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KVM: guest_memfd: Remove RCU-protected attribute from slot->gmem.file

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Remove the RCU-protected attribute from slot->gmem.file. No need to use RCU
primitives rcu_assign_pointer()/synchronize_rcu() to update this pointer.

- slot->gmem.file is updated in 3 places:
  kvm_gmem_bind(), kvm_gmem_unbind(), kvm_gmem_release().
  All of them are protected by kvm->slots_lock.

- slot->gmem.file is read in 2 paths:
  (1) kvm_gmem_populate
        kvm_gmem_get_file
        __kvm_gmem_get_pfn

  (2) kvm_gmem_get_pfn
         kvm_gmem_get_file
         __kvm_gmem_get_pfn

  Path (1) kvm_gmem_populate() requires holding kvm->slots_lock, so
  slot->gmem.file is protected by the kvm->slots_lock in this path.

  Path (2) kvm_gmem_get_pfn() does not require holding kvm->slots_lock.
  However, it's also not guarded by rcu_read_lock() and rcu_read_unlock().
  So synchronize_rcu() in kvm_gmem_unbind()/kvm_gmem_release() actually
  will not wait for the readers in kvm_gmem_get_pfn() due to lack of RCU
  read-side critical section.

  The path (2) kvm_gmem_get_pfn() is safe without RCU protection because:
  a) kvm_gmem_bind() is called on a new memslot, before the memslot is
     visible to kvm_gmem_get_pfn().
  b) kvm->srcu ensures that kvm_gmem_unbind() and freeing of a memslot
     occur after the memslot is no longer visible to kvm_gmem_get_pfn().
  c) get_file_active() ensures that kvm_gmem_get_pfn() will not access the
     stale file if kvm_gmem_release() sets it to NULL.  This is because if
     kvm_gmem_release() occurs before kvm_gmem_get_pfn(), get_file_active()
     will return NULL; if get_file_active() does not return NULL,
     kvm_gmem_release() should not occur until after kvm_gmem_get_pfn()
     releases the file reference.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Message-ID: <20241104084303.29909-1-yan.y.zhao@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Yan Zhao 2024-11-04 16:43:03 +08:00 committed by Paolo Bonzini
parent c50be1c945
commit 67b43038ce
2 changed files with 27 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
include/linux/kvm_host.h
View File

@ -596,7 +596,12 @@ struct kvm_memory_slot {
#ifdef CONFIG_KVM_PRIVATE_MEM
struct {
struct file __rcu *file;
/*
* Writes protected by kvm->slots_lock. Acquiring a
* reference via kvm_gmem_get_file() is protected by
* either kvm->slots_lock or kvm->srcu.
*/
struct file *file;
pgoff_t pgoff;
} gmem;
#endif

34
virt/kvm/guest_memfd.c
View File

@ -259,15 +259,19 @@ static int kvm_gmem_release(struct inode *inode, struct file *file)
* dereferencing the slot for existing bindings needs to be protected
* against memslot updates, specifically so that unbind doesn't race
* and free the memslot (kvm_gmem_get_file() will return NULL).
*
* Since .release is called only when the reference count is zero,
* after which file_ref_get() and get_file_active() fail,
* kvm_gmem_get_pfn() cannot be using the file concurrently.
* file_ref_put() provides a full barrier, and get_file_active() the
* matching acquire barrier.
*/
mutex_lock(&kvm->slots_lock);
filemap_invalidate_lock(inode->i_mapping);
xa_for_each(&gmem->bindings, index, slot)
rcu_assign_pointer(slot->gmem.file, NULL);
synchronize_rcu();
WRITE_ONCE(slot->gmem.file, NULL);
/*
* All in-flight operations are gone and new bindings can be created.
@ -296,8 +300,7 @@ static inline struct file *kvm_gmem_get_file(struct kvm_memory_slot *slot)
/*
* Do not return slot->gmem.file if it has already been closed;
* there might be some time between the last fput() and when
* kvm_gmem_release() clears slot->gmem.file, and you do not
* want to spin in the meanwhile.
* kvm_gmem_release() clears slot->gmem.file.
*/
return get_file_active(&slot->gmem.file);
}
@ -508,11 +511,11 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
}
/*
* No synchronize_rcu() needed, any in-flight readers are guaranteed to
* be see either a NULL file or this new file, no need for them to go
* away.
* memslots of flag KVM_MEM_GUEST_MEMFD are immutable to change, so
* kvm_gmem_bind() must occur on a new memslot. Because the memslot
* is not visible yet, kvm_gmem_get_pfn() is guaranteed to see the file.
*/
rcu_assign_pointer(slot->gmem.file, file);
WRITE_ONCE(slot->gmem.file, file);
slot->gmem.pgoff = start;
xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
@ -548,8 +551,12 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
filemap_invalidate_lock(file->f_mapping);
xa_store_range(&gmem->bindings, start, end - 1, NULL, GFP_KERNEL);
rcu_assign_pointer(slot->gmem.file, NULL);
synchronize_rcu();
/*
* synchronize_srcu(&kvm->srcu) ensured that kvm_gmem_get_pfn()
* cannot see this memslot.
*/
WRITE_ONCE(slot->gmem.file, NULL);
filemap_invalidate_unlock(file->f_mapping);
fput(file);
@ -561,11 +568,12 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file,
pgoff_t index, kvm_pfn_t *pfn,
bool *is_prepared, int *max_order)
{
struct file *gmem_file = READ_ONCE(slot->gmem.file);
struct kvm_gmem *gmem = file->private_data;
struct folio *folio;
if (file != slot->gmem.file) {
WARN_ON_ONCE(slot->gmem.file);
if (file != gmem_file) {
WARN_ON_ONCE(gmem_file);
return ERR_PTR(-EFAULT);
}