Re: [PATCH v7 3/6] mce: fix set_mce_nospec to always unmap the whole page

Dan Williams
2022-04-11T23:27:48Z

On Tue, Apr 5, 2022 at 12:48 PM Jane Chu <jane.chu@oracle.com> wrote:
>
> The set_memory_uc() approach doesn't work well in all cases.
> For example, 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,

I think a patch is needed before this one to make this statement true? I.e.:

diff --git a/drivers/acpi/nfit/mce.c b/drivers/acpi/nfit/mce.c
index ee8d9973f60b..11641f55025a 100644
--- a/drivers/acpi/nfit/mce.c
+++ b/drivers/acpi/nfit/mce.c
@@ -32,6 +32,7 @@ static int nfit_handle_mce(struct notifier_block
*nb, unsigned long val,
*/
mutex_lock(&acpi_desc_lock);
list_for_each_entry(acpi_desc, &acpi_descs, list) {
+ unsigned int align = 1UL << MCI_MISC_ADDR_LSB(mce->misc);
struct device *dev = acpi_desc->dev;
int found_match = 0;

@@ -63,8 +64,7 @@ static int nfit_handle_mce(struct notifier_block
*nb, unsigned long val,

/* If this fails due to an -ENOMEM, there is little we can do */
nvdimm_bus_add_badrange(acpi_desc->nvdimm_bus,
- ALIGN(mce->addr, L1_CACHE_BYTES),
- L1_CACHE_BYTES);
+ ALIGN(mce->addr, align), align);
nvdimm_region_notify(nfit_spa->nd_region,
NVDIMM_REVALIDATE_POISON);

> let's 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 'np' 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>
> ---
> arch/x86/kernel/cpu/mce/core.c | 6 +++---
> arch/x86/mm/pat/set_memory.c | 18 ++++++------------
> drivers/nvdimm/pmem.c | 31 +++++++------------------------
> include/linux/set_memory.h | 4 ++--
> 4 files changed, 18 insertions(+), 41 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 93dde949f224..404ffcb3f2cb 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -1926,13 +1926,8 @@ int set_memory_wb(unsigned long addr, int numpages)
> EXPORT_SYMBOL(set_memory_wb);
>
> #ifdef CONFIG_X86_64
> -/*
> - * 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;
> @@ -1954,10 +1949,7 @@ 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;
> @@ -1966,7 +1958,9 @@ int set_mce_nospec(unsigned long pfn, bool unmap)
> /* 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 change_page_attr_set(&addr, 1, __pgprot(_PAGE_PRESENT), 0);

This probably warrants a set_memory_present() helper.

> }
> EXPORT_SYMBOL_GPL(clear_mce_nospec);
>
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index 58d95242a836..30c71a68175b 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -158,36 +158,19 @@ 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);
>
> - /*
> - * 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.
> - */
> + if (rc != BLK_STS_OK)
> + pr_warn_ratelimited("%s: failed to clear poison\n", __func__);

This should be either "dev_warn_ratelimited(to_dev(pmem), ...", or a
trace point similar to trace_block_rq_complete() that tells userspace
about adverse I/O completion results.

However, that's probably a discussion for another patch, so I would
just drop this new addition for now and we can discuss the logging in
a follow-on patch.

> + return rc;
> + }
> 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 d6263d7afb55..cde2d8687a7b 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)

Looks like after this change the "whole_page()" helper can be deleted
and the ->mce_whole_page flag in the task_struct can also go, right?
In a follow-on of course.

Other than those small issues, this looks good!