Messages in this thread Patch in this message | | | Subject | Re: [PATCH v8 -tip 13/26] kernel/entry: Add support for core-wide protection of kernel-mode | From | Alexandre Chartre <> | Date | Tue, 10 Nov 2020 10:35:17 +0100 |
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On 11/3/20 2:20 AM, Joel Fernandes wrote: > Hi Alexandre, > > Sorry for late reply as I was working on the snapshotting patch... > > On Fri, Oct 30, 2020 at 11:29:26AM +0100, Alexandre Chartre wrote: >> >> On 10/20/20 3:43 AM, Joel Fernandes (Google) wrote: >>> Core-scheduling prevents hyperthreads in usermode from attacking each >>> other, but it does not do anything about one of the hyperthreads >>> entering the kernel for any reason. This leaves the door open for MDS >>> and L1TF attacks with concurrent execution sequences between >>> hyperthreads. >>> >>> This patch therefore adds support for protecting all syscall and IRQ >>> kernel mode entries. Care is taken to track the outermost usermode exit >>> and entry using per-cpu counters. In cases where one of the hyperthreads >>> enter the kernel, no additional IPIs are sent. Further, IPIs are avoided >>> when not needed - example: idle and non-cookie HTs do not need to be >>> forced into kernel mode. >> >> Hi Joel, >> >> In order to protect syscall/IRQ kernel mode entries, shouldn't we have a >> call to sched_core_unsafe_enter() in the syscall/IRQ entry code? I don't >> see such a call. Am I missing something? > > Yes, this is known bug and fixed in v9 which I'll post soon. Meanwhile > updated patch is appended below: >
See comments below about the updated patch.
> ---8<----------------------- > > From b2835a587a28405ffdf8fc801e798129a014a8c8 Mon Sep 17 00:00:00 2001 > From: "Joel Fernandes (Google)" <joel@joelfernandes.org> > Date: Mon, 27 Jul 2020 17:56:14 -0400 > Subject: [PATCH] kernel/entry: Add support for core-wide protection of > kernel-mode > > Core-scheduling prevents hyperthreads in usermode from attacking each > other, but it does not do anything about one of the hyperthreads > entering the kernel for any reason. This leaves the door open for MDS > and L1TF attacks with concurrent execution sequences between > hyperthreads. > > This patch therefore adds support for protecting all syscall and IRQ > kernel mode entries. Care is taken to track the outermost usermode exit > and entry using per-cpu counters. In cases where one of the hyperthreads > enter the kernel, no additional IPIs are sent. Further, IPIs are avoided > when not needed - example: idle and non-cookie HTs do not need to be > forced into kernel mode. > > More information about attacks: > For MDS, it is possible for syscalls, IRQ and softirq handlers to leak > data to either host or guest attackers. For L1TF, it is possible to leak > to guest attackers. There is no possible mitigation involving flushing > of buffers to avoid this since the execution of attacker and victims > happen concurrently on 2 or more HTs. > > Cc: Julien Desfossez <jdesfossez@digitalocean.com> > Cc: Tim Chen <tim.c.chen@linux.intel.com> > Cc: Aaron Lu <aaron.lwe@gmail.com> > Cc: Aubrey Li <aubrey.li@linux.intel.com> > Cc: Tim Chen <tim.c.chen@intel.com> > Cc: Paul E. McKenney <paulmck@kernel.org> > Co-developed-by: Vineeth Pillai <viremana@linux.microsoft.com> > Tested-by: Julien Desfossez <jdesfossez@digitalocean.com> > Signed-off-by: Vineeth Pillai <viremana@linux.microsoft.com> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > --- > .../admin-guide/kernel-parameters.txt | 9 + > include/linux/entry-common.h | 6 +- > include/linux/sched.h | 12 + > kernel/entry/common.c | 28 ++- > kernel/sched/core.c | 230 ++++++++++++++++++ > kernel/sched/sched.h | 3 + > 6 files changed, 285 insertions(+), 3 deletions(-) > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > index 3236427e2215..a338d5d64c3d 100644 > --- a/Documentation/admin-guide/kernel-parameters.txt > +++ b/Documentation/admin-guide/kernel-parameters.txt > @@ -4678,6 +4678,15 @@ > > sbni= [NET] Granch SBNI12 leased line adapter > > + sched_core_protect_kernel= > + [SCHED_CORE] Pause SMT siblings of a core running in > + user mode, if at least one of the siblings of the core > + is running in kernel mode. This is to guarantee that > + kernel data is not leaked to tasks which are not trusted > + by the kernel. A value of 0 disables protection, 1 > + enables protection. The default is 1. Note that protection > + depends on the arch defining the _TIF_UNSAFE_RET flag. > + > sched_debug [KNL] Enables verbose scheduler debug messages. > > schedstats= [KNL,X86] Enable or disable scheduled statistics. > diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h > index 474f29638d2c..62278c5b3b5f 100644 > --- a/include/linux/entry-common.h > +++ b/include/linux/entry-common.h > @@ -33,6 +33,10 @@ > # define _TIF_PATCH_PENDING (0) > #endif > > +#ifndef _TIF_UNSAFE_RET > +# define _TIF_UNSAFE_RET (0) > +#endif > + > #ifndef _TIF_UPROBE > # define _TIF_UPROBE (0) > #endif > @@ -69,7 +73,7 @@ > > #define EXIT_TO_USER_MODE_WORK \ > (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE | \ > - _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | \ > + _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | _TIF_UNSAFE_RET | \ > ARCH_EXIT_TO_USER_MODE_WORK) > > /** > diff --git a/include/linux/sched.h b/include/linux/sched.h > index d38e904dd603..fe6f225bfbf9 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -2071,4 +2071,16 @@ int sched_trace_rq_nr_running(struct rq *rq); > > const struct cpumask *sched_trace_rd_span(struct root_domain *rd); > > +#ifdef CONFIG_SCHED_CORE > +void sched_core_unsafe_enter(void); > +void sched_core_unsafe_exit(void); > +bool sched_core_wait_till_safe(unsigned long ti_check); > +bool sched_core_kernel_protected(void); > +#else > +#define sched_core_unsafe_enter(ignore) do { } while (0) > +#define sched_core_unsafe_exit(ignore) do { } while (0) > +#define sched_core_wait_till_safe(ignore) do { } while (0) > +#define sched_core_kernel_protected(ignore) do { } while (0) > +#endif > + > #endif > diff --git a/kernel/entry/common.c b/kernel/entry/common.c > index 0a1e20f8d4e8..a18ed60cedea 100644 > --- a/kernel/entry/common.c > +++ b/kernel/entry/common.c > @@ -28,6 +28,8 @@ static __always_inline void enter_from_user_mode(struct pt_regs *regs) > > instrumentation_begin(); > trace_hardirqs_off_finish(); > + if (_TIF_UNSAFE_RET) /* Kernel protection depends on arch defining the flag. */ > + sched_core_unsafe_enter(); > instrumentation_end(); > } > > @@ -137,6 +139,27 @@ static __always_inline void exit_to_user_mode(void) > /* Workaround to allow gradual conversion of architecture code */ > void __weak arch_do_signal(struct pt_regs *regs) { } > > +unsigned long exit_to_user_get_work(void)
Function should be static.
> +{ > + unsigned long ti_work = READ_ONCE(current_thread_info()->flags); > + > + if ((IS_ENABLED(CONFIG_SCHED_CORE) && !sched_core_kernel_protected()) > + || !_TIF_UNSAFE_RET) > + return ti_work; > + > +#ifdef CONFIG_SCHED_CORE > + ti_work &= EXIT_TO_USER_MODE_WORK; > + if ((ti_work & _TIF_UNSAFE_RET) == ti_work) { > + sched_core_unsafe_exit(); > + if (sched_core_wait_till_safe(EXIT_TO_USER_MODE_WORK)) { > + sched_core_unsafe_enter(); /* not exiting to user yet. */ > + } > + } > + > + return READ_ONCE(current_thread_info()->flags); > +#endif > +} > + > static unsigned long exit_to_user_mode_loop(struct pt_regs *regs, > unsigned long ti_work) > { > @@ -175,7 +198,7 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs, > * enabled above. > */ > local_irq_disable_exit_to_user(); > - ti_work = READ_ONCE(current_thread_info()->flags); > + ti_work = exit_to_user_get_work(); > }
What happen if the task is scheduled out in exit_to_user_mode_loop? (e.g. if it has _TIF_NEED_RESCHED set). It will have call sched_core_unsafe_enter() and force siblings to wait for it. So shouldn't sched_core_unsafe_exit() be called when the task is scheduled out? (because it won't run anymore) And sched_core_unsafe_enter() when the task is scheduled back in?
> /* Return the latest work state for arch_exit_to_user_mode() */ > @@ -184,9 +207,10 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs, > > static void exit_to_user_mode_prepare(struct pt_regs *regs) > { > - unsigned long ti_work = READ_ONCE(current_thread_info()->flags); > + unsigned long ti_work; > > lockdep_assert_irqs_disabled(); > + ti_work = exit_to_user_get_work(); > > if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK)) > ti_work = exit_to_user_mode_loop(regs, ti_work); > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index e05728bdb18c..bd206708fac2 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -76,6 +76,27 @@ __read_mostly int scheduler_running; > > #ifdef CONFIG_SCHED_CORE > > +DEFINE_STATIC_KEY_TRUE(sched_core_protect_kernel); > +static int __init set_sched_core_protect_kernel(char *str) > +{ > + unsigned long val = 0; > + > + if (!str) > + return 0; > + > + if (!kstrtoul(str, 0, &val) && !val) > + static_branch_disable(&sched_core_protect_kernel); > + > + return 1; > +} > +__setup("sched_core_protect_kernel=", set_sched_core_protect_kernel); > + > +/* Is the kernel protected by core scheduling? */ > +bool sched_core_kernel_protected(void) > +{ > + return static_branch_likely(&sched_core_protect_kernel); > +} > + > DEFINE_STATIC_KEY_FALSE(__sched_core_enabled); > > /* kernel prio, less is more */ > @@ -4596,6 +4617,214 @@ static inline bool cookie_match(struct task_struct *a, struct task_struct *b) > return a->core_cookie == b->core_cookie; > } > > +/* > + * Handler to attempt to enter kernel. It does nothing because the exit to > + * usermode or guest mode will do the actual work (of waiting if needed). > + */ > +static void sched_core_irq_work(struct irq_work *work) > +{ > + return; > +} > + > +static inline void init_sched_core_irq_work(struct rq *rq) > +{ > + init_irq_work(&rq->core_irq_work, sched_core_irq_work); > +} > + > +/* > + * sched_core_wait_till_safe - Pause the caller's hyperthread until the core > + * exits the core-wide unsafe state. Obviously the CPU calling this function > + * should not be responsible for the core being in the core-wide unsafe state > + * otherwise it will deadlock. > + * > + * @ti_check: We spin here with IRQ enabled and preempt disabled. Break out of > + * the loop if TIF flags are set and notify caller about it. > + * > + * IRQs should be disabled. > + */ > +bool sched_core_wait_till_safe(unsigned long ti_check) > +{ > + bool restart = false; > + struct rq *rq; > + int cpu; > + > + /* We clear the thread flag only at the end, so need to check for it. */
Do you mean "no need to check for it" ?
> + ti_check &= ~_TIF_UNSAFE_RET; > + > + cpu = smp_processor_id(); > + rq = cpu_rq(cpu); > + > + if (!sched_core_enabled(rq)) > + goto ret; > + > + /* Down grade to allow interrupts to prevent stop_machine lockups.. */ > + preempt_disable(); > + local_irq_enable(); > + > + /* > + * Wait till the core of this HT is not in an unsafe state. > + * > + * Pair with smp_store_release() in sched_core_unsafe_exit(). > + */ > + while (smp_load_acquire(&rq->core->core_unsafe_nest) > 0) { > + cpu_relax(); > + if (READ_ONCE(current_thread_info()->flags) & ti_check) { > + restart = true; > + break; > + } > + } > + > + /* Upgrade it back to the expectations of entry code. */ > + local_irq_disable(); > + preempt_enable(); > + > +ret: > + if (!restart) > + clear_tsk_thread_flag(current, TIF_UNSAFE_RET); > + > + return restart; > +} > + > +/* > + * Enter the core-wide IRQ state. Sibling will be paused if it is running > + * 'untrusted' code, until sched_core_unsafe_exit() is called. Every attempt to > + * avoid sending useless IPIs is made. Must be called only from hard IRQ > + * context. > + */ > +void sched_core_unsafe_enter(void) > +{ > + const struct cpumask *smt_mask; > + unsigned long flags; > + struct rq *rq; > + int i, cpu; > + > + if (!static_branch_likely(&sched_core_protect_kernel)) > + return; > + > + /* Ensure that on return to user/guest, we check whether to wait. */ > + if (current->core_cookie) > + set_tsk_thread_flag(current, TIF_UNSAFE_RET); > + > + local_irq_save(flags); > + cpu = smp_processor_id(); > + rq = cpu_rq(cpu); > + if (!sched_core_enabled(rq)) > + goto ret;
Should we clear TIF_UNSAFE_RET if (!sched_core_enabled(rq))? This would avoid calling sched_core_wait_till_safe().
> + > + /* Count unsafe_enter() calls received without unsafe_exit() on this CPU. */ > + rq->core_this_unsafe_nest++; > + > + /* Should not nest: enter() should only pair with exit(). */ > + if (WARN_ON_ONCE(rq->core_this_unsafe_nest != 1)) > + goto ret;
I would be more precise about the nesting comment: we don't nest not only because each enter() is paired with an exit() but because each enter()/exit() is for a user context. We can have nested interrupts but they will be for a kernel context so they won't enter/exit.
So I would say something like:
/* * Should not nest: each enter() is paired with an exit(), and enter()/exit() * are done when coming from userspace. We can have nested interrupts between * enter()/exit() but they will originate from the kernel so they won't enter() * nor exit(). */
> + > + raw_spin_lock(rq_lockp(rq)); > + smt_mask = cpu_smt_mask(cpu); > + > + /* Contribute this CPU's unsafe_enter() to core-wide unsafe_enter() count. */ > + WRITE_ONCE(rq->core->core_unsafe_nest, rq->core->core_unsafe_nest + 1);
We are protected by the rq_lockp(rq) spinlock, but we still need to use WRITE_ONCE() because sched_core_wait_till_safe() checks core_unsafe_next without taking rq_lockp(rq), right? Shouldn't we be using smp_store_release() like sched_core_unsafe_exit() does?
In any case, it is worth having a comment why WRITE_ONCE() or smp_store_release() is used.
> + > + if (WARN_ON_ONCE(rq->core->core_unsafe_nest == UINT_MAX)) > + goto unlock;
It might be better checking if (rq->core->core_unsafe_nest >= cpumask_weight(smt_mask)) because we shouldn't exceed the number of siblings.
alex.
> + > + if (irq_work_is_busy(&rq->core_irq_work)) { > + /* > + * Do nothing more since we are in an IPI sent from another > + * sibling to enforce safety. That sibling would have sent IPIs > + * to all of the HTs. > + */ > + goto unlock; > + } > + > + /* > + * If we are not the first ones on the core to enter core-wide unsafe > + * state, do nothing. > + */ > + if (rq->core->core_unsafe_nest > 1) > + goto unlock; > + > + /* Do nothing more if the core is not tagged. */ > + if (!rq->core->core_cookie) > + goto unlock; > + > + for_each_cpu(i, smt_mask) { > + struct rq *srq = cpu_rq(i); > + > + if (i == cpu || cpu_is_offline(i)) > + continue; > + > + if (!srq->curr->mm || is_task_rq_idle(srq->curr)) > + continue; > + > + /* Skip if HT is not running a tagged task. */ > + if (!srq->curr->core_cookie && !srq->core_pick) > + continue; > + > + /* > + * Force sibling into the kernel by IPI. If work was already > + * pending, no new IPIs are sent. This is Ok since the receiver > + * would already be in the kernel, or on its way to it. > + */ > + irq_work_queue_on(&srq->core_irq_work, i); > + } > +unlock: > + raw_spin_unlock(rq_lockp(rq)); > +ret: > + local_irq_restore(flags); > +} > + > +/* > + * Process any work need for either exiting the core-wide unsafe state, or for > + * waiting on this hyperthread if the core is still in this state. > + * > + * @idle: Are we called from the idle loop? > + */ > +void sched_core_unsafe_exit(void) > +{ > + unsigned long flags; > + unsigned int nest; > + struct rq *rq; > + int cpu; > + > + if (!static_branch_likely(&sched_core_protect_kernel)) > + return; > + > + local_irq_save(flags); > + cpu = smp_processor_id(); > + rq = cpu_rq(cpu); > + > + /* Do nothing if core-sched disabled. */ > + if (!sched_core_enabled(rq)) > + goto ret; > + > + /* > + * Can happen when a process is forked and the first return to user > + * mode is a syscall exit. Either way, there's nothing to do. > + */ > + if (rq->core_this_unsafe_nest == 0) > + goto ret; > + > + rq->core_this_unsafe_nest--; > + > + /* enter() should be paired with exit() only. */ > + if (WARN_ON_ONCE(rq->core_this_unsafe_nest != 0)) > + goto ret; > + > + raw_spin_lock(rq_lockp(rq)); > + /* > + * Core-wide nesting counter can never be 0 because we are > + * still in it on this CPU. > + */ > + nest = rq->core->core_unsafe_nest; > + WARN_ON_ONCE(!nest); > + > + /* Pair with smp_load_acquire() in sched_core_wait_till_safe(). */ > + smp_store_release(&rq->core->core_unsafe_nest, nest - 1); > + raw_spin_unlock(rq_lockp(rq)); > +ret: > + local_irq_restore(flags); > +} > + > // XXX fairness/fwd progress conditions > /* > * Returns > @@ -5019,6 +5248,7 @@ static inline void sched_core_cpu_starting(unsigned int cpu) > rq = cpu_rq(i); > if (rq->core && rq->core == rq) > core_rq = rq; > + init_sched_core_irq_work(rq); > } > > if (!core_rq) > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index f7e2d8a3be8e..4bcf3b1ddfb3 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -1059,12 +1059,15 @@ struct rq { > unsigned int core_enabled; > unsigned int core_sched_seq; > struct rb_root core_tree; > + struct irq_work core_irq_work; /* To force HT into kernel */ > + unsigned int core_this_unsafe_nest; > > /* shared state */ > unsigned int core_task_seq; > unsigned int core_pick_seq; > unsigned long core_cookie; > unsigned char core_forceidle; > + unsigned int core_unsafe_nest; > #endif > }; > >
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