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Re: [PATCH v4 2/2] arm64: kvm: Introduce MTE VCPU feature

From: Steven Price
Subject: Re: [PATCH v4 2/2] arm64: kvm: Introduce MTE VCPU feature
Date: Fri, 20 Nov 2020 09:33:51 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 19/11/2020 16:24, Catalin Marinas wrote:
On Thu, Nov 19, 2020 at 12:45:52PM +0000, Steven Price wrote:
On 18/11/2020 17:05, Andrew Jones wrote:
On Wed, Nov 18, 2020 at 04:50:01PM +0000, Catalin Marinas wrote:
On Wed, Nov 18, 2020 at 04:01:20PM +0000, Steven Price wrote:
On 17/11/2020 16:07, Catalin Marinas wrote:
On Mon, Oct 26, 2020 at 03:57:27PM +0000, Steven Price wrote:
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 19aacc7d64de..38fe25310ca1 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -862,6 +862,26 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
phys_addr_t fault_ipa,
        if (vma_pagesize == PAGE_SIZE && !force_pte)
                vma_pagesize = transparent_hugepage_adjust(memslot, hva,
                                                           &pfn, &fault_ipa);
+       /*
+        * The otherwise redundant test for system_supports_mte() allows the
+        * code to be compiled out when CONFIG_ARM64_MTE is not present.
+        */
+       if (system_supports_mte() && kvm->arch.mte_enabled && pfn_valid(pfn)) {
+               /*
+                * VM will be able to see the page's tags, so we must ensure
+                * they have been initialised.
+                */
+               struct page *page = pfn_to_page(pfn);
+               long i, nr_pages = compound_nr(page);
+               /* if PG_mte_tagged is set, tags have already been initialised 
+               for (i = 0; i < nr_pages; i++, page++) {
+                       if (!test_and_set_bit(PG_mte_tagged, &page->flags))
+                               mte_clear_page_tags(page_address(page));
+               }
+       }

If this page was swapped out and mapped back in, where does the
restoring from swap happen?

Restoring from swap happens above this in the call to gfn_to_pfn_prot()

Looking at the call chain, gfn_to_pfn_prot() ends up with
get_user_pages() using the current->mm (the VMM) and that does a
set_pte_at(), presumably restoring the tags. Does this mean that all
memory mapped by the VMM in user space should have PROT_MTE set?
Otherwise we don't take the mte_sync_tags() path in set_pte_at() and no
tags restored from swap (we do save them since when they were mapped,
PG_mte_tagged was set).

So I think the code above should be similar to mte_sync_tags(), even
calling a common function, but I'm not sure where to get the swap pte

You're right - the code is broken as it stands. I've just been able to
reproduce the loss of tags due to swap.

The problem is that we also don't have a suitable pte to do the restore from
swap from. So either set_pte_at() would have to unconditionally check for
MTE tags for all previous swap entries as you suggest below. I had a quick
go at testing this and hit issues with the idle task getting killed during
boot - I fear there are some fun issues regarding initialisation order here.

My attempt here but not fully tested (just booted, no swap support):

Ah, very similar to what I had, just without the silly mistake... ;)

I just did a quick test with this and it seems to work. I obviously should have looked harder before giving up on this approach.



diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index b35833259f08..27d7fd336a16 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -304,7 +304,7 @@ static inline void set_pte_at(struct mm_struct *mm, 
unsigned long addr,
if (system_supports_mte() &&
-           pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
+           pte_present(pte) && pte_valid_user(pte) && !pte_special(pte))
                mte_sync_tags(ptep, pte);
__check_racy_pte_update(mm, ptep, pte);
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 52a0638ed967..bbd6c56d33d9 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -20,18 +20,24 @@
  #include <asm/ptrace.h>
  #include <asm/sysreg.h>
-static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
+static void mte_sync_page_tags(struct page *page, pte_t *ptep, pte_t pte,
+                              bool check_swap)
        pte_t old_pte = READ_ONCE(*ptep);
if (check_swap && is_swap_pte(old_pte)) {
                swp_entry_t entry = pte_to_swp_entry(old_pte);
- if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
+               if (!non_swap_entry(entry) && mte_restore_tags(entry, page)) {
+                       set_bit(PG_mte_tagged, &page->flags);
+               }
- mte_clear_page_tags(page_address(page));
+       if (pte_tagged(pte)) {
+               mte_clear_page_tags(page_address(page));
+               set_bit(PG_mte_tagged, &page->flags);
+       }
void mte_sync_tags(pte_t *ptep, pte_t pte)
@@ -42,8 +48,8 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
/* if PG_mte_tagged is set, tags have already been initialised */
        for (i = 0; i < nr_pages; i++, page++) {
-               if (!test_and_set_bit(PG_mte_tagged, &page->flags))
-                       mte_sync_page_tags(page, ptep, check_swap);
+               if (!test_bit(PG_mte_tagged, &page->flags))
+                       mte_sync_page_tags(page, ptep, pte, check_swap);

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