Re: [v1] security: add trace event for cap_capable

[Jordan Rome <linux@jordanrome.com>]

On Fri, Oct 25, 2024 at 7:18 AM <sergeh@kernel.org> wrote:
>
> On Thu, Oct 24, 2024 at 09:15:56PM -0400, Jordan Rome wrote:
> > On Thu, Oct 24, 2024 at 4:28 PM <sergeh@kernel.org> wrote:
> > >
> > > On Thu, Oct 24, 2024 at 03:40:12AM -0700, Jordan Rome 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 which needs the capability" is not quite the
> > > right way to put this.  It's the user namespace against which the
> > > capability is needed.  It's an object, not a subject.  Or maybe
> > > "the user namespace of the resource being accessed".
> > >
> >
> > I like "The user namespace of the resource being accessed"
> >
> > > > - The user namespace that actually has the capability (if one exists)
> > >
> > > How about "the user namespace in which the task has the
> > > capability targeted at the resource"?  (It's not the user
> > > namespace itself that has the capability)
> > >
> >
> > This phrasing seems a little confusing. How about:
> > "The user namespace that has the capability to access the targeted resource" ?
>
> The problem again is that it's not the user namespace that has the
> capability.  The cred has the capability in that user namespace.
>
> "The user namespace in which the credential has the capability to access
> the targeted resource"?
>
> Or maybe s/has/provides/.
>

This sounds good to me with "provides"

> > > > - 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@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@hallyn.com>
> > > >  L:   linux-security-module@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
> > >
> > > (same here)
> > >
> > > > + * @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) {
> > > > +                     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))
> > > > -                     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 */
> > >
> > > With this change, I become less comfortable with us assuming that it is
> > > the case that we'll never just drop off the end of the while loop.  I'd
> > > be more comfortable if you set ret = -EPERM at the top, and set it to 0
> > > in the last break.
> > >
> >
> > Sure. I can make this change.
> >
> > > > +     trace_capable(cred, targ_ns, ret == 0 ? ns : NULL, cap, opts, ret);
> > > > +     return ret;
> > > >  }
> > > >
> > > >  /**
> > > > --
> > > > 2.43.5
> > > >