From: Yan Zhao yan.y.zhao@intel.com
[ Upstream commit 67b43038ce14d6b0673bdffb2052d879065c94ae ]
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 Stable-dep-of: ae431059e75d ("KVM: guest_memfd: Remove bindings on memslot deletion when gmem is dying") Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/kvm_host.h | 7 ++++++- virt/kvm/guest_memfd.c | 34 +++++++++++++++++++++------------- 2 files changed, 27 insertions(+), 14 deletions(-)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 2e836d44f7386..a2ebc37a29ff4 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -608,7 +608,12 @@ 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 diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index 73e5db8ef1611..343ed456b89b0 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -261,15 +261,19 @@ 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. @@ -298,8 +302,7 @@ 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); } @@ -510,11 +513,11 @@ 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); @@ -550,8 +553,12 @@ 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); @@ -563,11 +570,12 @@ 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); }