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Date: Wed, 7 Nov 2018 13:33:08 -0500
From: Waiman Long <longman@...hat.com>
To: Tim Chen <tim.c.chen@...ux.intel.com>,
Thomas Gleixner <tglx@...utronix.de>
Cc: Jiri Kosina <jikos@...nel.org>,
Tom Lendacky <thomas.lendacky@....com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Andrea Arcangeli <aarcange@...hat.com>,
David Woodhouse <dwmw@...zon.co.uk>,
Andi Kleen <ak@...ux.intel.com>,
Dave Hansen <dave.hansen@...el.com>,
Casey Schaufler <casey.schaufler@...el.com>,
Asit Mallick <asit.k.mallick@...el.com>,
Arjan van de Ven <arjan@...ux.intel.com>,
Jon Masters <jcm@...hat.com>,
LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
Kees Cook <keescook@...omium.org>
Subject: Re: [Patch v4 17/18] x86/speculation: Update SPEC_CTRL MSRs of remote
CPUs
On 11/06/2018 07:18 PM, Tim Chen wrote:
> Thomas,
>
>>>> 2) Add _TIF_UPDATE_SPEC_CTRL to the SYSCALL_EXIT_WORK_FLAGS and handle it
>>>> in the slow work path.
>>> There can be tasks that don't do any syscalls, and it seems like we can
>>> have MSRs getting out of sync?
>> Setting the TIF flag directly in a remote task is wrong. It needs to be
>> handled when the _TIF_UPDATE_SPEC_CTRL is evaluated, i.e. the information
>> needs to be stored process wide e.g. in task->mm.
>>
>> But yes, if the remote task runs in user space forever, it won't
>> help. Though the point is that dumpable is usually set when the process
>> starts, so it's probably mostly a theoretical issue.
>>
> I took a crack to implement what you suggested to update
> remote task's flag and remote SPEC_CTRL MSR on the syscall exit slow path.
>
> This looks reasobale?
>
> Tim
>
>
> ------------
>
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 3b2490b..614594a 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -216,7 +216,7 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
>
> #define SYSCALL_EXIT_WORK_FLAGS \
> (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
> - _TIF_SINGLESTEP | _TIF_SYSCALL_TRACEPOINT)
> + _TIF_SINGLESTEP | _TIF_SYSCALL_TRACEPOINT | _TIF_UPDATE_SPEC_CTRL)
>
> static void syscall_slow_exit_work(struct pt_regs *regs, u32 cached_flags)
> {
> @@ -227,6 +227,8 @@ static void syscall_slow_exit_work(struct pt_regs *regs, u32 cached_flags)
> if (cached_flags & _TIF_SYSCALL_TRACEPOINT)
> trace_sys_exit(regs, regs->ax);
>
> + if (cached_flags & _TIF_UPDATE_SPEC_CTRL)
> + spec_ctrl_do_pending_update();
> /*
> * If TIF_SYSCALL_EMU is set, we only get here because of
> * TIF_SINGLESTEP (i.e. this is PTRACE_SYSEMU_SINGLESTEP).
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index c59a6c4..f124597 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -276,6 +276,8 @@ static inline void indirect_branch_prediction_barrier(void)
> alternative_msr_write(MSR_IA32_PRED_CMD, val, X86_FEATURE_USE_IBPB);
> }
>
> +void spec_ctrl_do_pending_update(void);
> +
> /* The Intel SPEC CTRL MSR base value cache */
> extern u64 x86_spec_ctrl_base;
>
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index 4f6a7a9..b78db59 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -97,6 +97,7 @@ struct thread_info {
> #define TIF_USER_RETURN_NOTIFY 14 /* Notify kernel of userspace return */
> #define TIF_PATCH_PENDING 15 /* Pending live patching update */
> #define TIF_FSCHECK 16 /* Check FS is USER_DS on return */
> +#define TIF_UPDATE_SPEC_CTRL 17 /* Pending update of speculation control */
>
> /* Task status */
> #define TIF_UPROBE 18 /* Breakpointed or singlestepping */
> @@ -131,6 +132,7 @@ struct thread_info {
> #define _TIF_USER_RETURN_NOTIFY (1 << TIF_USER_RETURN_NOTIFY)
> #define _TIF_PATCH_PENDING (1 << TIF_PATCH_PENDING)
> #define _TIF_FSCHECK (1 << TIF_FSCHECK)
> +#define _TIF_UPDATE_SPEC_CTRL (1 << TIF_UPDATE_SPEC_CTRL)
>
> #define _TIF_UPROBE (1 << TIF_UPROBE)
> #define _TIF_MEMDIE (1 << TIF_MEMDIE)
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 4c15c86..d82d3f8 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -14,6 +14,8 @@
> #include <linux/module.h>
> #include <linux/nospec.h>
> #include <linux/prctl.h>
> +#include <linux/sched/coredump.h>
> +#include <linux/sched/signal.h>
>
> #include <asm/spec-ctrl.h>
> #include <asm/cmdline.h>
> @@ -770,6 +772,69 @@ static int ssb_prctl_set(struct task_struct *task, unsigned long ctrl)
> return 0;
> }
>
> +static void set_task_stibp(struct task_struct *tsk, bool stibp_on)
> +{
> + bool update = false;
> +
> + if (stibp_on)
> + update = !test_and_set_tsk_thread_flag(tsk, TIF_STIBP);
> + else
> + update = test_and_clear_tsk_thread_flag(tsk, TIF_STIBP);
> +
> + if (tsk == current && update)
> + speculation_ctrl_update_current();
> +}
> +
> +void spec_ctrl_do_pending_update(void)
> +{
> + if (!static_branch_unlikely(&spectre_v2_app_lite))
> + return;
> +
> + if (!current->mm)
> + return;
> +
> + if (get_dumpable(current->mm) != SUID_DUMP_USER)
> + set_tsk_thread_flag(current, TIF_STIBP);
> + else
> + clear_tsk_thread_flag(current, TIF_STIBP);
> +
> + clear_tsk_thread_flag(current, TIF_UPDATE_SPEC_CTRL);
> + speculation_ctrl_update_current();
> +}
> +
> +int arch_update_spec_ctrl_restriction(struct task_struct *task)
> +{
> + unsigned long flags;
> + struct task_struct *t;
> + bool stibp_on = false;
> +
> + if (!static_branch_unlikely(&spectre_v2_app_lite))
> + return 0;
> +
> + if (!task->mm)
> + return -EINVAL;
> +
> + if (!lock_task_sighand(task, &flags))
> + return -ESRCH;
> +
> + if (get_dumpable(task->mm) != SUID_DUMP_USER)
> + stibp_on = true;
> +
> + for_each_thread(task, t) {
> + if (task_cpu(task) == smp_processor_id())
> + set_task_stibp(task, stibp_on);
I think "t" is the iterator, not "task". BTW, a thread is on the same
CPU doesn't mean it is running. Should you just check "(t == current)" here?
> + else if (test_tsk_thread_flag(task, TIF_STIBP) != stibp_on)
> + set_tsk_thread_flag(task, TIF_UPDATE_SPEC_CTRL);
> + }
> +
> + unlock_task_sighand(task, &flags);
> + return 0;
> +}
> +
> int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
> unsigned long ctrl)
> {
Cheers,
Longman
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