[PATCH v2] kernel/sys.c: fix and improve control flow in __sys_setres[ug]id()

[Ondrej Mosnacek]

On Mon, Mar 13, 2023 at 7:29 PM Andrew Morton <akpm at linux-foundation.org> wrote:
>
> On Fri, 17 Feb 2023 17:21:54 +0100 Ondrej Mosnacek <omosnace at redhat.com> wrote:
>
> > Linux Security Modules (LSMs) that implement the "capable" hook will
> > usually emit an access denial message to the audit log whenever they
> > "block" the current task from using the given capability based on their
> > security policy.
> >
> > The occurrence of a denial is used as an indication that the given task
> > has attempted an operation that requires the given access permission, so
> > the callers of functions that perform LSM permission checks must take
> > care to avoid calling them too early (before it is decided if the
> > permission is actually needed to perform the requested operation).
> >
> > The __sys_setres[ug]id() functions violate this convention by first
> > calling ns_capable_setid() and only then checking if the operation
> > requires the capability or not. It means that any caller that has the
> > capability granted by DAC (task's capability set) but not by MAC (LSMs)
> > will generate a "denied" audit record, even if is doing an operation for
> > which the capability is not required.
> >
> > Fix this by reordering the checks such that ns_capable_setid() is
> > checked last and -EPERM is returned immediately if it returns false.
> >
> > While there, also do two small optimizations:
> > * move the capability check before prepare_creds() and
> > * bail out early in case of a no-op.
> >
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>
> Looks and sounds good to me, so I queued it up for some testing.  I'd
> ask that someone more familiar with this code perform review, please.
>
> I assume that you believe that a -stable backport is desirable?  I'll
> add a cc:stable to the patch for now.

Yes, it's a minor bug, but we hit it while testing on Fedora and it's
better for us to have the fix in stable kernels than adding a
workaround elsewhere.

Thanks,

-- 
Ondrej Mosnacek
Senior Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.