On Fri, 2017-07-14 at 11:52 +1000, Benjamin Herrenschmidt wrote:
> There's a somewhat architectural issue with Radix MMU and KVM.
>
> When coming out of a guest with AIL (ie, MMU enabled), we start
> executing hypervisor code with the PID register still containing
> whatever the guest has been using.
>
> The problem is that the CPU can (and will) then start prefetching
> or speculatively load from whatever host context has that same
> PID (if any), thus bringing translations for that context into
> the TLB, which Linux doesn't know about.
>
> This can cause stale translations and subsequent crashes.
>
> Fixing this in a way that is neither racy nor a huge performance
> impact is difficult. We could just make the host invalidations
> always use broadcast forms but that would hurt single threaded
> programs for example.
>
> We chose to fix it instead by partitioning the PID space between
> guest and host. This is possible because today Linux only use 19
> out of the 20 bits of PID space, so existing guests will work
> if we make the host use the top half of the 20 bits space.
>
> We additionally add a property to indicate to Linux the size of
> the PID register which will be useful if we eventually have
> processors with a larger PID space available.
>
> There is still an issue with malicious guests purposefully setting
> the PID register to a value in the host range. Hopefully future HW
> can prevent that, but in the meantime, we handle it with a pair of
> kludges:
>
> - On the way out of a guest, before we clear the current VCPU
> in the PACA, we check the PID and if it's outside of the permitted
> range we flush the TLB for that PID.
>
> - When context switching, if the mm is "new" on that CPU (the
> corresponding bit was set for the first time in the mm cpumask), we
> check if any sibling thread is in KVM (has a non-NULL VCPU pointer
> in the PACA). If that is the case, we also flush the PID for that
> CPU (core).
>
> This second part is needed to handle the case where a process is
> migrated (or starts a new pthread) on a sibling thread of the CPU
> coming out of KVM, as there's a window where stale translations
> can exist before we detect it and flush them out.
>
> A future optimization could be added by keeping track of whether
> the PID has ever been used and avoid doing that for completely
> fresh PIDs. We could similarily mark PIDs that have been the subject of
> a global invalidation as "fresh". But for now this will do.
>
> Signed-off-by: Benjamin Herrenschmidt <b...@kernel.crashing.org>
> ---
> ---
> arch/powerpc/include/asm/book3s/64/mmu.h | 15 ++++----
> .../powerpc/include/asm/book3s/64/tlbflush-radix.h | 3 ++
> arch/powerpc/include/asm/mmu_context.h | 25 +++++++++----
> arch/powerpc/kernel/head_32.S | 6 +--
> arch/powerpc/kernel/swsusp.c | 2 +-
> arch/powerpc/kvm/book3s_32_sr.S | 2 +-
> arch/powerpc/kvm/book3s_hv_rmhandlers.S | 12 ++++++
> arch/powerpc/mm/mmu_context_book3s64.c | 43
> ++++++++++++++++++++--
> arch/powerpc/mm/mmu_context_nohash.c | 4 +-
> arch/powerpc/mm/pgtable-radix.c | 31 +++++++++++++++-
> arch/powerpc/mm/pgtable_64.c | 3 ++
> arch/powerpc/mm/tlb-radix.c | 10 +++--
> drivers/cpufreq/pmac32-cpufreq.c | 2 +-
> drivers/macintosh/via-pmu.c | 4 +-
> 14 files changed, 131 insertions(+), 31 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h
> b/arch/powerpc/include/asm/book3s/64/mmu.h
> index 77529a3..5b4023c 100644
> --- a/arch/powerpc/include/asm/book3s/64/mmu.h
> +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
> @@ -59,13 +59,14 @@ extern struct patb_entry *partition_tb;
> #define PRTS_MASK 0x1f /* process table size field */
> #define PRTB_MASK 0x0ffffffffffff000UL
>
> -/*
> - * Limit process table to PAGE_SIZE table. This
> - * also limit the max pid we can support.
> - * MAX_USER_CONTEXT * 16 bytes of space.
> - */
> -#define PRTB_SIZE_SHIFT (CONTEXT_BITS + 4)
> -#define PRTB_ENTRIES (1ul << CONTEXT_BITS)
> +/* Number of supported PID bits */
> +extern unsigned int mmu_pid_bits;
> +
> +/* Base PID to allocate from */
> +extern unsigned int mmu_base_pid;
> +
> +#define PRTB_SIZE_SHIFT (mmu_pid_bits + 4)
> +#define PRTB_ENTRIES (1ul << mmu_pid_bits)
>
> /*
> * Power9 currently only support 64K partition table size.
> diff --git a/arch/powerpc/include/asm/book3s/64/tlbflush-radix.h
> b/arch/powerpc/include/asm/book3s/64/tlbflush-radix.h
> index 9b433a6..2e0a6b4 100644
> --- a/arch/powerpc/include/asm/book3s/64/tlbflush-radix.h
> +++ b/arch/powerpc/include/asm/book3s/64/tlbflush-radix.h
> @@ -21,10 +21,13 @@ extern void radix__flush_tlb_range(struct vm_area_struct
> *vma, unsigned long sta
> extern void radix__flush_tlb_kernel_range(unsigned long start, unsigned long
> end);
>
> extern void radix__local_flush_tlb_mm(struct mm_struct *mm);
> +extern void radix__local_flush_all_mm(struct mm_struct *mm);
> extern void radix__local_flush_tlb_page(struct vm_area_struct *vma, unsigned
> long vmaddr);
> extern void radix__local_flush_tlb_page_psize(struct mm_struct *mm, unsigned
> long vmaddr,
> int psize);
> extern void radix__tlb_flush(struct mmu_gather *tlb);
> +extern void radix_flush_pid(unsigned long pid);
> +
> #ifdef CONFIG_SMP
> extern void radix__flush_tlb_mm(struct mm_struct *mm);
> extern void radix__flush_tlb_page(struct vm_area_struct *vma, unsigned long
> vmaddr);
> diff --git a/arch/powerpc/include/asm/mmu_context.h
> b/arch/powerpc/include/asm/mmu_context.h
> index da7e943..26a587a 100644
> --- a/arch/powerpc/include/asm/mmu_context.h
> +++ b/arch/powerpc/include/asm/mmu_context.h
> @@ -45,13 +45,15 @@ extern void set_context(unsigned long id, pgd_t *pgd);
>
> #ifdef CONFIG_PPC_BOOK3S_64
> extern void radix__switch_mmu_context(struct mm_struct *prev,
> - struct mm_struct *next);
> + struct mm_struct *next,
> + bool new_on_cpu);
> static inline void switch_mmu_context(struct mm_struct *prev,
> struct mm_struct *next,
> - struct task_struct *tsk)
> + struct task_struct *tsk,
> + bool new_on_cpu)
> {
> if (radix_enabled())
> - return radix__switch_mmu_context(prev, next);
> + return radix__switch_mmu_context(prev, next, new_on_cpu);
> return switch_slb(tsk, next);
> }
>
> @@ -60,8 +62,13 @@ extern void hash__reserve_context_id(int id);
> extern void __destroy_context(int context_id);
> static inline void mmu_context_init(void) { }
> #else
> -extern void switch_mmu_context(struct mm_struct *prev, struct mm_struct
> *next,
> - struct task_struct *tsk);
> +extern void __switch_mmu_context(struct mm_struct *prev, struct mm_struct
> *next,
> + struct task_struct *tsk);
> +static inline void switch_mmu_context(struct mm_struct *prev, struct
> mm_struct *next,
> + struct task_struct *tsk, bool new_on_cpu)
> +{
> + __switch_mmu_context(prev, next, tsk);
> +}
> extern unsigned long __init_new_context(void);
> extern void __destroy_context(unsigned long context_id);
> extern void mmu_context_init(void);
> @@ -79,9 +86,13 @@ static inline void switch_mm_irqs_off(struct mm_struct
> *prev,
> struct mm_struct *next,
> struct task_struct *tsk)
> {
> + bool new_on_cpu = false;
> +
> /* Mark this context has been used on the new CPU */
> - if (!cpumask_test_cpu(smp_processor_id(), mm_cpumask(next)))
> + if (!cpumask_test_cpu(smp_processor_id(), mm_cpumask(next))) {
> cpumask_set_cpu(smp_processor_id(), mm_cpumask(next));
> + new_on_cpu = true;
> + }
>
> /* 32-bit keeps track of the current PGDIR in the thread struct */
> #ifdef CONFIG_PPC32
> @@ -113,7 +124,7 @@ static inline void switch_mm_irqs_off(struct mm_struct
> *prev,
> * The actual HW switching method differs between the various
> * sub architectures. Out of line for now
> */
> - switch_mmu_context(prev, next, tsk);
> + switch_mmu_context(prev, next, tsk, new_on_cpu);
> }
>
> static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> diff --git a/arch/powerpc/kernel/head_32.S b/arch/powerpc/kernel/head_32.S
> index e227342..a530124 100644
> --- a/arch/powerpc/kernel/head_32.S
> +++ b/arch/powerpc/kernel/head_32.S
> @@ -1003,11 +1003,11 @@ start_here:
> RFI
>
> /*
> - * void switch_mmu_context(struct mm_struct *prev, struct mm_struct *next);
> + * void __switch_mmu_context(struct mm_struct *prev, struct mm_struct *next);
> *
> * Set up the segment registers for a new context.
> */
> -_ENTRY(switch_mmu_context)
> +_ENTRY(__switch_mmu_context)
> lwz r3,MMCONTEXTID(r4)
> cmpwi cr0,r3,0
> blt- 4f
> @@ -1040,7 +1040,7 @@ _ENTRY(switch_mmu_context)
> 4: trap
> EMIT_BUG_ENTRY 4b,__FILE__,__LINE__,0
> blr
> -EXPORT_SYMBOL(switch_mmu_context)
> +EXPORT_SYMBOL(__switch_mmu_context)
>
> /*
> * An undocumented "feature" of 604e requires that the v bit
> diff --git a/arch/powerpc/kernel/swsusp.c b/arch/powerpc/kernel/swsusp.c
> index 0050b2d..9c32cfd 100644
> --- a/arch/powerpc/kernel/swsusp.c
> +++ b/arch/powerpc/kernel/swsusp.c
> @@ -32,6 +32,6 @@ void save_processor_state(void)
> void restore_processor_state(void)
> {
> #ifdef CONFIG_PPC32
> - switch_mmu_context(current->active_mm, current->active_mm, NULL);
> + __switch_mmu_context(current->active_mm, current->active_mm, NULL);
> #endif
> }
> diff --git a/arch/powerpc/kvm/book3s_32_sr.S b/arch/powerpc/kvm/book3s_32_sr.S
> index 7e06a6f..a1705d4 100644
> --- a/arch/powerpc/kvm/book3s_32_sr.S
> +++ b/arch/powerpc/kvm/book3s_32_sr.S
> @@ -138,6 +138,6 @@
> lwz r4, MM(r4)
> tophys(r4, r4)
> /* This only clobbers r0, r3, r4 and r5 */
> - bl switch_mmu_context
> + bl __switch_mmu_context
>
> .endm
> diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> index 6ea4b53..4fb3581b 100644
> --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> @@ -1522,6 +1522,18 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_207S)
> std r6, VCPU_BESCR(r9)
> stw r7, VCPU_GUEST_PID(r9)
> std r8, VCPU_WORT(r9)
> +
> + /* Handle the case where the guest used an illegal PID */
> + LOAD_REG_ADDR(r4, mmu_base_pid)
> + lwz r3, 0(r4)
> + cmpw cr0,r7,r3
> + blt 1f
So the boundary is [1..(1<<mmu_pid_bits-1)]? Do we flush the tlb
for pid 0 always?
> +
> + /* Illegal PID, flush the TLB */
> + bl radix_flush_pid
> + ld r9, HSTATE_KVM_VCPU(r13)
> +1:
> +
> BEGIN_FTR_SECTION
> mfspr r5, SPRN_TCSCR
> mfspr r6, SPRN_ACOP
> diff --git a/arch/powerpc/mm/mmu_context_book3s64.c
> b/arch/powerpc/mm/mmu_context_book3s64.c
> index abed1fe..183a67b 100644
> --- a/arch/powerpc/mm/mmu_context_book3s64.c
> +++ b/arch/powerpc/mm/mmu_context_book3s64.c
> @@ -25,6 +25,10 @@
> #include <asm/mmu_context.h>
> #include <asm/pgalloc.h>
>
> +#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
> +#include <asm/kvm_book3s_asm.h>
> +#endif
> +
> #include "icswx.h"
>
> static DEFINE_SPINLOCK(mmu_context_lock);
> @@ -126,9 +130,10 @@ static int hash__init_new_context(struct mm_struct *mm)
> static int radix__init_new_context(struct mm_struct *mm)
> {
> unsigned long rts_field;
> - int index;
> + int index, max_id;
>
> - index = alloc_context_id(1, PRTB_ENTRIES - 1);
> + max_id = (1 << mmu_pid_bits) - 1;
> + index = alloc_context_id(mmu_base_pid, max_id);
> if (index < 0)
> return index;
>
> @@ -247,8 +252,40 @@ void destroy_context(struct mm_struct *mm)
> }
>
> #ifdef CONFIG_PPC_RADIX_MMU
> -void radix__switch_mmu_context(struct mm_struct *prev, struct mm_struct
> *next)
> +void radix__switch_mmu_context(struct mm_struct *prev, struct mm_struct
> *next,
> + bool new_on_cpu)
> {
> + /*
> + * If this context hasn't run on that CPU before and KVM is
> + * around, there's a slim chance that the guest on another
> + * CPU just brought in obsolete translation into the TLB of
> + * this CPU due to a bad prefetch using the guest PID on
> + * the way into the hypervisor.
> + *
> + * We work around this here. If KVM is possible, we check if
> + * any sibling thread is in KVM. If it is, the window may exist
> + * and thus we flush that PID from the core.
> + *
> + * A potential future improvement would be to mark which PIDs
> + * have never been used on the system and avoid it if the PID
> + * is new and the process has no other cpumask bit set.
Also due to the pid split, the chances of the context bringing in
TLB entries is low, but a bad guest could bring in stale entries
> + */
> +#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
> + if (cpu_has_feature(CPU_FTR_HVMODE) && new_on_cpu) {
> + int cpu = smp_processor_id();
> + int sib = cpu_first_thread_sibling(cpu);
> + bool flush = false;
> +
> + for (; sib <= cpu_last_thread_sibling(cpu) && !flush; sib++) {
> + if (sib == cpu)
> + continue;
> + if (paca[sib].kvm_hstate.kvm_vcpu)
> + flush = true;
> + }
> + if (flush)
> + radix__local_flush_all_mm(next);
> + }
> +#endif /* CONFIG_KVM_BOOK3S_HV_POSSIBLE */
>
