Re: [PATCH v2 0/3] Implement LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS

["Mickaël Salaün" <mic@digikod.net>]

On Fri, Jul 17, 2026 at 06:03:16PM -0400, Justin Suess wrote:
> Howdy
> 
> This series adds a new landlock_restrict_self(2) flag:
> LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS.
> 
> This is a redesign of v1 [1], which added
> LANDLOCK_RESTRICT_SELF_NNP_ON_EXEC, a flag staging no_new_privs so that
> it was only set at the next execve(2).  Following Mickaël's feedback,
> [2] the staging mechanism (credential bit and bprm_committing_creds hook)
> is dropped entirely.  The new flag instead sets the no_new_privs
> attribute of the calling thread atomically with the enforcement of the
> ruleset: no_new_privs is set if and only if the
> landlock_restrict_self(2) call succeeds.
> 
> Semantics:
> 
> A single call replaces the usual prctl(PR_SET_NO_NEW_PRIVS) +
> landlock_restrict_self(2) pair.  Because no_new_privs is set by the
> call itself, the no_new_privs/CAP_SYS_ADMIN precondition is fulfilled
> by construction, so the flag is usable by unprivileged processes.  This
> is safe for the same reason the prctl(2) pair is: the executed programs
> can either gain privileges or be restricted, never both.
> 
> The two states cannot diverge.  A failed call (invalid ruleset FD,
> E2BIG, ENOMEM, interrupted TSYNC, ...) leaves no_new_privs unchanged,
> and a successful call never returns without no_new_privs set: the
> attribute is set past the last point of failure, right before
> commit_creds(), which cannot fail.
> 
> Combined with LANDLOCK_RESTRICT_SELF_TSYNC, no_new_privs is set on all
> threads with the same guarantee: each sibling thread sets it in the
> commit phase of the TSYNC protocol, after its all-or-nothing barrier,
> so either every thread gets both the domain and no_new_privs, or none
> does.  This also makes it possible to atomically set no_new_privs
> process-wide, which prctl(2) cannot do.
> 
> Unlike the v1 flag, this flag requires a ruleset: calls with a
> ruleset_fd of -1 are rejected.  As a consequence of the fulfilled
> precondition, an unprivileged caller passing unknown flag bits together
> with this flag receives EINVAL instead of EPERM; the selftests pin this
> error ordering as well.
> 
> The reason why -1 ruleset_fd is rejected is basically then we are
> making a Landlock-flaved prctl(nnp) call that doesn't do anything
> special. It seems better to be able to have the option to define
> behavior later rather than have a useless feature stuck in the
> syscall abi. So we reject the -1 ruleset_fd for now.

I think it's the right approach to always require a valid ruleset (see
my reply on the previous thread), and this should be explained in the
commit message.  I don't see valid use cases where someone would like to
set NNP+TSYNC but not add any Landlock restrictions as well.

> 
> The Landlock ABI version is bumped to 11.
> 
> Test coverage:
> 
> base_test checks that a successful call sets no_new_privs without a
> prior prctl(2) nor CAP_SYS_ADMIN, that a failed call leaves it
> unchanged, that the flag requires a ruleset FD, and the updated
> EPERM/EINVAL ordering.  tsync_test checks that TSYNC sets no_new_privs
> on sibling threads along with the domain.
> 
> Changes since v1:
> 
> - Renamed LANDLOCK_RESTRICT_SELF_NNP_ON_EXEC to
>   LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS, per Mickaël's feedback.
> - Dropped the staged "on exec" design: removed the credential bit and
>   the bprm_committing_creds hook; no_new_privs is now set atomically
>   with the ruleset enforcement.
> - The flag now fulfills the no_new_privs/CAP_SYS_ADMIN precondition,
>   a significant departure from the v1.
> - The flag now requires a ruleset (no ruleset_fd = -1). Otherwise such
>   a call would just == a vanilla prctl no-new-privs call. This is left
>   in case we want to repurpose it later rather than defining useless
>   redundant behavior.
> 
> [1] https://lore.kernel.org/linux-security-module/20260708133928.852999-1-utilityemal77@gmail.com/
> [2] https://lore.kernel.org/linux-security-module/20260709.Eaphooyoh6sh@digikod.net/
> 
> Justin Suess (3):
>   landlock: Add LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
>   selftests/landlock: Test LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
>   landlock: Document LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
> 
>  Documentation/userspace-api/landlock.rst      | 14 +++-
>  include/uapi/linux/landlock.h                 | 13 ++++
>  security/landlock/limits.h                    |  2 +-
>  security/landlock/syscalls.c                  | 28 ++++++--
>  security/landlock/tsync.c                     |  8 ++-
>  security/landlock/tsync.h                     |  4 +-
>  tools/testing/selftests/landlock/base_test.c  | 65 +++++++++++++++++--
>  tools/testing/selftests/landlock/tsync_test.c | 33 ++++++++++
>  8 files changed, 150 insertions(+), 17 deletions(-)
> 
> 
> base-commit: 55f82176ef8dde632ea3eb94a6224950ed809d7c
> -- 
> 2.54.0
> 
>