KVM: s390: Properly lock mm context allow_gmap_hpage_1m setting

We have to do down_write on the mm semaphore to set a bitfield in the
mm context.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Fixes: a4499382 ("KVM: s390: Add huge page enablement control")
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

2 files changed, 9 insertions, 1 deletions

diff --git a/arch/s390/include/asm/mmu.h b/arch/s390/include/asm/mmu.h
index f31a15044c24..a8418e1379eb 100644
--- a/arch/s390/include/asm/mmu.h
+++ b/arch/s390/include/asm/mmu.h
@@ -16,7 +16,13 @@ typedef struct {
         unsigned long asce;
         unsigned long asce_limit;
         unsigned long vdso_base;
-        /* The mmu context allocates 4K page tables. */
+        /*
+         * The following bitfields need a down_write on the mm
+         * semaphore when they are written to. As they are only
+         * written once, they can be read without a lock.
+         *
+         * The mmu context allocates 4K page tables.
+         */
         unsigned int alloc_pgste:1;
         /* The mmu context uses extended page tables. */
         unsigned int has_pgste:1;
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 91ad4a9425c0..f69333fd2fa3 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -695,7 +695,9 @@ static int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
                         r = -EINVAL;
                 else {
                         r = 0;
+                        down_write(&kvm->mm->mmap_sem);
                         kvm->mm->context.allow_gmap_hpage_1m = 1;
+                        up_write(&kvm->mm->mmap_sem);
                         /*
                          * We might have to create fake 4k page
                          * tables. To avoid that the hardware works on