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From: | Steven Price |
Subject: | Re: [PATCH v11 1/6] arm64: mte: Sync tags for pages where PTE is untagged |
Date: | Thu, 29 Apr 2021 17:06:05 +0100 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 |
On 27/04/2021 18:43, Catalin Marinas wrote:
On Fri, Apr 16, 2021 at 04:43:04PM +0100, Steven Price wrote:A KVM guest could store tags in a page even if the VMM hasn't mapped the page with PROT_MTE. So when restoring pages from swap we will need to check to see if there are any saved tags even if !pte_tagged(). However don't check pages which are !pte_valid_user() as these will not have been swapped out.You should remove the pte_valid_user() mention from the commit log as well.
Good spot - sorry about that. I really must get better at reading my own commit messages.
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index e17b96d0e4b5..cf4b52a33b3c 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h @@ -312,7 +312,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr, __sync_icache_dcache(pte);if (system_supports_mte() &&- pte_present(pte) && pte_tagged(pte) && !pte_special(pte)) + pte_present(pte) && (pte_val(pte) & PTE_USER) && !pte_special(pte))I would add a pte_user() macro here or, if we restore the tags only when the page is readable, use pte_access_permitted(pte, false). Also add a comment why we do this.
pte_access_permitted() looks like it describes what we want (user space can access the memory). I'll add the following comment:
/* * If the PTE would provide user space will access to the tags * associated with it then ensure that the MTE tags are synchronised. * Exec-only mappings don't expose tags (instruction fetches don't * check tags). */
There's also the pte_user_exec() case which may not have the PTE_USER set (exec-only permission) but I don't think it matters. We don't do tag checking on instruction fetches, so if the user adds a PROT_READ to it, it would go through set_pte_at() again. I'm not sure KVM does anything special with exec-only mappings at stage 2, I suspect they won't be accessible by the guest (but needs checking).
It comes down to the behaviour of get_user_pages(). AFAICT that will fail if the memory is exec-only, so no stage 2 mapping will be created. Which of course means the guest can't do anything with that memory. That certainly seems like the only sane behaviour even without MTE.
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 b3c70a612c7a..e016ab57ea36 100644 --- a/arch/arm64/kernel/mte.c +++ b/arch/arm64/kernel/mte.c @@ -26,17 +26,23 @@ u64 gcr_kernel_excl __ro_after_init;static bool report_fault_once = true; -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, bool check_swap, + bool pte_is_tagged) { 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); return; + } }+ if (!pte_is_tagged || test_and_set_bit(PG_mte_tagged, &page->flags))+ return;I don't think we need another test_bit() here, it was done in the caller (bar potential races which need more thought).
Good point - I'll change that to a straight set_bit().
+ page_kasan_tag_reset(page); /* * We need smp_wmb() in between setting the flags and clearing the @@ -54,11 +60,13 @@ void mte_sync_tags(pte_t *ptep, pte_t pte) struct page *page = pte_page(pte); long i, nr_pages = compound_nr(page); bool check_swap = nr_pages == 1; + bool pte_is_tagged = pte_tagged(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, check_swap, + pte_is_tagged); } }You were right in the previous thread that if we have a race, it's already there even without your patches KVM patches. If it's the same pte in a multithreaded app, we should be ok as the core code holds the ptl (the arch code also holds the mmap_lock during exception handling but only as a reader, so you can have multiple holders). If there are multiple ptes to the same page, for example mapped with MAP_ANONYMOUS | MAP_SHARED, metadata recovery is done via arch_swap_restore() before we even set the pte and with the page locked. So calling lock_page() again in mte_restore_tags() would deadlock. I can see that do_swap_page() also holds the page lock around set_pte_at(), so I think we are covered. Any other scenario I may have missed? My understanding is that if the pte is the same, we have the ptl. Otherwise we have the page lock for shared pages.
That is my understanding - either the PTL is held or the page is locked. But I am aware I was partly basing that on an assumption that the existing code is correct. If there's a way that a new PTE can be created which races with the arch_swap_restore() path then there is a problem. I'm not aware of how that would happen though.
Steve
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