Re: [PATCH v3 1/4] landlock: Add LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS

["Mickaël Salaün" <mic@digikod.net>]

On Sun, Aug 09, 2026 at 11:31:01AM -0400, Justin Suess wrote:
> On Fri, Aug 07, 2026 at 03:18:07PM +0200, Mickaël Salaün wrote:
> > On Mon, Aug 03, 2026 at 06:31:05PM -0400, Justin Suess wrote:
> > > Add a landlock_restrict_self(2) flag to set the no_new_privs attribute
> > > of the calling thread only after enforcement of the ruleset:
> > > no_new_privs is set if and only if the call succeeds.  This removes the
> > > need for a prior prctl(2) PR_SET_NO_NEW_PRIVS call and guarantees that
> > > a failed enforcement leaves the attribute unchanged.
> > > 
> > > Because no_new_privs is set by the call itself, the no_new_privs /
> > > CAP_SYS_ADMIN requirement of landlock_restrict_self(2) is fulfilled by
> > > construction, and the related EPERM check is skipped.  As a consequence,
> > > an unprivileged caller passing unknown flags along with this flag gets
> > > EINVAL instead of EPERM.
> > > 
> > > Unlike LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF, this flag always
> > > requires a valid ruleset: with a ruleset_fd of -1, such a call would be
> > > nothing more than a Landlock-flavored prctl(2) PR_SET_NO_NEW_PRIVS, and
> > > there is no valid use case for setting no_new_privs (possibly with
> > > LANDLOCK_RESTRICT_SELF_TSYNC) without also enforcing Landlock
> > > restrictions.  Rejecting these calls also keeps the option of giving
> > > them a meaning later.
> > > 
> > > The attribute is only set past the last point of failure, just before
> > > committing the new credentials.  When combined with
> > > LANDLOCK_RESTRICT_SELF_TSYNC, no_new_privs is set on the sibling threads
> > > as well, in their commit phase, with the same ordering.
> > > 
> > > Bump the Landlock ABI version to 11.
> > > 
> > > Cc: Mickaël Salaün <mic@digikod.net>
> > > Signed-off-by: Justin Suess <utilityemal77@gmail.com>
> > > ---
> > > 
> > > Notes:
> > >     v2->v3:
> > >     - Reword "atomically" to the ordering guarantee (no_new_privs is only set
> > >       once enforcement succeeded) in the commit message and both kdocs
> > >     - Explain in the commit message why the flag requires a valid ruleset
> > > 
> > >  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 +++-
> > >  5 files changed, 44 insertions(+), 11 deletions(-)
> > > 
> > > diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
> > > index 27ae3f39cafb..11bf600698f0 100644
> > > --- a/include/uapi/linux/landlock.h
> > > +++ b/include/uapi/linux/landlock.h
> > > @@ -191,12 +191,25 @@ struct landlock_ruleset_attr {
> > >   *
> > >   *     If the calling thread is running with no_new_privs, this operation
> > >   *     enables no_new_privs on the sibling threads as well.
> > > + *
> > > + * The following flag ties the no_new_privs attribute to the ruleset
> > > + * enforcement:
> > > + *
> > > + * %LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
> > > + *     Sets the no_new_privs attribute of the calling thread only once the
> > > + *     enforcement of the ruleset succeeded: no_new_privs is set if and only
> > > + *     if sys_landlock_restrict_self() succeeds.  This removes the need for a
> > > + *     prior :manpage:`prctl(2)` ``PR_SET_NO_NEW_PRIVS`` call, and with it the
> > > + *     %CAP_SYS_ADMIN requirement.  This flag requires a ruleset.  When
> > > + *     combined with %LANDLOCK_RESTRICT_SELF_TSYNC, no_new_privs is set on the
> > > + *     sibling threads as well.
> > >   */
> > >  /* clang-format off */
> > >  #define LANDLOCK_RESTRICT_SELF_LOG_SAME_EXEC_OFF		(1U << 0)
> > >  #define LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON			(1U << 1)
> > >  #define LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF		(1U << 2)
> > >  #define LANDLOCK_RESTRICT_SELF_TSYNC				(1U << 3)
> > > +#define LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS			(1U << 4)
> > >  /* clang-format on */
> > >  
> > >  /**
> > > diff --git a/security/landlock/limits.h b/security/landlock/limits.h
> > > index 08d5f2f6d321..1a7c5fb8f6fd 100644
> > > --- a/security/landlock/limits.h
> > > +++ b/security/landlock/limits.h
> > > @@ -34,7 +34,7 @@
> > >  #define LANDLOCK_NUM_ACCESS_MAX \
> > >  	MAX(MAX(LANDLOCK_NUM_ACCESS_FS, LANDLOCK_NUM_ACCESS_NET), LANDLOCK_NUM_SCOPE)
> > >  
> > > -#define LANDLOCK_LAST_RESTRICT_SELF	LANDLOCK_RESTRICT_SELF_TSYNC
> > > +#define LANDLOCK_LAST_RESTRICT_SELF	LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
> > >  #define LANDLOCK_MASK_RESTRICT_SELF	((LANDLOCK_LAST_RESTRICT_SELF << 1) - 1)
> > >  
> > >  /* clang-format on */
> > > diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
> > > index 36b02892c62f..e97f944109f9 100644
> > > --- a/security/landlock/syscalls.c
> > > +++ b/security/landlock/syscalls.c
> > > @@ -169,7 +169,7 @@ static const struct file_operations ruleset_fops = {
> > >   * If the change involves a fix that requires userspace awareness, also update
> > >   * the errata documentation in Documentation/userspace-api/landlock.rst .
> > >   */
> > > -const int landlock_abi_version = 10;
> > > +const int landlock_abi_version = 11;
> > >  
> > >  /**
> > >   * sys_landlock_create_ruleset - Create a new ruleset
> > > @@ -502,21 +502,28 @@ SYSCALL_DEFINE4(landlock_add_rule, const int, ruleset_fd,
> > >   *         - %LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON
> > >   *         - %LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF
> > >   *         - %LANDLOCK_RESTRICT_SELF_TSYNC
> > > + *         - %LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
> > >   *
> > >   * This system call enforces a Landlock ruleset on the current thread.
> > >   * Enforcing a ruleset requires that the task has %CAP_SYS_ADMIN in its
> > >   * namespace or is running with no_new_privs.  This avoids scenarios where
> > >   * unprivileged tasks can affect the behavior of privileged children.
> > >   *
> > > + * With %LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS, the no_new_privs attribute of the
> > > + * calling thread is set only once the enforcement of the ruleset succeeded,
> > > + * which fulfills the above requirement: no_new_privs is set if and only if the
> > > + * call succeeds.
> > > + *
> > >   * Return: 0 on success, or -errno on failure.  Possible returned errors are:
> > >   *
> > >   * - %EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
> > >   * - %EINVAL: @flags contains an unknown bit.
> > >   * - %EBADF: @ruleset_fd is not a file descriptor for the current thread;
> > >   * - %EBADFD: @ruleset_fd is not a ruleset file descriptor;
> > > - * - %EPERM: @ruleset_fd has no read access to the underlying ruleset, or the
> > > - *   current thread is not running with no_new_privs, or it doesn't have
> > > - *   %CAP_SYS_ADMIN in its namespace.
> > > + * - %EPERM: @ruleset_fd has no read access to the underlying ruleset, or
> > > + *   %LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS is not set while the current thread
> > > + *   is not running with no_new_privs and doesn't have %CAP_SYS_ADMIN in its
> > > + *   namespace.
> > >   * - %E2BIG: The maximum number of stacked rulesets is reached for the current
> > >   *   thread.
> > >   *
> > > @@ -529,6 +536,8 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
> > >  	struct landlock_ruleset *ruleset __free(landlock_put_ruleset) = NULL;
> > >  	struct cred *new_cred;
> > >  	struct landlock_cred_security *new_llcred;
> > > +	const bool set_no_new_privs =
> > > +		!!(flags & LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS);
> > 
> > This variable should be set later (or not exist at all), see my next
> > comment.
> > 
> > >  	bool __maybe_unused log_same_exec, log_new_exec, log_subdomains,
> > >  		prev_log_subdomains;
> > >  
> > > @@ -537,9 +546,10 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
> > >  
> > >  	/*
> > >  	 * Similar checks as for seccomp(2), except that an -EPERM may be
> > > -	 * returned.
> > > +	 * returned.  LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS fulfills this
> > > +	 * requirement.
> > >  	 */
> > > -	if (!task_no_new_privs(current) &&
> > > +	if (!set_no_new_privs && !task_no_new_privs(current) &&
> > 
> > This is correct according to the current code, but kind of inconsistent
> > wrt previous kernels (e.g. an unprivileged caller *without* NNP already
> > set would get EPERM if it sets LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS,
> > whereas it will now get EINVAL).  In fact, a dedicated patch should move
> > the NNP/CAP checks just after the flags check.  This is a visible change
> > but I think it would be cleaner this way.  BTW, seccomp check flags in
> > the same order.
> >
> 
> If I understand you correctly:
> 
> This technically is an ABI change to existing behavior, but it's pretty
> niche (for callers passing unknown flags w/o CAP_SYS_ADMIN ||
> no_new_privs).

I prefer not to change the ordering priority of errnos but it's OK and
not considered an ABI change.

> 
> I'll add the tests in that commit as well to make it bisectable.

Good

> > 
> > >  	    !ns_capable_noaudit(current_user_ns(), CAP_SYS_ADMIN))
> > >  		return -EPERM;
> > >  
> > > @@ -620,12 +630,16 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
> > >  
> > >  	if (flags & LANDLOCK_RESTRICT_SELF_TSYNC) {
> > >  		const int err = landlock_restrict_sibling_threads(
> > > -			current_cred(), new_cred);
> > > +			current_cred(), new_cred, flags);
> > >  		if (err) {
> > >  			abort_creds(new_cred);
> > >  			return err;
> > >  		}
> > >  	}
> > >  
> > > +	/* Sets no_new_privs past the last point of failure. */
> > > +	if (set_no_new_privs)
> > > +		task_set_no_new_privs(current);
> > > +
> > >  	return commit_creds(new_cred);
> > >  }
> > > diff --git a/security/landlock/tsync.c b/security/landlock/tsync.c
> > > index c5730bbd9ed3..0b71e158c3f5 100644
> > > --- a/security/landlock/tsync.c
> > > +++ b/security/landlock/tsync.c
> > > @@ -17,6 +17,7 @@
> > >  #include <linux/sched/task.h>
> > >  #include <linux/slab.h>
> > >  #include <linux/task_work.h>
> > > +#include <uapi/linux/landlock.h>
> > >  
> > >  #include "cred.h"
> > >  #include "tsync.h"
> > > @@ -466,7 +467,8 @@ static void cancel_tsync_works(const struct tsync_works *works,
> > >   * restrict_sibling_threads - enables a Landlock policy for all sibling threads
> > >   */
> > >  int landlock_restrict_sibling_threads(const struct cred *old_cred,
> > > -				      const struct cred *new_cred)
> > > +				      const struct cred *new_cred,
> > > +				      const u32 restrict_flags)
> > >  {
> > >  	int err;
> > >  	struct tsync_shared_context shared_ctx;
> > > @@ -481,7 +483,9 @@ int landlock_restrict_sibling_threads(const struct cred *old_cred,
> > >  	init_completion(&shared_ctx.all_finished);
> > >  	shared_ctx.old_cred = old_cred;
> > >  	shared_ctx.new_cred = new_cred;
> > > -	shared_ctx.set_no_new_privs = task_no_new_privs(current);
> > > +	shared_ctx.set_no_new_privs =
> > > +		(restrict_flags & LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS) ||
> > > +		task_no_new_privs(current);
> > >  
> > >  	/*
> > >  	 * Serialize concurrent TSYNC operations to prevent deadlocks when
> > > diff --git a/security/landlock/tsync.h b/security/landlock/tsync.h
> > > index ef86bb61c2f6..2ae4f938ca00 100644
> > > --- a/security/landlock/tsync.h
> > > +++ b/security/landlock/tsync.h
> > > @@ -9,8 +9,10 @@
> > >  #define _SECURITY_LANDLOCK_TSYNC_H
> > >  
> > >  #include <linux/cred.h>
> > > +#include <linux/types.h>
> > >  
> > >  int landlock_restrict_sibling_threads(const struct cred *old_cred,
> > > -				      const struct cred *new_cred);
> > > +				      const struct cred *new_cred,
> > > +				      u32 restrict_flags);
> > >  
> > >  #endif /* _SECURITY_LANDLOCK_TSYNC_H */
> > > -- 
> > > 2.54.0
> > > 
> > > 
>