Messages in this thread | | | Date | Thu, 31 Mar 2022 15:13:41 +0200 | Subject | Re: [PATCH v9 01/18] KVM: s390: pv: leak the topmost page table when destroy fails | From | Janosch Frank <> |
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On 3/30/22 14:25, Claudio Imbrenda wrote: > Each secure guest must have a unique ASCE (address space control > element); we must avoid that new guests use the same page for their > ASCE, to avoid errors. > > Since the ASCE mostly consists of the address of the topmost page table > (plus some flags), we must not return that memory to the pool unless > the ASCE is no longer in use. > > Only a successful Destroy Secure Configuration UVC will make the ASCE > reusable again. > > If the Destroy Configuration UVC fails, the ASCE cannot be reused for a > secure guest (either for the ASCE or for other memory areas). To avoid > a collision, it must not be used again. This is a permanent error and > the page becomes in practice unusable, so we set it aside and leak it. > On failure we already leak other memory that belongs to the ultravisor > (i.e. the variable and base storage for a guest) and not leaking the > topmost page table was an oversight. > > This error (and thus the leakage) should not happen unless the hardware > is broken or KVM has some unknown serious bug. > > Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com> > Fixes: 29b40f105ec8d55 ("KVM: s390: protvirt: Add initial vm and cpu lifecycle handling") > --- > arch/s390/include/asm/gmap.h | 2 + > arch/s390/kvm/pv.c | 9 +++-- > arch/s390/mm/gmap.c | 71 ++++++++++++++++++++++++++++++++++++ > 3 files changed, 79 insertions(+), 3 deletions(-) > > diff --git a/arch/s390/include/asm/gmap.h b/arch/s390/include/asm/gmap.h > index 40264f60b0da..746e18bf8984 100644 > --- a/arch/s390/include/asm/gmap.h > +++ b/arch/s390/include/asm/gmap.h > @@ -148,4 +148,6 @@ void gmap_sync_dirty_log_pmd(struct gmap *gmap, unsigned long dirty_bitmap[4], > unsigned long gaddr, unsigned long vmaddr); > int gmap_mark_unmergeable(void); > void s390_reset_acc(struct mm_struct *mm); > +void s390_remove_old_asce(struct gmap *gmap); > +int s390_replace_asce(struct gmap *gmap); > #endif /* _ASM_S390_GMAP_H */ > diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c > index 7f7c0d6af2ce..3c59ef763dde 100644 > --- a/arch/s390/kvm/pv.c > +++ b/arch/s390/kvm/pv.c > @@ -166,10 +166,13 @@ int kvm_s390_pv_deinit_vm(struct kvm *kvm, u16 *rc, u16 *rrc) > atomic_set(&kvm->mm->context.is_protected, 0); > KVM_UV_EVENT(kvm, 3, "PROTVIRT DESTROY VM: rc %x rrc %x", *rc, *rrc); > WARN_ONCE(cc, "protvirt destroy vm failed rc %x rrc %x", *rc, *rrc); > - /* Inteded memory leak on "impossible" error */ > - if (!cc) > + /* Intended memory leak on "impossible" error */ > + if (!cc) { > kvm_s390_pv_dealloc_vm(kvm); > - return cc ? -EIO : 0; > + return 0; > + } > + s390_replace_asce(kvm->arch.gmap); > + return -EIO; > } > > int kvm_s390_pv_init_vm(struct kvm *kvm, u16 *rc, u16 *rrc) > diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c > index dfee0ebb2fac..3b42bf7adb77 100644 > --- a/arch/s390/mm/gmap.c > +++ b/arch/s390/mm/gmap.c > @@ -2714,3 +2714,74 @@ void s390_reset_acc(struct mm_struct *mm) > mmput(mm); > } > EXPORT_SYMBOL_GPL(s390_reset_acc); > + > +/** > + * s390_remove_old_asce - Remove the topmost level of page tables from the > + * list of page tables of the gmap. > + * @gmap the gmap whose table is to be removed > + * > + * This means that it will not be freed when the VM is torn down, and needs > + * to be handled separately by the caller, unless an intentional leak is > + * intended. Notice that this function will only remove the page from the > + * list, the page will still be used as a top level page table (and ASCE). > + */ > +void s390_remove_old_asce(struct gmap *gmap) > +{ > + struct page *old; > + > + old = virt_to_page(gmap->table); > + spin_lock(&gmap->guest_table_lock); > + list_del(&old->lru); > + /* > + * in case the ASCE needs to be "removed" multiple times, for example > + * if the VM is rebooted into secure mode several times > + * concurrently. > + */
How can that happen, what are we protecting against here?
> + INIT_LIST_HEAD(&old->lru); > + spin_unlock(&gmap->guest_table_lock); > +} > +EXPORT_SYMBOL_GPL(s390_remove_old_asce); > + > +/** > + * s390_replace_asce - Try to replace the current ASCE of a gmap with > + * another equivalent one. > + * @gmap the gmap > + * > + * If the allocation of the new top level page table fails, the ASCE is not > + * replaced. > + * In any case, the old ASCE is always removed from the list. Therefore the > + * caller has to make sure to save a pointer to it beforehands, unless an > + * intentional leak is intended. > + */ > +int s390_replace_asce(struct gmap *gmap) > +{ > + unsigned long asce; > + struct page *page; > + void *table; > + > + s390_remove_old_asce(gmap); > + > + page = alloc_pages(GFP_KERNEL_ACCOUNT, CRST_ALLOC_ORDER); > + if (!page) > + return -ENOMEM; > + table = page_to_virt(page); > + memcpy(table, gmap->table, 1UL << (CRST_ALLOC_ORDER + PAGE_SHIFT)); > + > + /* > + * The caller has to deal with the old ASCE, but here we make sure > + * the new one is properly added to the list of page tables, so that > + * it will be freed when the VM is torn down. > + */ > + spin_lock(&gmap->guest_table_lock); > + list_add(&page->lru, &gmap->crst_list); > + spin_unlock(&gmap->guest_table_lock); > + > + /* Set new table origin while preserving existing ASCE control bits */ > + asce = (gmap->asce & ~_ASCE_ORIGIN) | __pa(table); > + WRITE_ONCE(gmap->asce, asce); > + WRITE_ONCE(gmap->mm->context.gmap_asce, asce); > + WRITE_ONCE(gmap->table, table); > + > + return 0; > +} > +EXPORT_SYMBOL_GPL(s390_replace_asce);
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