[PATCH v8 3/8] KVM: mmu: Make __kvm_follow_pfn not imply FOLL_GET
David Stevens
stevensd at chromium.org
Thu Aug 24 18:04:03 AEST 2023
From: David Stevens <stevensd at chromium.org>
Make it so that __kvm_follow_pfn does not imply FOLL_GET. This allows
callers to resolve a gfn when the associated pfn has a valid struct page
that isn't being actively refcounted (e.g. tail pages of non-compound
higher order pages). For a caller to safely omit FOLL_GET, all usages of
the returned pfn must be guarded by a mmu notifier.
This also adds a is_refcounted_page out parameter to kvm_follow_pfn that
is set when the returned pfn has an associated struct page with a valid
refcount. Callers that don't pass FOLL_GET should remember this value
and use it to avoid places like kvm_is_ad_tracked_page that assume a
non-zero refcount.
Signed-off-by: David Stevens <stevensd at chromium.org>
---
include/linux/kvm_host.h | 7 ++++
virt/kvm/kvm_main.c | 84 ++++++++++++++++++++++++----------------
virt/kvm/pfncache.c | 2 +-
3 files changed, 58 insertions(+), 35 deletions(-)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 59d9b5e5db33..713fc2d91f95 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1164,10 +1164,17 @@ struct kvm_follow_pfn {
bool atomic;
/* Try to create a writable mapping even for a read fault */
bool try_map_writable;
+ /*
+ * Usage of the returned pfn will be guared by a mmu notifier. Must
+ * be true if FOLL_GET is not set.
+ */
+ bool guarded_by_mmu_notifier;
/* Outputs of __kvm_follow_pfn */
hva_t hva;
bool writable;
+ /* True if the returned pfn is for a page with a valid refcount. */
+ bool is_refcounted_page;
};
kvm_pfn_t __kvm_follow_pfn(struct kvm_follow_pfn *foll);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 5fde46f05117..963b96cd8ff9 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2481,6 +2481,25 @@ static inline int check_user_page_hwpoison(unsigned long addr)
return rc == -EHWPOISON;
}
+static kvm_pfn_t kvm_follow_refcounted_pfn(struct kvm_follow_pfn *foll,
+ struct page *page)
+{
+ kvm_pfn_t pfn = page_to_pfn(page);
+
+ foll->is_refcounted_page = true;
+
+ /*
+ * FIXME: Ideally, KVM wouldn't pass FOLL_GET to gup() when the caller
+ * doesn't want to grab a reference, but gup() doesn't support getting
+ * just the pfn, i.e. FOLL_GET is effectively mandatory. If that ever
+ * changes, drop this and simply don't pass FOLL_GET to gup().
+ */
+ if (!(foll->flags & FOLL_GET))
+ put_page(page);
+
+ return pfn;
+}
+
/*
* The fast path to get the writable pfn which will be stored in @pfn,
* true indicates success, otherwise false is returned. It's also the
@@ -2499,8 +2518,8 @@ static bool hva_to_pfn_fast(struct kvm_follow_pfn *foll, kvm_pfn_t *pfn)
return false;
if (get_user_page_fast_only(foll->hva, FOLL_WRITE, page)) {
- *pfn = page_to_pfn(page[0]);
foll->writable = true;
+ *pfn = kvm_follow_refcounted_pfn(foll, page[0]);
return true;
}
@@ -2513,7 +2532,7 @@ static bool hva_to_pfn_fast(struct kvm_follow_pfn *foll, kvm_pfn_t *pfn)
*/
static int hva_to_pfn_slow(struct kvm_follow_pfn *foll, kvm_pfn_t *pfn)
{
- unsigned int flags = FOLL_HWPOISON | foll->flags;
+ unsigned int flags = FOLL_HWPOISON | FOLL_GET | foll->flags;
struct page *page;
int npages;
@@ -2535,7 +2554,7 @@ static int hva_to_pfn_slow(struct kvm_follow_pfn *foll, kvm_pfn_t *pfn)
page = wpage;
}
}
- *pfn = page_to_pfn(page);
+ *pfn = kvm_follow_refcounted_pfn(foll, page);
return npages;
}
@@ -2550,16 +2569,6 @@ static bool vma_is_valid(struct vm_area_struct *vma, bool write_fault)
return true;
}
-static int kvm_try_get_pfn(kvm_pfn_t pfn)
-{
- struct page *page = kvm_pfn_to_refcounted_page(pfn);
-
- if (!page)
- return 1;
-
- return get_page_unless_zero(page);
-}
-
static int hva_to_pfn_remapped(struct vm_area_struct *vma,
struct kvm_follow_pfn *foll, kvm_pfn_t *p_pfn)
{
@@ -2568,6 +2577,7 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
pte_t pte;
spinlock_t *ptl;
bool write_fault = foll->flags & FOLL_WRITE;
+ struct page *page;
int r;
r = follow_pte(vma->vm_mm, foll->hva, &ptep, &ptl);
@@ -2601,28 +2611,29 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
pfn = pte_pfn(pte);
/*
- * Get a reference here because callers of *hva_to_pfn* and
- * *gfn_to_pfn* ultimately call kvm_release_pfn_clean on the
- * returned pfn. This is only needed if the VMA has VM_MIXEDMAP
- * set, but the kvm_try_get_pfn/kvm_release_pfn_clean pair will
- * simply do nothing for reserved pfns.
- *
- * Whoever called remap_pfn_range is also going to call e.g.
- * unmap_mapping_range before the underlying pages are freed,
- * causing a call to our MMU notifier.
+ * Now deal with reference counting. If kvm_pfn_to_refcounted_page
+ * returns NULL, then there's no refcount to worry about.
*
- * Certain IO or PFNMAP mappings can be backed with valid
- * struct pages, but be allocated without refcounting e.g.,
- * tail pages of non-compound higher order allocations, which
- * would then underflow the refcount when the caller does the
- * required put_page. Don't allow those pages here.
+ * Otherwise, certain IO or PFNMAP mappings can be backed with valid
+ * struct pages but be allocated without refcounting e.g., tail pages of
+ * non-compound higher order allocations. If FOLL_GET is set and we
+ * increment such a refcount, then when that pfn is eventually passed to
+ * kvm_release_pfn_clean, its refcount would hit zero and be incorrectly
+ * freed. Therefore don't allow those pages here when FOLL_GET is set.
*/
- if (!kvm_try_get_pfn(pfn))
- r = -EFAULT;
+ page = kvm_pfn_to_refcounted_page(pfn);
+ if (!page)
+ goto out;
+
+ if (get_page_unless_zero(page))
+ WARN_ON_ONCE(kvm_follow_refcounted_pfn(foll, page) != pfn);
out:
pte_unmap_unlock(ptep, ptl);
- *p_pfn = pfn;
+ if (!foll->is_refcounted_page && !foll->guarded_by_mmu_notifier)
+ r = -EFAULT;
+ else
+ *p_pfn = pfn;
return r;
}
@@ -2696,6 +2707,11 @@ kvm_pfn_t hva_to_pfn(struct kvm_follow_pfn *foll)
kvm_pfn_t __kvm_follow_pfn(struct kvm_follow_pfn *foll)
{
foll->writable = false;
+ foll->is_refcounted_page = false;
+
+ if (WARN_ON_ONCE(!(foll->flags & FOLL_GET) && !foll->guarded_by_mmu_notifier))
+ return KVM_PFN_ERR_FAULT;
+
foll->hva = __gfn_to_hva_many(foll->slot, foll->gfn, NULL,
foll->flags & FOLL_WRITE);
@@ -2720,7 +2736,7 @@ kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
struct kvm_follow_pfn foll = {
.slot = slot,
.gfn = gfn,
- .flags = 0,
+ .flags = FOLL_GET,
.atomic = atomic,
.try_map_writable = !!writable,
};
@@ -2752,7 +2768,7 @@ kvm_pfn_t gfn_to_pfn_prot(struct kvm *kvm, gfn_t gfn, bool write_fault,
struct kvm_follow_pfn foll = {
.slot = gfn_to_memslot(kvm, gfn),
.gfn = gfn,
- .flags = write_fault ? FOLL_WRITE : 0,
+ .flags = FOLL_GET | (write_fault ? FOLL_WRITE : 0),
.try_map_writable = !!writable,
};
pfn = __kvm_follow_pfn(&foll);
@@ -2767,7 +2783,7 @@ kvm_pfn_t gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn)
struct kvm_follow_pfn foll = {
.slot = slot,
.gfn = gfn,
- .flags = FOLL_WRITE,
+ .flags = FOLL_GET | FOLL_WRITE,
};
return __kvm_follow_pfn(&foll);
}
@@ -2778,7 +2794,7 @@ kvm_pfn_t gfn_to_pfn_memslot_atomic(const struct kvm_memory_slot *slot, gfn_t gf
struct kvm_follow_pfn foll = {
.slot = slot,
.gfn = gfn,
- .flags = FOLL_WRITE,
+ .flags = FOLL_GET | FOLL_WRITE,
.atomic = true,
};
return __kvm_follow_pfn(&foll);
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 86cd40acad11..c558f510ab51 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -147,7 +147,7 @@ static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
struct kvm_follow_pfn foll = {
.slot = gpc->memslot,
.gfn = gpa_to_gfn(gpc->gpa),
- .flags = FOLL_WRITE,
+ .flags = FOLL_WRITE | FOLL_GET,
.hva = gpc->uhva,
};
--
2.42.0.rc1.204.g551eb34607-goog
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