bpf: Take mmap_lock in zap_pages()

zap_vma_range() requires the owning mm's mmap_lock to be held.

Taking mmap_read_lock under arena->lock would AB-BA against
arena_vm_close() and arena_map_mmap(), both of which run with
mmap_write_lock held and then acquire arena->lock. Instead drop
arena->lock, mmget_not_zero() the vma's mm, take mmap_read_lock, and
re-resolve the vma via find_vma() since it may have been unmapped or
replaced while waiting.

Track processed vmls with a per-call generation in vml->zap_gen and
serialize zap_pages() callers with a new arena->zap_mutex so
concurrent callers on different uaddr ranges do not mark each other's
vmls processed before the zap is done.

Reported-by: David Hildenbrand <david@kernel.org>
Fixes: 317460317a02 ("bpf: Introduce bpf_arena.")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260528222014.38980-1-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 57 insertions, 4 deletions

diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index 1727503b25d8..9b2dea229b38 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -60,6 +60,8 @@ struct bpf_arena {
 	struct list_head vma_list;
 	/* protects vma_list */
 	struct mutex lock;
+	u64 zap_gen;
+	struct mutex zap_mutex;
 	struct irq_work     free_irq;
 	struct work_struct  free_work;
 	struct llist_head   free_spans;
@@ -289,6 +291,7 @@ static struct bpf_map *arena_map_alloc(union bpf_attr *attr)
 	if (err)
 		goto err_free_scratch;
 	mutex_init(&arena->lock);
+	mutex_init(&arena->zap_mutex);
 	raw_res_spin_lock_init(&arena->spinlock);
 	err = populate_pgtable_except_pte(arena);
 	if (err)
@@ -391,6 +394,7 @@ struct vma_list {
 	struct vm_area_struct *vma;
 	struct list_head head;
 	refcount_t mmap_count;
+	u64 zap_gen;
 };
 
 static int remember_vma(struct bpf_arena *arena, struct vm_area_struct *vma)
@@ -403,6 +407,7 @@ static int remember_vma(struct bpf_arena *arena, struct vm_area_struct *vma)
 	refcount_set(&vml->mmap_count, 1);
 	vma->vm_private_data = vml;
 	vml->vma = vma;
+	vml->zap_gen = 0;
 	list_add(&vml->head, &arena->vma_list);
 	return 0;
 }
@@ -746,12 +751,60 @@ out_free_pages:
  */
 static void zap_pages(struct bpf_arena *arena, long uaddr, long page_cnt)
 {
+	unsigned long size = (unsigned long)page_cnt << PAGE_SHIFT;
+	struct vm_area_struct *vma;
+	struct mm_struct *mm;
 	struct vma_list *vml;
+	unsigned long vm_start;
+	u64 my_gen;
 
-	guard(mutex)(&arena->lock);
-	/* iterate link list under lock */
-	list_for_each_entry(vml, &arena->vma_list, head)
-		zap_vma_range(vml->vma, uaddr, PAGE_SIZE * page_cnt);
+	/*
+	 * Taking mmap_read_lock() under arena->lock would deadlock against
+	 * arena_vm_close(), which runs with mmap_write_lock held and then
+	 * acquires arena->lock. Drop arena->lock for mmap_read_lock().
+	 *
+	 * Use per-call my_gen, recorded in vml->zap_gen, to remember which
+	 * vmls this invocation has already processed across the lock drop.
+	 * Hold zap_mutex around the whole walk so concurrent zap_pages()
+	 * callers cannot overwrite each other's marks on shared vmls --
+	 * otherwise call B's mark would make call A skip a vml that A has
+	 * not yet zapped for A's uaddr range.
+	 */
+	mutex_lock(&arena->zap_mutex);
+	mutex_lock(&arena->lock);
+	my_gen = ++arena->zap_gen;
+	for (;;) {
+		mm = NULL;
+		list_for_each_entry(vml, &arena->vma_list, head) {
+			if (vml->zap_gen >= my_gen)
+				continue;
+			vml->zap_gen = my_gen;
+			if (!mmget_not_zero(vml->vma->vm_mm))
+				continue;
+			mm = vml->vma->vm_mm;
+			vm_start = vml->vma->vm_start;
+			break;
+		}
+		if (!mm)
+			break;
+		mutex_unlock(&arena->lock);
+
+		mmap_read_lock(mm);
+		/*
+		 * Re-resolve: while we waited the VMA could have been unmapped
+		 * and a different mapping installed at the same address.
+		 */
+		vma = find_vma(mm, vm_start);
+		if (vma && vma->vm_start == vm_start &&
+		    vma->vm_file && vma->vm_file->private_data == &arena->map)
+			zap_vma_range(vma, uaddr, size);
+		mmap_read_unlock(mm);
+		mmput(mm);
+
+		mutex_lock(&arena->lock);
+	}
+	mutex_unlock(&arena->lock);
+	mutex_unlock(&arena->zap_mutex);
 }
 
 static void arena_free_pages(struct bpf_arena *arena, long uaddr, long page_cnt, bool sleepable)