[PATCH 5.16 071/227] KVM: x86/xen: Fix runstate updates to be atomic when preempting vCPU

Greg Kroah-Hartman
2022-02-21T09:58:51Z

From: David Woodhouse <dwmw@amazon.co.uk>

commit fcb732d8f8cf6084f8480015ad41d25fb023a4dd upstream.

There are circumstances whem kvm_xen_update_runstate_guest() should not
sleep because it ends up being called from __schedule() when the vCPU
is preempted:

[ 222.830825] kvm_xen_update_runstate_guest+0x24/0x100
[ 222.830878] kvm_arch_vcpu_put+0x14c/0x200
[ 222.830920] kvm_sched_out+0x30/0x40
[ 222.830960] __schedule+0x55c/0x9f0

To handle this, make it use the same trick as __kvm_xen_has_interrupt(),
of using the hva from the gfn_to_hva_cache directly. Then it can use
pagefault_disable() around the accesses and just bail out if the page
is absent (which is unlikely).

I almost switched to using a gfn_to_pfn_cache here and bailing out if
kvm_map_gfn() fails, like kvm_steal_time_set_preempted() does — but on
closer inspection it looks like kvm_map_gfn() will *always* fail in
atomic context for a page in IOMEM, which means it will silently fail
to make the update every single time for such guests, AFAICT. So I
didn't do it that way after all. And will probably fix that one too.

Cc: stable@vger.kernel.org
Fixes: 30b5c851af79 ("KVM: x86/xen: Add support for vCPU runstate information")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <b17a93e5ff4561e57b1238e3e7ccd0b613eb827e.camel@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kvm/xen.c | 97 ++++++++++++++++++++++++++++++++++++-----------------
1 file changed, 67 insertions(+), 30 deletions(-)

--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -93,32 +93,57 @@ static void kvm_xen_update_runstate(stru
void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, int state)
{
struct kvm_vcpu_xen *vx = &v->arch.xen;
+ struct gfn_to_hva_cache *ghc = &vx->runstate_cache;
+ struct kvm_memslots *slots = kvm_memslots(v->kvm);
+ bool atomic = (state == RUNSTATE_runnable);
uint64_t state_entry_time;
- unsigned int offset;
+ int __user *user_state;
+ uint64_t __user *user_times;

kvm_xen_update_runstate(v, state);

if (!vx->runstate_set)
return;

- BUILD_BUG_ON(sizeof(struct compat_vcpu_runstate_info) != 0x2c);
+ if (unlikely(slots->generation != ghc->generation || kvm_is_error_hva(ghc->hva)) &&
+ kvm_gfn_to_hva_cache_init(v->kvm, ghc, ghc->gpa, ghc->len))
+ return;
+
+ /* We made sure it fits in a single page */
+ BUG_ON(!ghc->memslot);
+
+ if (atomic)
+ pagefault_disable();

- offset = offsetof(struct compat_vcpu_runstate_info, state_entry_time);
-#ifdef CONFIG_X86_64
/*
- * The only difference is alignment of uint64_t in 32-bit.
- * So the first field 'state' is accessed directly using
- * offsetof() (where its offset happens to be zero), while the
- * remaining fields which are all uint64_t, start at 'offset'
- * which we tweak here by adding 4.
+ * The only difference between 32-bit and 64-bit versions of the
+ * runstate struct us the alignment of uint64_t in 32-bit, which
+ * means that the 64-bit version has an additional 4 bytes of
+ * padding after the first field 'state'.
+ *
+ * So we use 'int __user *user_state' to point to the state field,
+ * and 'uint64_t __user *user_times' for runstate_entry_time. So
+ * the actual array of time[] in each state starts at user_times[1].
*/
+ BUILD_BUG_ON(offsetof(struct vcpu_runstate_info, state) != 0);
+ BUILD_BUG_ON(offsetof(struct compat_vcpu_runstate_info, state) != 0);
+ user_state = (int __user *)ghc->hva;
+
+ BUILD_BUG_ON(sizeof(struct compat_vcpu_runstate_info) != 0x2c);
+
+ user_times = (uint64_t __user *)(ghc->hva +
+ offsetof(struct compat_vcpu_runstate_info,
+ state_entry_time));
+#ifdef CONFIG_X86_64
BUILD_BUG_ON(offsetof(struct vcpu_runstate_info, state_entry_time) !=
offsetof(struct compat_vcpu_runstate_info, state_entry_time) + 4);
BUILD_BUG_ON(offsetof(struct vcpu_runstate_info, time) !=
offsetof(struct compat_vcpu_runstate_info, time) + 4);

if (v->kvm->arch.xen.long_mode)
- offset = offsetof(struct vcpu_runstate_info, state_entry_time);
+ user_times = (uint64_t __user *)(ghc->hva +
+ offsetof(struct vcpu_runstate_info,
+ state_entry_time));
#endif
/*
* First write the updated state_entry_time at the appropriate
@@ -132,10 +157,8 @@ void kvm_xen_update_runstate_guest(struc
BUILD_BUG_ON(sizeof_field(struct compat_vcpu_runstate_info, state_entry_time) !=
sizeof(state_entry_time));

- if (kvm_write_guest_offset_cached(v->kvm, &v->arch.xen.runstate_cache,
- &state_entry_time, offset,
- sizeof(state_entry_time)))
- return;
+ if (__put_user(state_entry_time, user_times))
+ goto out;
smp_wmb();

/*
@@ -149,11 +172,8 @@ void kvm_xen_update_runstate_guest(struc
BUILD_BUG_ON(sizeof_field(struct compat_vcpu_runstate_info, state) !=
sizeof(vx->current_runstate));

- if (kvm_write_guest_offset_cached(v->kvm, &v->arch.xen.runstate_cache,
- &vx->current_runstate,
- offsetof(struct vcpu_runstate_info, state),
- sizeof(vx->current_runstate)))
- return;
+ if (__put_user(vx->current_runstate, user_state))
+ goto out;

/*
* Write the actual runstate times immediately after the
@@ -168,24 +188,23 @@ void kvm_xen_update_runstate_guest(struc
BUILD_BUG_ON(sizeof_field(struct vcpu_runstate_info, time) !=
sizeof(vx->runstate_times));

- if (kvm_write_guest_offset_cached(v->kvm, &v->arch.xen.runstate_cache,
- &vx->runstate_times[0],
- offset + sizeof(u64),
- sizeof(vx->runstate_times)))
- return;
-
+ if (__copy_to_user(user_times + 1, vx->runstate_times, sizeof(vx->runstate_times)))
+ goto out;
smp_wmb();

/*
* Finally, clear the XEN_RUNSTATE_UPDATE bit in the guest's
* runstate_entry_time field.
*/
-
state_entry_time &= ~XEN_RUNSTATE_UPDATE;
- if (kvm_write_guest_offset_cached(v->kvm, &v->arch.xen.runstate_cache,
- &state_entry_time, offset,
- sizeof(state_entry_time)))
- return;
+ __put_user(state_entry_time, user_times);
+ smp_wmb();
+
+ out:
+ mark_page_dirty_in_slot(v->kvm, ghc->memslot, ghc->gpa >> PAGE_SHIFT);
+
+ if (atomic)
+ pagefault_enable();
}

int __kvm_xen_has_interrupt(struct kvm_vcpu *v)
@@ -337,6 +356,12 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcp
break;
}

+ /* It must fit within a single page */
+ if ((data->u.gpa & ~PAGE_MASK) + sizeof(struct vcpu_info) > PAGE_SIZE) {
+ r = -EINVAL;
+ break;
+ }
+
r = kvm_gfn_to_hva_cache_init(vcpu->kvm,
&vcpu->arch.xen.vcpu_info_cache,
data->u.gpa,
@@ -354,6 +379,12 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcp
break;
}

+ /* It must fit within a single page */
+ if ((data->u.gpa & ~PAGE_MASK) + sizeof(struct pvclock_vcpu_time_info) > PAGE_SIZE) {
+ r = -EINVAL;
+ break;
+ }
+
r = kvm_gfn_to_hva_cache_init(vcpu->kvm,
&vcpu->arch.xen.vcpu_time_info_cache,
data->u.gpa,
@@ -375,6 +406,12 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcp
break;
}

+ /* It must fit within a single page */
+ if ((data->u.gpa & ~PAGE_MASK) + sizeof(struct vcpu_runstate_info) > PAGE_SIZE) {
+ r = -EINVAL;
+ break;
+ }
+
r = kvm_gfn_to_hva_cache_init(vcpu->kvm,
&vcpu->arch.xen.runstate_cache,
data->u.gpa,