From 23e06b14354b328a100229c2d5c66b1d1ffc7a6f Mon Sep 17 00:00:00 2001 From: Andrei Vagin Date: Tue, 13 Sep 2022 14:10:28 +0000 Subject: [PATCH] fs/exec: switch timens when a task gets a new mm Changing a time namespace requires remapping a vvar page, so we don't want to allow doing that if any other tasks can use the same mm. Currently, we install a time namespace when a task is cloned without CLONE_VM. exec() is another case when a task gets a new mm and so it can switch a time namespace safely, but this case isn't handled now. One more issue of the current interface is that clone() with CLONE_VM isn't allowed if the current task has unshared a time namespace (timens_for_children doesn't match the current timens). Both these issues make some inconvenience for users. For example, Florian reported that posix_spawn() uses vfork+exec and this pattern doesn't work well with time namespaces due to the both described issues. LXC needed to workaround the exec() issue by calling setns. In 133e2d3e81de5 "fs/exec: allow to unshare a time namespace on vfork+exec", we tried to fix these issues with minimal impact on UAPI. But it adds extra complexity and some undesirable side effects. Eric suggested fixing the issues properly because here are all the reasons to suppose that there are no users that depend on the old behavior. Cc: Alexey Izbyshev Cc: Christian Brauner Cc: Dmitry Safonov <0x7f454c46@gmail.com> Cc: "Eric W. Biederman" Cc: Florian Weimer Cc: Kees Cook Suggested-by: "Eric W. Biederman" Original-author: "Eric W. Biederman" Signed-off-by: Andrei Vagin --- fs/exec.c | 5 +++++ include/linux/nsproxy.h | 1 + kernel/fork.c | 9 --------- kernel/nsproxy.c | 23 +++++++++++++++++++++-- 4 files changed, 27 insertions(+), 11 deletions(-) diff --git a/fs/exec.c b/fs/exec.c index d046dbb9cbd0..71284188b96d 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -65,6 +65,7 @@ #include #include #include +#include #include #include @@ -1296,6 +1297,10 @@ int begin_new_exec(struct linux_binprm * bprm) bprm->mm = NULL; + retval = exec_task_namespaces(); + if (retval) + goto out_unlock; + #ifdef CONFIG_POSIX_TIMERS spin_lock_irq(&me->sighand->siglock); posix_cpu_timers_exit(me); diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h index cdb171efc7cb..fee881cded01 100644 --- a/include/linux/nsproxy.h +++ b/include/linux/nsproxy.h @@ -94,6 +94,7 @@ static inline struct cred *nsset_cred(struct nsset *set) int copy_namespaces(unsigned long flags, struct task_struct *tsk); void exit_task_namespaces(struct task_struct *tsk); void switch_task_namespaces(struct task_struct *tsk, struct nsproxy *new); +int exec_task_namespaces(void); void free_nsproxy(struct nsproxy *ns); int unshare_nsproxy_namespaces(unsigned long, struct nsproxy **, struct cred *, struct fs_struct *); diff --git a/kernel/fork.c b/kernel/fork.c index 2b6bd511c6ed..4eb803f75225 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2044,15 +2044,6 @@ static __latent_entropy struct task_struct *copy_process( return ERR_PTR(-EINVAL); } - /* - * If the new process will be in a different time namespace - * do not allow it to share VM or a thread group with the forking task. - */ - if (clone_flags & (CLONE_THREAD | CLONE_VM)) { - if (nsp->time_ns != nsp->time_ns_for_children) - return ERR_PTR(-EINVAL); - } - if (clone_flags & CLONE_PIDFD) { /* * - CLONE_DETACHED is blocked so that we can potentially diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c index eec72ca962e2..a487ff24129b 100644 --- a/kernel/nsproxy.c +++ b/kernel/nsproxy.c @@ -157,7 +157,8 @@ int copy_namespaces(unsigned long flags, struct task_struct *tsk) if (likely(!(flags & (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC | CLONE_NEWPID | CLONE_NEWNET | CLONE_NEWCGROUP | CLONE_NEWTIME)))) { - if (likely(old_ns->time_ns_for_children == old_ns->time_ns)) { + if ((flags & CLONE_VM) || + likely(old_ns->time_ns_for_children == old_ns->time_ns)) { get_nsproxy(old_ns); return 0; } @@ -179,7 +180,8 @@ int copy_namespaces(unsigned long flags, struct task_struct *tsk) if (IS_ERR(new_ns)) return PTR_ERR(new_ns); - timens_on_fork(new_ns, tsk); + if ((flags & CLONE_VM) == 0) + timens_on_fork(new_ns, tsk); tsk->nsproxy = new_ns; return 0; @@ -254,6 +256,23 @@ void exit_task_namespaces(struct task_struct *p) switch_task_namespaces(p, NULL); } +int exec_task_namespaces(void) +{ + struct task_struct *tsk = current; + struct nsproxy *new; + + if (tsk->nsproxy->time_ns_for_children == tsk->nsproxy->time_ns) + return 0; + + new = create_new_namespaces(0, tsk, current_user_ns(), tsk->fs); + if (IS_ERR(new)) + return PTR_ERR(new); + + timens_on_fork(new, tsk); + switch_task_namespaces(tsk, new); + return 0; +} + static int check_setns_flags(unsigned long flags) { if (!flags || (flags & ~(CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC | -- 2.37.2.789.g6183377224-goog