[v1] security: add trace event for cap_capable

[Andrii Nakryiko]

On Thu, Oct 24, 2024 at 3:40 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 which needs the capability
> - The user namespace that actually has the capability (if one exists)
> - 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 | 58 +++++++++++++++++++++++++++++++
>  security/commoncap.c              | 21 +++++++----
>  3 files changed, 74 insertions(+), 6 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..092b8e77063a
> --- /dev/null
> +++ b/include/trace/events/capability.h
> @@ -0,0 +1,58 @@
> +/* 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>
> +
> +/**
> + * capable - called after it's determined if a task has a particular
> + * effective capability
> + *
> + * @cred: The credentials used
> + * @targ_ns:  The user namespace which needs the capability
> + * @capable_ns:  The user namespace that actually has the capability
> + *               if ret is 0 otherwise this will be NULL
> + * @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(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("cap %d, opts %u, ret %d",
> +               __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..675d40fbaa77 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.
> @@ -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 = 0;
>
>         /* See if cred has the capability in the target user namespace
>          * by examining the target user namespace and all of the target
> @@ -75,22 +79,26 @@ 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) {
> +                       ret = cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;
> +                       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) {

I'd do

ns = NULL;

here

> +                       ret = -EPERM;
> +                       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))

and this needs `ns = ns->parent;` (because that's the namespace that
grants capability, is that right?)

> -                       return 0;
> +                       break;
>
>                 /*
>                  * If you have a capability in a parent user ns, then you have
> @@ -99,7 +107,8 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
>                 ns = ns->parent;
>         }
>
> -       /* We never get here */
> +       trace_capable(cred, targ_ns, ret == 0 ? ns : NULL, cap, opts, ret);

with the above changes just pass ns directly, no need for `ret == 0`
check (and it seems it's wrong for one of the case due to ns->parent
use)

> +       return ret;
>  }
>
>  /**
> --
> 2.43.5
>