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

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 17 Feb 2023 17:21:54 +0100 Ondrej Mosnacek <omosnace@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.