diff mbox series

[v1,01/13] KVM: Add KVM_MEM_USERFAULT memslot flag and bitmap

Message ID 20241204191349.1730936-2-jthoughton@google.com (mailing list archive)
State New
Headers show
Series KVM: Introduce KVM Userfault | expand

Commit Message

James Houghton Dec. 4, 2024, 7:13 p.m. UTC 
Use one of the 14 reserved u64s in struct kvm_userspace_memory_region2
for the user to provide `userfault_bitmap`.

The memslot flag indicates if KVM should be reading from the
`userfault_bitmap` field from the memslot. The user is permitted to
provide a bogus pointer. If the pointer cannot be read from, we will
return -EFAULT (with no other information) back to the user.

Signed-off-by: James Houghton <jthoughton@google.com>
---
 include/linux/kvm_host.h | 14 ++++++++++++++
 include/uapi/linux/kvm.h |  4 +++-
 virt/kvm/Kconfig         |  3 +++
 virt/kvm/kvm_main.c      | 28 ++++++++++++++++++++++++++++
 4 files changed, 48 insertions(+), 1 deletion(-)

Comments

kernel test robot Dec. 5, 2024, 11:52 a.m. UTC | #1 
Hi James,

kernel test robot noticed the following build warnings:

[auto build test WARNING on 4d911c7abee56771b0219a9fbf0120d06bdc9c14]

url:    https://github.com/intel-lab-lkp/linux/commits/James-Houghton/KVM-Add-KVM_MEM_USERFAULT-memslot-flag-and-bitmap/20241205-032516
base:   4d911c7abee56771b0219a9fbf0120d06bdc9c14
patch link:    https://lore.kernel.org/r/20241204191349.1730936-2-jthoughton%40google.com
patch subject: [PATCH v1 01/13] KVM: Add KVM_MEM_USERFAULT memslot flag and bitmap
config: x86_64-randconfig-121 (https://download.01.org/0day-ci/archive/20241205/202412051904.GNL7BE1X-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20241205/202412051904.GNL7BE1X-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202412051904.GNL7BE1X-lkp@intel.com/

sparse warnings: (new ones prefixed by >>)
   arch/x86/kvm/../../../virt/kvm/kvm_main.c: note: in included file:
   include/linux/kvm_host.h:2080:54: sparse: sparse: array of flexible structures
   include/linux/kvm_host.h:2082:56: sparse: sparse: array of flexible structures
>> arch/x86/kvm/../../../virt/kvm/kvm_main.c:2049:39: sparse: sparse: incorrect type in assignment (different address spaces) @@     expected unsigned long [noderef] __user *userfault_bitmap @@     got unsigned long * @@
   arch/x86/kvm/../../../virt/kvm/kvm_main.c:2049:39: sparse:     expected unsigned long [noderef] __user *userfault_bitmap
   arch/x86/kvm/../../../virt/kvm/kvm_main.c:2049:39: sparse:     got unsigned long *
   arch/x86/kvm/../../../virt/kvm/kvm_main.c:626:25: sparse: sparse: context imbalance in 'kvm_mmu_notifier_invalidate_range_start' - different lock contexts for basic block
   arch/x86/kvm/../../../virt/kvm/kvm_main.c:626:25: sparse: sparse: context imbalance in 'kvm_mmu_notifier_invalidate_range_end' - different lock contexts for basic block
   arch/x86/kvm/../../../virt/kvm/kvm_main.c:626:25: sparse: sparse: context imbalance in 'kvm_mmu_notifier_clear_flush_young' - different lock contexts for basic block
   arch/x86/kvm/../../../virt/kvm/kvm_main.c:626:25: sparse: sparse: context imbalance in 'kvm_mmu_notifier_clear_young' - different lock contexts for basic block
   arch/x86/kvm/../../../virt/kvm/kvm_main.c:626:25: sparse: sparse: context imbalance in 'kvm_mmu_notifier_test_young' - different lock contexts for basic block
   arch/x86/kvm/../../../virt/kvm/kvm_main.c: note: in included file (through include/linux/mutex.h, include/linux/kvm_types.h, include/kvm/iodev.h):
   include/linux/list.h:83:21: sparse: sparse: self-comparison always evaluates to true
   arch/x86/kvm/../../../virt/kvm/kvm_main.c:1960:49: sparse: sparse: self-comparison always evaluates to false
   include/linux/list.h:83:21: sparse: sparse: self-comparison always evaluates to true

vim +2049 arch/x86/kvm/../../../virt/kvm/kvm_main.c

  1931	
  1932	/*
  1933	 * Allocate some memory and give it an address in the guest physical address
  1934	 * space.
  1935	 *
  1936	 * Discontiguous memory is allowed, mostly for framebuffers.
  1937	 *
  1938	 * Must be called holding kvm->slots_lock for write.
  1939	 */
  1940	int __kvm_set_memory_region(struct kvm *kvm,
  1941				    const struct kvm_userspace_memory_region2 *mem)
  1942	{
  1943		struct kvm_memory_slot *old, *new;
  1944		struct kvm_memslots *slots;
  1945		enum kvm_mr_change change;
  1946		unsigned long npages;
  1947		gfn_t base_gfn;
  1948		int as_id, id;
  1949		int r;
  1950	
  1951		r = check_memory_region_flags(kvm, mem);
  1952		if (r)
  1953			return r;
  1954	
  1955		as_id = mem->slot >> 16;
  1956		id = (u16)mem->slot;
  1957	
  1958		/* General sanity checks */
  1959		if ((mem->memory_size & (PAGE_SIZE - 1)) ||
  1960		    (mem->memory_size != (unsigned long)mem->memory_size))
  1961			return -EINVAL;
  1962		if (mem->guest_phys_addr & (PAGE_SIZE - 1))
  1963			return -EINVAL;
  1964		/* We can read the guest memory with __xxx_user() later on. */
  1965		if ((mem->userspace_addr & (PAGE_SIZE - 1)) ||
  1966		    (mem->userspace_addr != untagged_addr(mem->userspace_addr)) ||
  1967		     !access_ok((void __user *)(unsigned long)mem->userspace_addr,
  1968				mem->memory_size))
  1969			return -EINVAL;
  1970		if (mem->flags & KVM_MEM_GUEST_MEMFD &&
  1971		    (mem->guest_memfd_offset & (PAGE_SIZE - 1) ||
  1972		     mem->guest_memfd_offset + mem->memory_size < mem->guest_memfd_offset))
  1973			return -EINVAL;
  1974		if (as_id >= kvm_arch_nr_memslot_as_ids(kvm) || id >= KVM_MEM_SLOTS_NUM)
  1975			return -EINVAL;
  1976		if (mem->guest_phys_addr + mem->memory_size < mem->guest_phys_addr)
  1977			return -EINVAL;
  1978		if ((mem->memory_size >> PAGE_SHIFT) > KVM_MEM_MAX_NR_PAGES)
  1979			return -EINVAL;
  1980	
  1981		slots = __kvm_memslots(kvm, as_id);
  1982	
  1983		/*
  1984		 * Note, the old memslot (and the pointer itself!) may be invalidated
  1985		 * and/or destroyed by kvm_set_memslot().
  1986		 */
  1987		old = id_to_memslot(slots, id);
  1988	
  1989		if (!mem->memory_size) {
  1990			if (!old || !old->npages)
  1991				return -EINVAL;
  1992	
  1993			if (WARN_ON_ONCE(kvm->nr_memslot_pages < old->npages))
  1994				return -EIO;
  1995	
  1996			return kvm_set_memslot(kvm, old, NULL, KVM_MR_DELETE);
  1997		}
  1998	
  1999		base_gfn = (mem->guest_phys_addr >> PAGE_SHIFT);
  2000		npages = (mem->memory_size >> PAGE_SHIFT);
  2001	
  2002		if (!old || !old->npages) {
  2003			change = KVM_MR_CREATE;
  2004	
  2005			/*
  2006			 * To simplify KVM internals, the total number of pages across
  2007			 * all memslots must fit in an unsigned long.
  2008			 */
  2009			if ((kvm->nr_memslot_pages + npages) < kvm->nr_memslot_pages)
  2010				return -EINVAL;
  2011		} else { /* Modify an existing slot. */
  2012			/* Private memslots are immutable, they can only be deleted. */
  2013			if (mem->flags & KVM_MEM_GUEST_MEMFD)
  2014				return -EINVAL;
  2015			if ((mem->userspace_addr != old->userspace_addr) ||
  2016			    (npages != old->npages) ||
  2017			    ((mem->flags ^ old->flags) & KVM_MEM_READONLY))
  2018				return -EINVAL;
  2019	
  2020			if (base_gfn != old->base_gfn)
  2021				change = KVM_MR_MOVE;
  2022			else if (mem->flags != old->flags)
  2023				change = KVM_MR_FLAGS_ONLY;
  2024			else /* Nothing to change. */
  2025				return 0;
  2026		}
  2027	
  2028		if ((change == KVM_MR_CREATE || change == KVM_MR_MOVE) &&
  2029		    kvm_check_memslot_overlap(slots, id, base_gfn, base_gfn + npages))
  2030			return -EEXIST;
  2031	
  2032		/* Allocate a slot that will persist in the memslot. */
  2033		new = kzalloc(sizeof(*new), GFP_KERNEL_ACCOUNT);
  2034		if (!new)
  2035			return -ENOMEM;
  2036	
  2037		new->as_id = as_id;
  2038		new->id = id;
  2039		new->base_gfn = base_gfn;
  2040		new->npages = npages;
  2041		new->flags = mem->flags;
  2042		new->userspace_addr = mem->userspace_addr;
  2043		if (mem->flags & KVM_MEM_GUEST_MEMFD) {
  2044			r = kvm_gmem_bind(kvm, new, mem->guest_memfd, mem->guest_memfd_offset);
  2045			if (r)
  2046				goto out;
  2047		}
  2048		if (mem->flags & KVM_MEM_USERFAULT)
> 2049			new->userfault_bitmap = (unsigned long *)mem->userfault_bitmap;
  2050	
  2051		r = kvm_set_memslot(kvm, old, new, change);
  2052		if (r)
  2053			goto out_unbind;
  2054	
  2055		return 0;
  2056	
  2057	out_unbind:
  2058		if (mem->flags & KVM_MEM_GUEST_MEMFD)
  2059			kvm_gmem_unbind(new);
  2060	out:
  2061		kfree(new);
  2062		return r;
  2063	}
  2064	EXPORT_SYMBOL_GPL(__kvm_set_memory_region);
  2065
kernel test robot Dec. 5, 2024, 2:22 p.m. UTC | #2 
Hi James,

kernel test robot noticed the following build warnings:

[auto build test WARNING on 4d911c7abee56771b0219a9fbf0120d06bdc9c14]

url:    https://github.com/intel-lab-lkp/linux/commits/James-Houghton/KVM-Add-KVM_MEM_USERFAULT-memslot-flag-and-bitmap/20241205-032516
base:   4d911c7abee56771b0219a9fbf0120d06bdc9c14
patch link:    https://lore.kernel.org/r/20241204191349.1730936-2-jthoughton%40google.com
patch subject: [PATCH v1 01/13] KVM: Add KVM_MEM_USERFAULT memslot flag and bitmap
config: i386-buildonly-randconfig-006 (https://download.01.org/0day-ci/archive/20241205/202412052133.pTg3UAQm-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20241205/202412052133.pTg3UAQm-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202412052133.pTg3UAQm-lkp@intel.com/

All warnings (new ones prefixed by >>):

   arch/x86/kvm/../../../virt/kvm/kvm_main.c: In function '__kvm_set_memory_region':
>> arch/x86/kvm/../../../virt/kvm/kvm_main.c:2049:41: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
    2049 |                 new->userfault_bitmap = (unsigned long *)mem->userfault_bitmap;
         |                                         ^


vim +2049 arch/x86/kvm/../../../virt/kvm/kvm_main.c

  1931	
  1932	/*
  1933	 * Allocate some memory and give it an address in the guest physical address
  1934	 * space.
  1935	 *
  1936	 * Discontiguous memory is allowed, mostly for framebuffers.
  1937	 *
  1938	 * Must be called holding kvm->slots_lock for write.
  1939	 */
  1940	int __kvm_set_memory_region(struct kvm *kvm,
  1941				    const struct kvm_userspace_memory_region2 *mem)
  1942	{
  1943		struct kvm_memory_slot *old, *new;
  1944		struct kvm_memslots *slots;
  1945		enum kvm_mr_change change;
  1946		unsigned long npages;
  1947		gfn_t base_gfn;
  1948		int as_id, id;
  1949		int r;
  1950	
  1951		r = check_memory_region_flags(kvm, mem);
  1952		if (r)
  1953			return r;
  1954	
  1955		as_id = mem->slot >> 16;
  1956		id = (u16)mem->slot;
  1957	
  1958		/* General sanity checks */
  1959		if ((mem->memory_size & (PAGE_SIZE - 1)) ||
  1960		    (mem->memory_size != (unsigned long)mem->memory_size))
  1961			return -EINVAL;
  1962		if (mem->guest_phys_addr & (PAGE_SIZE - 1))
  1963			return -EINVAL;
  1964		/* We can read the guest memory with __xxx_user() later on. */
  1965		if ((mem->userspace_addr & (PAGE_SIZE - 1)) ||
  1966		    (mem->userspace_addr != untagged_addr(mem->userspace_addr)) ||
  1967		     !access_ok((void __user *)(unsigned long)mem->userspace_addr,
  1968				mem->memory_size))
  1969			return -EINVAL;
  1970		if (mem->flags & KVM_MEM_GUEST_MEMFD &&
  1971		    (mem->guest_memfd_offset & (PAGE_SIZE - 1) ||
  1972		     mem->guest_memfd_offset + mem->memory_size < mem->guest_memfd_offset))
  1973			return -EINVAL;
  1974		if (as_id >= kvm_arch_nr_memslot_as_ids(kvm) || id >= KVM_MEM_SLOTS_NUM)
  1975			return -EINVAL;
  1976		if (mem->guest_phys_addr + mem->memory_size < mem->guest_phys_addr)
  1977			return -EINVAL;
  1978		if ((mem->memory_size >> PAGE_SHIFT) > KVM_MEM_MAX_NR_PAGES)
  1979			return -EINVAL;
  1980	
  1981		slots = __kvm_memslots(kvm, as_id);
  1982	
  1983		/*
  1984		 * Note, the old memslot (and the pointer itself!) may be invalidated
  1985		 * and/or destroyed by kvm_set_memslot().
  1986		 */
  1987		old = id_to_memslot(slots, id);
  1988	
  1989		if (!mem->memory_size) {
  1990			if (!old || !old->npages)
  1991				return -EINVAL;
  1992	
  1993			if (WARN_ON_ONCE(kvm->nr_memslot_pages < old->npages))
  1994				return -EIO;
  1995	
  1996			return kvm_set_memslot(kvm, old, NULL, KVM_MR_DELETE);
  1997		}
  1998	
  1999		base_gfn = (mem->guest_phys_addr >> PAGE_SHIFT);
  2000		npages = (mem->memory_size >> PAGE_SHIFT);
  2001	
  2002		if (!old || !old->npages) {
  2003			change = KVM_MR_CREATE;
  2004	
  2005			/*
  2006			 * To simplify KVM internals, the total number of pages across
  2007			 * all memslots must fit in an unsigned long.
  2008			 */
  2009			if ((kvm->nr_memslot_pages + npages) < kvm->nr_memslot_pages)
  2010				return -EINVAL;
  2011		} else { /* Modify an existing slot. */
  2012			/* Private memslots are immutable, they can only be deleted. */
  2013			if (mem->flags & KVM_MEM_GUEST_MEMFD)
  2014				return -EINVAL;
  2015			if ((mem->userspace_addr != old->userspace_addr) ||
  2016			    (npages != old->npages) ||
  2017			    ((mem->flags ^ old->flags) & KVM_MEM_READONLY))
  2018				return -EINVAL;
  2019	
  2020			if (base_gfn != old->base_gfn)
  2021				change = KVM_MR_MOVE;
  2022			else if (mem->flags != old->flags)
  2023				change = KVM_MR_FLAGS_ONLY;
  2024			else /* Nothing to change. */
  2025				return 0;
  2026		}
  2027	
  2028		if ((change == KVM_MR_CREATE || change == KVM_MR_MOVE) &&
  2029		    kvm_check_memslot_overlap(slots, id, base_gfn, base_gfn + npages))
  2030			return -EEXIST;
  2031	
  2032		/* Allocate a slot that will persist in the memslot. */
  2033		new = kzalloc(sizeof(*new), GFP_KERNEL_ACCOUNT);
  2034		if (!new)
  2035			return -ENOMEM;
  2036	
  2037		new->as_id = as_id;
  2038		new->id = id;
  2039		new->base_gfn = base_gfn;
  2040		new->npages = npages;
  2041		new->flags = mem->flags;
  2042		new->userspace_addr = mem->userspace_addr;
  2043		if (mem->flags & KVM_MEM_GUEST_MEMFD) {
  2044			r = kvm_gmem_bind(kvm, new, mem->guest_memfd, mem->guest_memfd_offset);
  2045			if (r)
  2046				goto out;
  2047		}
  2048		if (mem->flags & KVM_MEM_USERFAULT)
> 2049			new->userfault_bitmap = (unsigned long *)mem->userfault_bitmap;
  2050	
  2051		r = kvm_set_memslot(kvm, old, new, change);
  2052		if (r)
  2053			goto out_unbind;
  2054	
  2055		return 0;
  2056	
  2057	out_unbind:
  2058		if (mem->flags & KVM_MEM_GUEST_MEMFD)
  2059			kvm_gmem_unbind(new);
  2060	out:
  2061		kfree(new);
  2062		return r;
  2063	}
  2064	EXPORT_SYMBOL_GPL(__kvm_set_memory_region);
  2065
James Houghton Dec. 6, 2024, 10:46 p.m. UTC | #3 
>    arch/x86/kvm/../../../virt/kvm/kvm_main.c: In function '__kvm_set_memory_region':
> >> arch/x86/kvm/../../../virt/kvm/kvm_main.c:2049:41: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
>     2049 |                 new->userfault_bitmap = (unsigned long *)mem->userfault_bitmap;
>          |                                         ^

I realize that, not only have I done this cast slightly wrong, I'm
missing a few checks on userfault_bitmap that I should have. Applying
this diff, or at least something like it, to fix it:

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index b552cdef2850..30f09141df64 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1977,6 +1977,12 @@ int __kvm_set_memory_region(struct kvm *kvm,
 		return -EINVAL;
 	if ((mem->memory_size >> PAGE_SHIFT) > KVM_MEM_MAX_NR_PAGES)
 		return -EINVAL;
+	if (mem->flags & KVM_MEM_USERFAULT &&
+	    ((mem->userfault_bitmap != untagged_addr(mem->userfault_bitmap)) ||
+	     !access_ok((void __user *)(unsigned long)mem->userfault_bitmap,
+			DIV_ROUND_UP(mem->memory_size >> PAGE_SHIFT, BITS_PER_LONG)
+			 * sizeof(long))))
+		return -EINVAL;
 
 	slots = __kvm_memslots(kvm, as_id);
 
@@ -2053,7 +2059,8 @@ int __kvm_set_memory_region(struct kvm *kvm,
 			goto out;
 	}
 	if (mem->flags & KVM_MEM_USERFAULT)
-		new->userfault_bitmap = (unsigned long *)mem->userfault_bitmap;
+		new->userfault_bitmap =
+		  (unsigned long __user *)(unsigned long)mem->userfault_bitmap;
 
 	r = kvm_set_memslot(kvm, old, new, change);
 	if (r)
diff mbox series

Patch

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 401439bb21e3..f7a3dfd5e224 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -590,6 +590,7 @@  struct kvm_memory_slot {
 	unsigned long *dirty_bitmap;
 	struct kvm_arch_memory_slot arch;
 	unsigned long userspace_addr;
+	unsigned long __user *userfault_bitmap;
 	u32 flags;
 	short id;
 	u16 as_id;
@@ -724,6 +725,11 @@  static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)
 }
 #endif
 
+static inline bool kvm_has_userfault(struct kvm *kvm)
+{
+	return IS_ENABLED(CONFIG_HAVE_KVM_USERFAULT);
+}
+
 struct kvm_memslots {
 	u64 generation;
 	atomic_long_t last_used_slot;
@@ -2553,4 +2559,12 @@  long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
 				    struct kvm_pre_fault_memory *range);
 #endif
 
+int kvm_gfn_userfault(struct kvm *kvm, struct kvm_memory_slot *memslot,
+		      gfn_t gfn);
+
+static inline bool kvm_memslot_userfault(struct kvm_memory_slot *memslot)
+{
+	return memslot->flags & KVM_MEM_USERFAULT;
+}
+
 #endif
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 502ea63b5d2e..94be7e8b46a4 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -40,7 +40,8 @@  struct kvm_userspace_memory_region2 {
 	__u64 guest_memfd_offset;
 	__u32 guest_memfd;
 	__u32 pad1;
-	__u64 pad2[14];
+	__u64 userfault_bitmap;
+	__u64 pad2[13];
 };
 
 /*
@@ -51,6 +52,7 @@  struct kvm_userspace_memory_region2 {
 #define KVM_MEM_LOG_DIRTY_PAGES	(1UL << 0)
 #define KVM_MEM_READONLY	(1UL << 1)
 #define KVM_MEM_GUEST_MEMFD	(1UL << 2)
+#define KVM_MEM_USERFAULT	(1UL << 3)
 
 /* for KVM_IRQ_LINE */
 struct kvm_irq_level {
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index 54e959e7d68f..9eb1fae238b1 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -124,3 +124,6 @@  config HAVE_KVM_ARCH_GMEM_PREPARE
 config HAVE_KVM_ARCH_GMEM_INVALIDATE
        bool
        depends on KVM_PRIVATE_MEM
+
+config HAVE_KVM_USERFAULT
+       bool
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index de2c11dae231..23fa3e911c4e 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1541,6 +1541,9 @@  static int check_memory_region_flags(struct kvm *kvm,
 	    !(mem->flags & KVM_MEM_GUEST_MEMFD))
 		valid_flags |= KVM_MEM_READONLY;
 
+	if (kvm_has_userfault(kvm))
+		valid_flags |= KVM_MEM_USERFAULT;
+
 	if (mem->flags & ~valid_flags)
 		return -EINVAL;
 
@@ -2042,6 +2045,8 @@  int __kvm_set_memory_region(struct kvm *kvm,
 		if (r)
 			goto out;
 	}
+	if (mem->flags & KVM_MEM_USERFAULT)
+		new->userfault_bitmap = (unsigned long *)mem->userfault_bitmap;
 
 	r = kvm_set_memslot(kvm, old, new, change);
 	if (r)
@@ -6426,3 +6431,26 @@  void kvm_exit(void)
 	kvm_irqfd_exit();
 }
 EXPORT_SYMBOL_GPL(kvm_exit);
+
+int kvm_gfn_userfault(struct kvm *kvm, struct kvm_memory_slot *memslot,
+		       gfn_t gfn)
+{
+	unsigned long bitmap_chunk = 0;
+	off_t offset;
+
+	if (!kvm_memslot_userfault(memslot))
+		return 0;
+
+	if (WARN_ON_ONCE(!memslot->userfault_bitmap))
+		return 0;
+
+	offset = gfn - memslot->base_gfn;
+
+	if (copy_from_user(&bitmap_chunk,
+			   memslot->userfault_bitmap + offset / BITS_PER_LONG,
+			   sizeof(bitmap_chunk)))
+		return -EFAULT;
+
+	/* Set in the bitmap means that the gfn is userfault */
+	return !!(bitmap_chunk & (1ul << (offset % BITS_PER_LONG)));
+}