[v2] security: add trace event for cap_capable
Andrii Nakryiko
andrii.nakryiko at gmail.com
Fri Oct 25 18:37:59 UTC 2024
On Fri, Oct 25, 2024 at 8:15 AM Jordan Rome <linux at jordanrome.com> wrote:
>
> In cases where we want a stable way to observe/trace
> cap_capable (e.g. protection from inlining and API updates)
> add a tracepoint that passes:
> - The credentials used
> - The user namespace of the resource being accessed
> - The user namespace that has the capability to access the
> targeted resource
> - The capability to check for
> - Bitmask of options defined in include/linux/security.h
> - The return value of the check
>
> Signed-off-by: Jordan Rome <linux at jordanrome.com>
> ---
> MAINTAINERS | 1 +
> include/trace/events/capability.h | 60 +++++++++++++++++++++++++++++++
> security/commoncap.c | 31 +++++++++++-----
> 3 files changed, 84 insertions(+), 8 deletions(-)
> create mode 100644 include/trace/events/capability.h
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index cc40a9d9b8cd..210e9076c858 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -4994,6 +4994,7 @@ M: Serge Hallyn <serge at hallyn.com>
> L: linux-security-module at vger.kernel.org
> S: Supported
> F: include/linux/capability.h
> +F: include/trace/events/capability.h
> F: include/uapi/linux/capability.h
> F: kernel/capability.c
> F: security/commoncap.c
> diff --git a/include/trace/events/capability.h b/include/trace/events/capability.h
> new file mode 100644
> index 000000000000..e706ce690c38
> --- /dev/null
> +++ b/include/trace/events/capability.h
> @@ -0,0 +1,60 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#undef TRACE_SYSTEM
> +#define TRACE_SYSTEM capability
> +
> +#if !defined(_TRACE_CAPABILITY_H) || defined(TRACE_HEADER_MULTI_READ)
> +#define _TRACE_CAPABILITY_H
> +
> +#include <linux/cred.h>
> +#include <linux/tracepoint.h>
> +#include <linux/user_namespace.h>
> +
> +/**
> + * cap_capable - called after it's determined if a task has a particular
> + * effective capability
> + *
> + * @cred: The credentials used
> + * @targ_ns: The user namespace of the resource being accessed
> + * @capable_ns: The user namespace in which the credential provides the
> + * capability to access the targeted resource.
> + * This will be NULL if ret is not 0.
> + * @cap: The capability to check for
> + * @opts: Bitmask of options defined in include/linux/security.h
> + * @ret: The return value of the check: 0 if it does, -ve if it does not
> + *
> + * Allows to trace calls to cap_capable in commoncap.c
> + */
> +TRACE_EVENT(cap_capable,
> +
> + TP_PROTO(const struct cred *cred, struct user_namespace *targ_ns,
> + struct user_namespace *capable_ns, int cap, unsigned int opts, int ret),
> +
> + TP_ARGS(cred, targ_ns, capable_ns, cap, opts, ret),
> +
> + TP_STRUCT__entry(
> + __field(const struct cred *, cred)
> + __field(struct user_namespace *, targ_ns)
> + __field(struct user_namespace *, capable_ns)
> + __field(int, cap)
> + __field(unsigned int, opts)
> + __field(int, ret)
> + ),
> +
> + TP_fast_assign(
> + __entry->cred = cred;
> + __entry->targ_ns = targ_ns;
> + __entry->capable_ns = capable_ns;
> + __entry->cap = cap;
> + __entry->opts = opts;
> + __entry->ret = ret;
> + ),
> +
> + TP_printk("cred %p, targ_ns %p, capable_ns %p, cap %d, opts %u, ret %d",
> + __entry->cred, __entry->targ_ns, __entry->capable_ns, __entry->cap,
> + __entry->opts, __entry->ret)
> +);
> +
> +#endif /* _TRACE_CAPABILITY_H */
> +
> +/* This part must be outside protection */
> +#include <trace/define_trace.h>
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 162d96b3a676..12c3ddfe0d6e 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -27,6 +27,9 @@
> #include <linux/mnt_idmapping.h>
> #include <uapi/linux/lsm.h>
>
> +#define CREATE_TRACE_POINTS
> +#include <trace/events/capability.h>
> +
> /*
> * If a non-root user executes a setuid-root binary in
> * !secure(SECURE_NOROOT) mode, then we raise capabilities.
> @@ -52,7 +55,7 @@ static void warn_setuid_and_fcaps_mixed(const char *fname)
> /**
> * cap_capable - Determine whether a task has a particular effective capability
> * @cred: The credentials to use
> - * @targ_ns: The user namespace in which we need the capability
> + * @targ_ns: The user namespace of the resource being accessed
> * @cap: The capability to check for
> * @opts: Bitmask of options defined in include/linux/security.h
> *
> @@ -68,6 +71,7 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
> int cap, unsigned int opts)
> {
> struct user_namespace *ns = targ_ns;
> + int ret = -EPERM;
>
> /* See if cred has the capability in the target user namespace
> * by examining the target user namespace and all of the target
> @@ -75,22 +79,32 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
> */
> for (;;) {
> /* Do we have the necessary capabilities? */
> - if (ns == cred->user_ns)
> - return cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;
> + if (ns == cred->user_ns) {
> + if (cap_raised(cred->cap_effective, cap))
> + ret = 0;
> + else
> + ns = NULL;
This is a bit unfortunate :( so maybe all we needed was `ns =
ns->parent` for that one use case, and keep the original `ret ? NULL :
ns` inside trace_cap_capable().
But whatever security folks prefer, I'm fine with either.
Acked-by: Andrii Nakryiko <andrii at kernel.org>
> + break;
> + }
>
> /*
> * If we're already at a lower level than we're looking for,
> * we're done searching.
> */
> - if (ns->level <= cred->user_ns->level)
> - return -EPERM;
> + if (ns->level <= cred->user_ns->level) {
> + ns = NULL;
> + break;
> + }
>
> /*
> * The owner of the user namespace in the parent of the
> * user namespace has all caps.
> */
> - if ((ns->parent == cred->user_ns) && uid_eq(ns->owner, cred->euid))
> - return 0;
> + if ((ns->parent == cred->user_ns) && uid_eq(ns->owner, cred->euid)) {
> + ns = ns->parent;
> + ret = 0;
> + break;
> + }
>
> /*
> * If you have a capability in a parent user ns, then you have
> @@ -99,7 +113,8 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
> ns = ns->parent;
> }
>
> - /* We never get here */
> + trace_cap_capable(cred, targ_ns, ns, cap, opts, ret);
> + return ret;
> }
>
> /**
> --
> 2.43.5
>
>
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