From: "Luck, Tony" tony.luck@intel.com

commit 17fae1294ad9d711b2c3dd0edef479d40c76a5e8 upstream

An interesting thing happened when a guest Linux instance took a machine check. The VMM unmapped the bad page from guest physical space and passed the machine check to the guest.

Linux took all the normal actions to offline the page from the process that was using it. But then guest Linux crashed because it said there was a second machine check inside the kernel with this stack trace:

do_memory_failure set_mce_nospec set_memory_uc _set_memory_uc change_page_attr_set_clr cpa_flush clflush_cache_range_opt

This was odd, because a CLFLUSH instruction shouldn't raise a machine check (it isn't consuming the data). Further investigation showed that the VMM had passed in another machine check because is appeared that the guest was accessing the bad page.

Fix is to check the scope of the poison by checking the MCi_MISC register. If the entire page is affected, then unmap the page. If only part of the page is affected, then mark the page as uncacheable.

This assumes that VMMs will do the logical thing and pass in the "whole page scope" via the MCi_MISC register (since they unmapped the entire page).

Reported-by: Jue Wang juew@google.com Tested-by: Jue Wang juew@google.com Fixes: 284ce4011ba6 ("x86/memory_failure: Introduce {set, clear}_mce_nospec()") Cc: stable@vger.kernel.org Signed-off-by: Tony Luck tony.luck@intel.com Signed-off-by: Tony Luck tony.luck@intel.com Link: https://lore.kernel.org/r/20200520163546.GA7977@agluck-desk2.amr.corp.intel.... --- arch/x86/include/asm/set_memory.h | 19 +++++++++++++------ arch/x86/kernel/cpu/mce/core.c | 11 +++++++++-- include/linux/set_memory.h | 2 +- 3 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/set_memory.h b/arch/x86/include/asm/set_memory.h index 2ee8e469dcf5..162128cdfbf2 100644 --- a/arch/x86/include/asm/set_memory.h +++ b/arch/x86/include/asm/set_memory.h @@ -85,28 +85,35 @@ void set_kernel_text_rw(void); void set_kernel_text_ro(void);

#ifdef CONFIG_X86_64 -static inline int set_mce_nospec(unsigned long pfn) +/* + * 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). + */ +static inline int set_mce_nospec(unsigned long pfn, bool unmap) { unsigned long decoy_addr; int rc;

/* - * Mark the linear address as UC to make sure we don't log more - * errors because of speculative access to the page. * We would like to just call: - * set_memory_uc((unsigned long)pfn_to_kaddr(pfn), 1); + * set_memory_XX((unsigned long)pfn_to_kaddr(pfn), 1); * but doing that would radically increase the odds of a * speculative access to the poison page because we'd have * the virtual address of the kernel 1:1 mapping sitting * around in registers. * Instead we get tricky. We create a non-canonical address * that looks just like the one we want, but has bit 63 flipped. - * This relies on set_memory_uc() properly sanitizing any __pa() + * This relies on set_memory_XX() properly sanitizing any __pa() * results with __PHYSICAL_MASK or PTE_PFN_MASK. */ decoy_addr = (pfn << PAGE_SHIFT) + (PAGE_OFFSET ^ BIT(63));

- rc = set_memory_uc(decoy_addr, 1); + if (unmap) + rc = set_memory_np(decoy_addr, 1); + else + rc = set_memory_uc(decoy_addr, 1); if (rc) pr_warn("Could not invalidate pfn=0x%lx from 1:1 map\n", pfn); return rc; diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c index aecb15ba66cd..fd76e3733dd3 100644 --- a/arch/x86/kernel/cpu/mce/core.c +++ b/arch/x86/kernel/cpu/mce/core.c @@ -533,6 +533,13 @@ bool mce_is_memory_error(struct mce *m) } EXPORT_SYMBOL_GPL(mce_is_memory_error);

+static bool whole_page(struct mce *m) +{ + if (!mca_cfg.ser || !(m->status & MCI_STATUS_MISCV)) + return true; + return MCI_MISC_ADDR_LSB(m->misc) >= PAGE_SHIFT; +} + bool mce_is_correctable(struct mce *m) { if (m->cpuvendor == X86_VENDOR_AMD && m->status & MCI_STATUS_DEFERRED) @@ -601,7 +608,7 @@ static int srao_decode_notifier(struct notifier_block *nb, unsigned long val, if (mce_usable_address(mce) && (mce->severity == MCE_AO_SEVERITY)) { pfn = mce->addr >> PAGE_SHIFT; if (!memory_failure(pfn, 0)) - set_mce_nospec(pfn); + set_mce_nospec(pfn, whole_page(mce)); }

return NOTIFY_OK; @@ -1103,7 +1110,7 @@ static int do_memory_failure(struct mce *m) if (ret) pr_err("Memory error not recovered"); else - set_mce_nospec(m->addr >> PAGE_SHIFT); + set_mce_nospec(m->addr >> PAGE_SHIFT, whole_page(m)); return ret; }

diff --git a/include/linux/set_memory.h b/include/linux/set_memory.h index 86281ac7c305..860e0f843c12 100644 --- a/include/linux/set_memory.h +++ b/include/linux/set_memory.h @@ -26,7 +26,7 @@ static inline int set_direct_map_default_noflush(struct page *page) #endif

#ifndef set_mce_nospec -static inline int set_mce_nospec(unsigned long pfn) +static inline int set_mce_nospec(unsigned long pfn, bool unmap) { return 0; }