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

[Ondrej Mosnacek]

On Fri, Feb 17, 2023 at 5:21 PM 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")
> Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
> ---
>
> v2: improve commit message
>
>  kernel/sys.c | 69 ++++++++++++++++++++++++++++++----------------------
>  1 file changed, 40 insertions(+), 29 deletions(-)

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