[PATCH v9 02/18] KVM: s390: pv: handle secure storage violations for protected guests

Claudio Imbrenda
2022-03-30T12:37:17Z

With upcoming patches, protected guests will be able to trigger secure
storage violations in normal operation.

A secure storage violation is triggered when a protected guest tries to
access secure memory that has been mapped erroneously, or that belongs
to a different protected guest or to the ultravisor.

With upcoming patches, protected guests will be able to trigger secure
storage violations in normal operation. This happens for example if a
protected guest is rebooted with lazy destroy enabled and the new guest
is also protected.

When the new protected guest touches pages that have not yet been
destroyed, and thus are accounted to the previous protected guest, a
secure storage violation is raised.

This patch adds handling of secure storage violations for protected
guests.

This exception is handled by first trying to destroy the page, because
it is expected to belong to a defunct protected guest where a destroy
should be possible. If that fails, a normal export of the page is
attempted.

Therefore, pages that trigger the exception will be made non-secure
before attempting to use them again for a different secure guest.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
---
arch/s390/include/asm/uv.h | 1 +
arch/s390/kernel/uv.c | 55 ++++++++++++++++++++++++++++++++++++++
arch/s390/mm/fault.c | 10 +++++++
3 files changed, 66 insertions(+)

diff --git a/arch/s390/include/asm/uv.h b/arch/s390/include/asm/uv.h
index 86218382d29c..6b2b33f19abe 100644
--- a/arch/s390/include/asm/uv.h
+++ b/arch/s390/include/asm/uv.h
@@ -356,6 +356,7 @@ static inline int is_prot_virt_host(void)
}

int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb);
+int gmap_destroy_page(struct gmap *gmap, unsigned long gaddr);
int uv_destroy_owned_page(unsigned long paddr);
int uv_convert_from_secure(unsigned long paddr);
int uv_convert_owned_from_secure(unsigned long paddr);
diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index a5425075dd25..2754471cc789 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c
@@ -334,6 +334,61 @@ int gmap_convert_to_secure(struct gmap *gmap, unsigned long gaddr)
}
EXPORT_SYMBOL_GPL(gmap_convert_to_secure);

+/**
+ * gmap_destroy_page - Destroy a guest page.
+ * @gmap the gmap of the guest
+ * @gaddr the guest address to destroy
+ *
+ * An attempt will be made to destroy the given guest page. If the attempt
+ * fails, an attempt is made to export the page. If both attempts fail, an
+ * appropriate error is returned.
+ */
+int gmap_destroy_page(struct gmap *gmap, unsigned long gaddr)
+{
+ struct vm_area_struct *vma;
+ unsigned long uaddr;
+ struct page *page;
+ int rc;
+
+ rc = -EFAULT;
+ mmap_read_lock(gmap->mm);
+
+ uaddr = __gmap_translate(gmap, gaddr);
+ if (IS_ERR_VALUE(uaddr))
+ goto out;
+ vma = vma_lookup(gmap->mm, uaddr);
+ if (!vma)
+ goto out;
+ /*
+ * Huge pages should not be able to become secure
+ */
+ if (is_vm_hugetlb_page(vma))
+ goto out;
+
+ rc = 0;
+ /* we take an extra reference here */
+ page = follow_page(vma, uaddr, FOLL_WRITE | FOLL_GET);
+ if (IS_ERR_OR_NULL(page))
+ goto out;
+ rc = uv_destroy_owned_page(page_to_phys(page));
+ /*
+ * Fault handlers can race; it is possible that two CPUs will fault
+ * on the same secure page. One CPU can destroy the page, reboot,
+ * re-enter secure mode and import it, while the second CPU was
+ * stuck at the beginning of the handler. At some point the second
+ * CPU will be able to progress, and it will not be able to destroy
+ * the page. In that case we do not want to terminate the process,
+ * we instead try to export the page.
+ */
+ if (rc)
+ rc = uv_convert_owned_from_secure(page_to_phys(page));
+ put_page(page);
+out:
+ mmap_read_unlock(gmap->mm);
+ return rc;
+}
+EXPORT_SYMBOL_GPL(gmap_destroy_page);
+
/*
* To be called with the page locked or with an extra reference! This will
* prevent gmap_make_secure from touching the page concurrently. Having 2
diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index ff16ce0d04ee..47b52e5384f8 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -853,6 +853,16 @@ NOKPROBE_SYMBOL(do_non_secure_storage_access);

void do_secure_storage_violation(struct pt_regs *regs)
{
+ unsigned long gaddr = regs->int_parm_long & __FAIL_ADDR_MASK;
+ struct gmap *gmap = (struct gmap *)S390_lowcore.gmap;
+
+ /*
+ * If the VM has been rebooted, its address space might still contain
+ * secure pages from the previous boot.
+ * Clear the page so it can be reused.
+ */
+ if (!gmap_destroy_page(gmap, gaddr))
+ return;
/*
* Either KVM messed up the secure guest mapping or the same
* page is mapped into multiple secure guests.
--
2.34.1