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References: <20220420020435.90326-1-jane.chu@oracle.com> <20220420020435.90326-4-jane.chu@oracle.com>
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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Thu, 21 Apr 2022 12:26:57 -0700
Message-ID: <CAPcyv4i0eWm9eeNpgZmJTnf+2ivumuKDhF2vj9GNcj2VX-Sfjw@mail.gmail.com>
Subject: Re: [PATCH v8 3/7] mce: fix set_mce_nospec to always unmap the whole page
To:     Jane Chu <jane.chu@oracle.com>
Cc:     Borislav Petkov <bp@alien8.de>,
        Christoph Hellwig <hch@infradead.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>, david <david@fromorbit.com>,
        "Darrick J. Wong" <djwong@kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux NVDIMM <nvdimm@lists.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>,
        Vishal L Verma <vishal.l.verma@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Alasdair Kergon <agk@redhat.com>,
        Mike Snitzer <snitzer@redhat.com>,
        device-mapper development <dm-devel@redhat.com>,
        "Weiny, Ira" <ira.weiny@intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vivek Goyal <vgoyal@redhat.com>
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On Tue, Apr 19, 2022 at 7:05 PM Jane Chu <jane.chu@oracle.com> wrote:
>
> The set_memory_uc() approach doesn't work well in all cases.
> As Dan pointed out when "The VMM unmapped the bad page from
> guest physical space and passed the machine check to the guest."
> "The guest gets virtual #MC on an access to that page. When
> the guest tries to do set_memory_uc() and instructs cpa_flush()
> to do clean caches that results in taking another fault / exception
> perhaps because the VMM unmapped the page from the guest."
>
> Since the driver has special knowledge to handle NP or UC,
> mark the poisoned page with NP and let driver handle it when
> it comes down to repair.
>
> Please refer to discussions here for more details.
> https://lore.kernel.org/all/CAPcyv4hrXPb1tASBZUg-GgdVs0OOFKXMXLiHmktg_kFi7YBMyQ@mail.gmail.com/
>
> Now since poisoned page is marked as not-present, in order to
> avoid writing to a not-present page and trigger kernel Oops,
> also fix pmem_do_write().
>
> Fixes: 284ce4011ba6 ("x86/memory_failure: Introduce {set, clear}_mce_nospec()")
> Signed-off-by: Jane Chu <jane.chu@oracle.com>

Looks good to me:

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

> ---
>  arch/x86/kernel/cpu/mce/core.c |  6 +++---
>  arch/x86/mm/pat/set_memory.c   | 23 +++++++++++------------
>  drivers/nvdimm/pmem.c          | 30 +++++++-----------------------
>  include/linux/set_memory.h     |  4 ++--
>  4 files changed, 23 insertions(+), 40 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
> index 981496e6bc0e..fa67bb9d1afe 100644
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -579,7 +579,7 @@ static int uc_decode_notifier(struct notifier_block *nb, unsigned long val,
>
>         pfn = mce->addr >> PAGE_SHIFT;
>         if (!memory_failure(pfn, 0)) {
> -               set_mce_nospec(pfn, whole_page(mce));
> +               set_mce_nospec(pfn);
>                 mce->kflags |= MCE_HANDLED_UC;
>         }
>
> @@ -1316,7 +1316,7 @@ static void kill_me_maybe(struct callback_head *cb)
>
>         ret = memory_failure(p->mce_addr >> PAGE_SHIFT, flags);
>         if (!ret) {
> -               set_mce_nospec(p->mce_addr >> PAGE_SHIFT, p->mce_whole_page);
> +               set_mce_nospec(p->mce_addr >> PAGE_SHIFT);
>                 sync_core();
>                 return;
>         }
> @@ -1342,7 +1342,7 @@ static void kill_me_never(struct callback_head *cb)
>         p->mce_count = 0;
>         pr_err("Kernel accessed poison in user space at %llx\n", p->mce_addr);
>         if (!memory_failure(p->mce_addr >> PAGE_SHIFT, 0))
> -               set_mce_nospec(p->mce_addr >> PAGE_SHIFT, p->mce_whole_page);
> +               set_mce_nospec(p->mce_addr >> PAGE_SHIFT);
>  }
>
>  static void queue_task_work(struct mce *m, char *msg, void (*func)(struct callback_head *))
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 978cf5bd2ab6..e3a5e55f3e08 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -1925,13 +1925,8 @@ int set_memory_wb(unsigned long addr, int numpages)
>  }
>  EXPORT_SYMBOL(set_memory_wb);
>
> -/*
> - * Prevent speculative access to the page by either unmapping
> - * it (if we do not require access to any part of the page) or
> - * marking it uncacheable (if we want to try to retrieve data
> - * from non-poisoned lines in the page).
> - */
> -int set_mce_nospec(unsigned long pfn, bool unmap)
> +/* Prevent speculative access to a page by marking it not-present */
> +int set_mce_nospec(unsigned long pfn)
>  {
>         unsigned long decoy_addr;
>         int rc;
> @@ -1956,19 +1951,23 @@ int set_mce_nospec(unsigned long pfn, bool unmap)
>          */
>         decoy_addr = (pfn << PAGE_SHIFT) + (PAGE_OFFSET ^ BIT(63));
>
> -       if (unmap)
> -               rc = set_memory_np(decoy_addr, 1);
> -       else
> -               rc = set_memory_uc(decoy_addr, 1);
> +       rc = set_memory_np(decoy_addr, 1);
>         if (rc)
>                 pr_warn("Could not invalidate pfn=0x%lx from 1:1 map\n", pfn);
>         return rc;
>  }
>
> +static int set_memory_present(unsigned long *addr, int numpages)
> +{
> +       return change_page_attr_set(addr, numpages, __pgprot(_PAGE_PRESENT), 0);
> +}
> +
>  /* Restore full speculative operation to the pfn. */
>  int clear_mce_nospec(unsigned long pfn)
>  {
> -       return set_memory_wb((unsigned long) pfn_to_kaddr(pfn), 1);
> +       unsigned long addr = (unsigned long) pfn_to_kaddr(pfn);
> +
> +       return set_memory_present(&addr, 1);
>  }
>  EXPORT_SYMBOL_GPL(clear_mce_nospec);
>
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index 58d95242a836..4aa17132a557 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -158,36 +158,20 @@ static blk_status_t pmem_do_write(struct pmem_device *pmem,
>                         struct page *page, unsigned int page_off,
>                         sector_t sector, unsigned int len)
>  {
> -       blk_status_t rc = BLK_STS_OK;
> -       bool bad_pmem = false;
>         phys_addr_t pmem_off = sector * 512 + pmem->data_offset;
>         void *pmem_addr = pmem->virt_addr + pmem_off;
>
> -       if (unlikely(is_bad_pmem(&pmem->bb, sector, len)))
> -               bad_pmem = true;
> +       if (unlikely(is_bad_pmem(&pmem->bb, sector, len))) {
> +               blk_status_t rc = pmem_clear_poison(pmem, pmem_off, len);
> +
> +               if (rc != BLK_STS_OK)
> +                       return rc;
> +       }
>
> -       /*
> -        * Note that we write the data both before and after
> -        * clearing poison.  The write before clear poison
> -        * handles situations where the latest written data is
> -        * preserved and the clear poison operation simply marks
> -        * the address range as valid without changing the data.
> -        * In this case application software can assume that an
> -        * interrupted write will either return the new good
> -        * data or an error.
> -        *
> -        * However, if pmem_clear_poison() leaves the data in an
> -        * indeterminate state we need to perform the write
> -        * after clear poison.
> -        */
>         flush_dcache_page(page);
>         write_pmem(pmem_addr, page, page_off, len);
> -       if (unlikely(bad_pmem)) {
> -               rc = pmem_clear_poison(pmem, pmem_off, len);
> -               write_pmem(pmem_addr, page, page_off, len);
> -       }
>
> -       return rc;
> +       return BLK_STS_OK;
>  }
>
>  static void pmem_submit_bio(struct bio *bio)
> diff --git a/include/linux/set_memory.h b/include/linux/set_memory.h
> index 683a6c3f7179..369769ce7399 100644
> --- a/include/linux/set_memory.h
> +++ b/include/linux/set_memory.h
> @@ -43,10 +43,10 @@ static inline bool can_set_direct_map(void)
>  #endif /* CONFIG_ARCH_HAS_SET_DIRECT_MAP */
>
>  #ifdef CONFIG_X86_64
> -int set_mce_nospec(unsigned long pfn, bool unmap);
> +int set_mce_nospec(unsigned long pfn);
>  int clear_mce_nospec(unsigned long pfn);
>  #else
> -static inline int set_mce_nospec(unsigned long pfn, bool unmap)
> +static inline int set_mce_nospec(unsigned long pfn)
>  {
>         return 0;
>  }
> --
> 2.18.4
>