Re: TLB flushes on fixmap changes

[Nadav Amit <nadav.amit@gmail.com>]

at 12:58 PM, Andy Lutomirski <luto@kernel.org> wrote:

> On Mon, Aug 27, 2018 at 12:43 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
>> at 12:10 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
>> 
>>> at 11:58 AM, Andy Lutomirski <luto@kernel.org> wrote:
>>> 
>>>> On Mon, Aug 27, 2018 at 11:54 AM, Nadav Amit <nadav.amit@gmail.com> wrote:
>>>>>> On Mon, Aug 27, 2018 at 10:34 AM, Nadav Amit <nadav.amit@gmail.com> wrote:
>>>>>> What do you all think?
>>>>> 
>>>>> I agree in general. But I think that current->mm would need to be loaded, as
>>>>> otherwise I am afraid it would break switch_mm_irqs_off().
>>>> 
>>>> What breaks?
>>> 
>>> Actually nothing. I just saw the IBPB stuff regarding tsk, but it should not
>>> matter.
>> 
>> So here is what I got. It certainly needs some cleanup, but it boots.
>> 
>> Let me know how crappy you find it...
>> 
>> 
>> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
>> index bbc796eb0a3b..336779650a41 100644
>> --- a/arch/x86/include/asm/mmu_context.h
>> +++ b/arch/x86/include/asm/mmu_context.h
>> @@ -343,4 +343,24 @@ static inline unsigned long __get_current_cr3_fast(void)
>>        return cr3;
>> }
>> 
>> +typedef struct {
>> +       struct mm_struct *prev;
>> +} temporary_mm_state_t;
>> +
>> +static inline temporary_mm_state_t use_temporary_mm(struct mm_struct *mm)
>> +{
>> +       temporary_mm_state_t state;
>> +
>> +       lockdep_assert_irqs_disabled();
>> +       state.prev = this_cpu_read(cpu_tlbstate.loaded_mm);
>> +       switch_mm_irqs_off(NULL, mm, current);
>> +       return state;
>> +}
>> +
>> +static inline void unuse_temporary_mm(temporary_mm_state_t prev)
>> +{
>> +       lockdep_assert_irqs_disabled();
>> +       switch_mm_irqs_off(NULL, prev.prev, current);
>> +}
>> +
>> #endif /* _ASM_X86_MMU_CONTEXT_H */
>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>> index 5715647fc4fe..ef62af9a0ef7 100644
>> --- a/arch/x86/include/asm/pgtable.h
>> +++ b/arch/x86/include/asm/pgtable.h
>> @@ -976,6 +976,10 @@ static inline void __meminit init_trampoline_default(void)
>>        /* Default trampoline pgd value */
>>        trampoline_pgd_entry = init_top_pgt[pgd_index(__PAGE_OFFSET)];
>> }
>> +
>> +void __init patching_mm_init(void);
>> +#define patching_mm_init patching_mm_init
>> +
>> # ifdef CONFIG_RANDOMIZE_MEMORY
>> void __meminit init_trampoline(void);
>> # else
>> diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
>> index 054765ab2da2..9f44262abde0 100644
>> --- a/arch/x86/include/asm/pgtable_64_types.h
>> +++ b/arch/x86/include/asm/pgtable_64_types.h
>> @@ -116,6 +116,9 @@ extern unsigned int ptrs_per_p4d;
>> #define LDT_PGD_ENTRY          (pgtable_l5_enabled() ? LDT_PGD_ENTRY_L5 : LDT_PGD_ENTRY_L4)
>> #define LDT_BASE_ADDR          (LDT_PGD_ENTRY << PGDIR_SHIFT)
>> 
>> +#define TEXT_POKE_PGD_ENTRY    -5UL
>> +#define TEXT_POKE_ADDR         (TEXT_POKE_PGD_ENTRY << PGDIR_SHIFT)
>> +
>> #define __VMALLOC_BASE_L4      0xffffc90000000000UL
>> #define __VMALLOC_BASE_L5      0xffa0000000000000UL
>> 
>> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
>> index 99fff853c944..840c72ec8c4f 100644
>> --- a/arch/x86/include/asm/pgtable_types.h
>> +++ b/arch/x86/include/asm/pgtable_types.h
>> @@ -505,6 +505,9 @@ pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>> /* Install a pte for a particular vaddr in kernel space. */
>> void set_pte_vaddr(unsigned long vaddr, pte_t pte);
>> 
>> +struct mm_struct;
>> +void set_mm_pte_vaddr(struct mm_struct *mm, unsigned long vaddr, pte_t pte);
>> +
>> #ifdef CONFIG_X86_32
>> extern void native_pagetable_init(void);
>> #else
>> diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
>> index 2ecd34e2d46c..cb364ea5b19d 100644
>> --- a/arch/x86/include/asm/text-patching.h
>> +++ b/arch/x86/include/asm/text-patching.h
>> @@ -38,4 +38,6 @@ extern void *text_poke(void *addr, const void *opcode, size_t len);
>> extern int poke_int3_handler(struct pt_regs *regs);
>> extern void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler);
>> 
>> +extern struct mm_struct *patching_mm;
>> +
>> #endif /* _ASM_X86_TEXT_PATCHING_H */
>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
>> index a481763a3776..fd8a950b0d62 100644
>> --- a/arch/x86/kernel/alternative.c
>> +++ b/arch/x86/kernel/alternative.c
>> @@ -11,6 +11,7 @@
>> #include <linux/stop_machine.h>
>> #include <linux/slab.h>
>> #include <linux/kdebug.h>
>> +#include <linux/mmu_context.h>
>> #include <asm/text-patching.h>
>> #include <asm/alternative.h>
>> #include <asm/sections.h>
>> @@ -701,8 +702,36 @@ void *text_poke(void *addr, const void *opcode, size_t len)
>>                WARN_ON(!PageReserved(pages[0]));
>>                pages[1] = virt_to_page(addr + PAGE_SIZE);
>>        }
>> -       BUG_ON(!pages[0]);
>> +
>>        local_irq_save(flags);
>> +       BUG_ON(!pages[0]);
>> +
>> +       /*
>> +        * During initial boot, it is hard to initialize patching_mm due to
>> +        * dependencies in boot order.
>> +        */
>> +       if (patching_mm) {
>> +               pte_t pte;
>> +               temporary_mm_state_t prev;
>> +
>> +               prev = use_temporary_mm(patching_mm);
>> +               pte = mk_pte(pages[0], PAGE_KERNEL);
>> +               set_mm_pte_vaddr(patching_mm, TEXT_POKE_ADDR, pte);
>> +               pte = mk_pte(pages[1], PAGE_KERNEL);
>> +               set_mm_pte_vaddr(patching_mm, TEXT_POKE_ADDR + PAGE_SIZE, pte);
>> +
>> +               memcpy((void *)(TEXT_POKE_ADDR | ((unsigned long)addr & ~PAGE_MASK)),
>> +                      opcode, len);
>> +
>> +               set_mm_pte_vaddr(patching_mm, TEXT_POKE_ADDR, __pte(0));
>> +               set_mm_pte_vaddr(patching_mm, TEXT_POKE_ADDR + PAGE_SIZE, __pte(0));
>> +               local_flush_tlb();
> 
> Hmm.  This is stuff busted on SMP, and it's IMO more complicated than
> needed.  How about getting rid of all the weird TLB flushing stuff and
> instead putting the mapping at vaddr - __START_KERNEL_map or whatever
> it is?  You *might* need to flush_tlb_mm_range() on module unload, but
> that's it.

I don’t see what’s wrong in SMP, since this entire piece of code should be
running under text_mutex.

I don’t quite understand your proposal. I really don’t want to have any
chance in which the page-tables for the poked address is not preallocated.

It is more complicated than needed, and there are redundant TLB flushes. The
reason I preferred to do it this way, is in order not to use other functions
that take locks during the software page-walk and not to duplicate existing
code. Yet, duplication might be the way to go.

>> +               sync_core();
> 
> I can't think of any case where sync_core() is needed.  The mm switch
> serializes.

Good point!

> 
> Also, is there any circumstance in which any of this is used before at
> least jump table init?  All the early stuff is text_poke_early(),
> right?

Not before jump_label_init. However, I did not manage to get rid of the two
code-patches in text_poke(), since text_poke is used relatively early by
x86_late_time_init(), and at this stage kmem_cache_alloc() - which is needed
to duplicate init_mm - still fails.