userns = cap_intersect(usermodehelper_userns,
> + new->cap_userns);
> +#endif
> spin_unlock(&umh_sysctl_lock);
>
> if (sub_info->init) {
> @@ -560,6 +567,15 @@ static struct ctl_table usermodehelper_table[] = {
> .mode = 0600,
> .proc_handler = proc_cap_handler,
> },
> +#ifdef CONFIG_USER_NS
> + {
> + .procname = "userns",
> + .data = &usermodehelper_userns,
> + .maxlen = 2 * sizeof(unsigned long),
> + .mode = 0600,
> + .proc_handler = proc_cap_handler,
> + },
> +#endif
> { }
> };
>
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 0b0b95418b16..7e624607330b 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -42,15 +42,13 @@ static void dec_user_namespaces(struct ucounts *ucounts)
>
> static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
> {
> - /* Start with the same capabilities as init but useless for doing
> - * anything as the capabilities are bound to the new user namespace.
> - */
> - cred->securebits = SECUREBITS_DEFAULT;
> + /* Start with the capabilities defined in the userns set. */
> + cred->cap_bset = cred->cap_userns;
> + cred->cap_permitted = cred->cap_userns;
> + cred->cap_effective = cred->cap_userns;
> cred->cap_inheritable = CAP_EMPTY_SET;
> - cred->cap_permitted = CAP_FULL_SET;
> - cred->cap_effective = CAP_FULL_SET;
> cred->cap_ambient = CAP_EMPTY_SET;
> - cred->cap_bset = CAP_FULL_SET;
> + cred->securebits = SECUREBITS_DEFAULT;
> #ifdef CONFIG_KEYS
> key_put(cred->request_key_auth);
> cred->request_key_auth = NULL;
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 162d96b3a676..b3d3372bf910 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -228,6 +228,28 @@ static inline int cap_inh_is_capped(void)
> return 1;
> }
>
> +/*
> + * Determine whether a userns capability can be raised.
> + * Returns 1 if it can, 0 otherwise.
> + */
> +#ifdef CONFIG_USER_NS
> +static inline int cap_uns_is_raiseable(unsigned long cap)
> +{
> + if (!!cap_raised(current_cred()->cap_userns, cap))
> + return 1;
> + /* a capability cannot be raised unless the current task has it in
> + * its bounding set and, without CAP_SETPCAP, its permitted set.
> + */
> + if (!cap_raised(current_cred()->cap_bset, cap))
> + return 0;
> + if (cap_capable(current_cred(), current_cred()->user_ns,
> + CAP_SETPCAP, CAP_OPT_NONE) != 0 &&
> + !cap_raised(current_cred()->cap_permitted, cap))
> + return 0;
> + return 1;
> +}
> +#endif
> +
> /**
> * cap_capset - Validate and apply proposed changes to current's capabilities
> * @new: The proposed new credentials; alterations should be made here
> @@ -1382,6 +1404,43 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> return commit_creds(new);
> }
>
> +#ifdef CONFIG_USER_NS
> + case PR_CAP_USERNS:
> + if (arg2 == PR_CAP_USERNS_CLEAR_ALL) {
> + if (arg3 | arg4 | arg5)
> + return -EINVAL;
> +
> + new = prepare_creds();
> + if (!new)
> + return -ENOMEM;
> + cap_clear(new->cap_userns);
> + return commit_creds(new);
> + }
> +
> + if (((!cap_valid(arg3)) | arg4 | arg5))
> + return -EINVAL;
> +
> + if (arg2 == PR_CAP_USERNS_IS_SET) {
> + return !!cap_raised(current_cred()->cap_userns, arg3);
> + } else if (arg2 != PR_CAP_USERNS_RAISE &&
> + arg2 != PR_CAP_USERNS_LOWER) {
> + return -EINVAL;
> + } else {
> + if (arg2 == PR_CAP_USERNS_RAISE &&
> + !cap_uns_is_raiseable(arg3))
> + return -EPERM;
> +
> + new = prepare_creds();
> + if (!new)
> + return -ENOMEM;
> + if (arg2 == PR_CAP_USERNS_RAISE)
> + cap_raise(new->cap_userns, arg3);
> + else
> + cap_lower(new->cap_userns, arg3);
> + return commit_creds(new);
> + }
> +#endif
> +
> default:
> /* No functionality available - continue with default */
> return -ENOSYS;
> diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
> index b5d5333ab330..e3670d815435 100644
> --- a/security/keys/process_keys.c
> +++ b/security/keys/process_keys.c
> @@ -944,6 +944,9 @@ void key_change_session_keyring(struct callback_head *twork)
> new->cap_effective = old->cap_effective;
> new->cap_ambient = old->cap_ambient;
> new->cap_bset = old->cap_bset;
> +#ifdef CONFIG_USER_NS
> + new->cap_userns = old->cap_userns;
> +#endif
>
> new->jit_keyring = old->jit_keyring;
> new->thread_keyring = key_get(old->thread_keyring);
> --
> 2.45.0
>

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