Re: [PATCH v4 5/6] Optionally flush L1D on context switch

From: Kees Cook
Date: Thu Apr 23 2020 - 15:19:55 EST

On Fri, Apr 24, 2020 at 12:01:24AM +1000, Balbir Singh wrote:
> Implement a mechanism to selectively flush the L1D cache. The goal is to
> allow tasks that are paranoid due to the recent snoop assisted data sampling
> vulnerabilites, to flush their L1D on being switched out. This protects
> their data from being snooped or leaked via side channels after the task
> has context switched out.
>
> There are two scenarios we might want to protect against, a task leaving
> the CPU with data still in L1D (which is the main concern of this patch),
> the second scenario is a malicious task coming in (not so well trusted)
> for which we want to clean up the cache before it starts. Only the case
> for the former is addressed.
>
> A new thread_info flag TIF_SPEC_FLUSH_L1D is added to track tasks which
> opt-into L1D flushing. cpu_tlbstate.last_user_mm_spec is used to convert
> the TIF flags into mm state (per cpu via last_user_mm_spec) in
> cond_mitigation(), which then used to do decide when to call flush_l1d().
>
> Add prctl()'s to opt-in to the L1D cache on context switch out, the
> existing mechanisms of tracking prev_mm via cpu_tlbstate is
> reused to track state of the tasks and to flush the L1D cache.
> The prctl interface is generic and can be ported over to other
> architectures.
>
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Balbir Singh <sblbir@xxxxxxxxxx>

I'm not a huge fan of __weak (I like CONFIGs better), but that's no
enough to NAK this. ;) Thanks for the prctl() change!

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
> arch/x86/include/asm/thread_info.h | 7 ++++-
> arch/x86/mm/tlb.c | 44 ++++++++++++++++++++++++++++--
> include/uapi/linux/prctl.h | 4 +++
> kernel/sys.c | 20 ++++++++++++++
> 4 files changed, 72 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index 8de8ceccb8bc..67de693d9ba1 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -84,7 +84,7 @@ struct thread_info {
> #define TIF_SYSCALL_AUDIT 7 /* syscall auditing active */
> #define TIF_SECCOMP 8 /* secure computing */
> #define TIF_SPEC_IB 9 /* Indirect branch speculation mitigation */
> -#define TIF_SPEC_FORCE_UPDATE 10 /* Force speculation MSR update in context switch */
> +#define TIF_SPEC_FLUSH_L1D 10 /* Flush L1D on mm switches (processes) */
> #define TIF_USER_RETURN_NOTIFY 11 /* notify kernel of userspace return */
> #define TIF_UPROBE 12 /* breakpointed or singlestepping */
> #define TIF_PATCH_PENDING 13 /* pending live patching update */
> @@ -96,6 +96,7 @@ struct thread_info {
> #define TIF_MEMDIE 20 /* is terminating due to OOM killer */
> #define TIF_POLLING_NRFLAG 21 /* idle is polling for TIF_NEED_RESCHED */
> #define TIF_IO_BITMAP 22 /* uses I/O bitmap */
> +#define TIF_SPEC_FORCE_UPDATE 23 /* Force speculation MSR update in context switch */
> #define TIF_FORCED_TF 24 /* true if TF in eflags artificially */
> #define TIF_BLOCKSTEP 25 /* set when we want DEBUGCTLMSR_BTF */
> #define TIF_LAZY_MMU_UPDATES 27 /* task is updating the mmu lazily */
> @@ -132,6 +133,7 @@ struct thread_info {
> #define _TIF_ADDR32 (1 << TIF_ADDR32)
> #define _TIF_X32 (1 << TIF_X32)
> #define _TIF_FSCHECK (1 << TIF_FSCHECK)
> +#define _TIF_SPEC_FLUSH_L1D (1 << TIF_SPEC_FLUSH_L1D)
>
> /* Work to do before invoking the actual syscall. */
> #define _TIF_WORK_SYSCALL_ENTRY \
> @@ -235,6 +237,9 @@ static inline int arch_within_stack_frames(const void * const stack,
> current_thread_info()->status & TS_COMPAT)
> #endif
>
> +extern int arch_prctl_l1d_flush_set(struct task_struct *tsk, unsigned long enable);
> +extern int arch_prctl_l1d_flush_get(struct task_struct *tsk);
> +
> extern void arch_task_cache_init(void);
> extern int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
> extern void arch_release_task_struct(struct task_struct *tsk);
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index da5c94286c7d..7778560760e6 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -13,6 +13,7 @@
> #include <asm/mmu_context.h>
> #include <asm/nospec-branch.h>
> #include <asm/cache.h>
> +#include <asm/cacheflush.h>
> #include <asm/apic.h>
> #include <asm/uv/uv.h>
>
> @@ -33,11 +34,12 @@
> */
>
> /*
> - * Bits to mangle the TIF_SPEC_IB state into the mm pointer which is
> + * Bits to mangle the TIF_SPEC_* state into the mm pointer which is
> * stored in cpu_tlb_state.last_user_mm_spec.
> */
> #define LAST_USER_MM_IBPB 0x1UL
> -#define LAST_USER_MM_SPEC_MASK (LAST_USER_MM_IBPB)
> +#define LAST_USER_MM_L1D_FLUSH 0x2UL
> +#define LAST_USER_MM_SPEC_MASK (LAST_USER_MM_IBPB | LAST_USER_MM_L1D_FLUSH)
>
> /*
> * We get here when we do something requiring a TLB invalidation
> @@ -152,6 +154,35 @@ void leave_mm(int cpu)
> }
> EXPORT_SYMBOL_GPL(leave_mm);
>
> +static int enable_l1d_flush_for_task(struct task_struct *tsk)
> +{
> + int ret = l1d_flush_init_once();
> +
> + if (ret < 0)
> + return ret;
> +
> + set_ti_thread_flag(&tsk->thread_info, TIF_SPEC_FLUSH_L1D);
> + return ret;
> +}
> +
> +static int disable_l1d_flush_for_task(struct task_struct *tsk)
> +{
> + clear_ti_thread_flag(&tsk->thread_info, TIF_SPEC_FLUSH_L1D);
> + return 0;
> +}
> +
> +int arch_prctl_l1d_flush_get(struct task_struct *tsk)
> +{
> + return test_ti_thread_flag(&tsk->thread_info, TIF_SPEC_FLUSH_L1D);
> +}
> +
> +int arch_prctl_l1d_flush_set(struct task_struct *tsk, unsigned long enable)
> +{
> + if (enable)
> + return enable_l1d_flush_for_task(tsk);
> + return disable_l1d_flush_for_task(tsk);
> +}
> +
> void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> struct task_struct *tsk)
> {
> @@ -268,6 +299,9 @@ static void cond_mitigation(struct task_struct *next)
> indirect_branch_prediction_barrier();
> }
>
> + if (prev_mm & LAST_USER_MM_L1D_FLUSH)
> + arch_l1d_flush(0); /* Just flush, don't populate the TLB */
> +
> this_cpu_write(cpu_tlbstate.last_user_mm_spec, next_mm);
> }
>
> @@ -502,6 +536,12 @@ void initialize_tlbstate_and_flush(void)
> write_cr3(build_cr3(mm->pgd, 0));
>
> /* Reinitialize tlbstate. */
> +
> + /*
> + * Leave last_user_mm_spec at LAST_USER_MM_IBPB, we don't
> + * want to set LAST_USER_MM_L1D_FLUSH and force a flush before
> + * we've allocated the flush pages.
> + */
> this_cpu_write(cpu_tlbstate.last_user_mm_spec, LAST_USER_MM_IBPB);
> this_cpu_write(cpu_tlbstate.loaded_mm_asid, 0);
> this_cpu_write(cpu_tlbstate.next_asid, 1);
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 07b4f8131e36..42cb3038c81a 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -238,4 +238,8 @@ struct prctl_mm_map {
> #define PR_SET_IO_FLUSHER 57
> #define PR_GET_IO_FLUSHER 58
>
> +/* Flush L1D on context switch (mm) */
> +#define PR_SET_L1D_FLUSH 59
> +#define PR_GET_L1D_FLUSH 60
> +
> #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index d325f3ab624a..578aa8b6d87e 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2262,6 +2262,16 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
> return -EINVAL;
> }
>
> +int __weak arch_prctl_l1d_flush_set(struct task_struct *tsk, unsigned long enable)
> +{
> + return -EINVAL;
> +}
> +
> +int __weak arch_prctl_l1d_flush_get(struct task_struct *t)
> +{
> + return -EINVAL;
> +}
> +
> #define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LESS_THROTTLE)
>
> SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> @@ -2514,6 +2524,16 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>
> error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER;
> break;
> + case PR_SET_L1D_FLUSH:
> + if (arg3 || arg4 || arg5)
> + return -EINVAL;
> + error = arch_prctl_l1d_flush_set(me, arg2);
> + break;
> + case PR_GET_L1D_FLUSH:
> + if (arg2 || arg3 || arg4 || arg5)
> + return -EINVAL;
> + error = arch_prctl_l1d_flush_get(me);
> + break;
> default:
> error = -EINVAL;
> break;
> --
> 2.17.1
>

--
Kees Cook