[PATCH bpf-next v3 2/4] bpf: arena: allocate the fault-in page outside the lock

[Jiayuan Chen <jiayuan.chen@linux.dev>]

arena_vm_fault() allocated the page while holding arena->spinlock, so it
could only use the non-blocking allocator. Once the memcg is at
memory.max that allocation just fails, the fault turns into
VM_FAULT_SIGSEGV, and the process gets a SIGSEGV on a perfectly valid
arena address. Hitting memory.max is routine (e.g. page cache from
reading a big file), so this kills innocent processes.

Rework the fault handler:

- Preallocate the page before taking the lock, like do_anonymous_page()
  does, so it can sleep, reclaim and go through the OOM path, and return
  VM_FAULT_OOM on failure so the memcg OOM handler runs instead of a fake
  segfault.

- A lockless probe skips that preallocation when a page is already mapped
  (e.g. allocated by the bpf program), so the common case wastes no
  allocation. The rare race where such a page is freed before we take the
  lock falls back to the non-blocking allocator under the lock.

- Return VM_FAULT_SIGBUS for the non-recoverable errors (lock failure,
  range-tree and page-table failures) instead of VM_FAULT_SIGSEGV; only
  BPF_F_SEGV_ON_FAULT, and a scratch-page hole under that flag, is a real
  user addressing error and keeps VM_FAULT_SIGSEGV.

- Tidy up the error labels.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 kernel/bpf/arena.c | 92 +++++++++++++++++++++++++++++++++++-----------
 1 file changed, 71 insertions(+), 21 deletions(-)

diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index 555ee2531ef9..09a718ca4c8b 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -481,7 +481,8 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf)
 	struct bpf_map *map = vmf->vma->vm_file->private_data;
 	struct bpf_arena *arena = container_of(map, struct bpf_arena, map);
 	struct mem_cgroup *new_memcg, *old_memcg;
-	struct page *page;
+	struct page *page, *new_page = NULL;
+	vm_fault_t fault_ret;
 	long kbase, kaddr;
 	unsigned long flags;
 	int ret;
@@ -489,59 +490,108 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf)
 	kbase = bpf_arena_get_kern_vm_start(arena);
 	kaddr = kbase + (u32)(vmf->address);
 
-	if (raw_res_spin_lock_irqsave(&arena->spinlock, flags))
+	page = vmalloc_to_page((void *)kaddr);
+	if (!page && !(arena->map.map_flags & BPF_F_SEGV_ON_FAULT)) {
+		/*
+		 * We run in process context here, so preallocate the page
+		 * outside the lock with an explicitly sleepable allocator. It
+		 * can then go through reclaim (both memcg and global) and the
+		 * OOM path, the way do_anonymous_page() does; under
+		 * arena->spinlock only the non-blocking allocator is available,
+		 * which never reclaims. That also decides the return value:
+		 * VM_FAULT_OOM below is only meaningful if the OOM machinery was
+		 * actually engaged, which the non-blocking allocator never does.
+		 */
+		bpf_map_memcg_enter(&arena->map, &old_memcg, &new_memcg);
+		new_page = bpf_map_alloc_page_sleepable(map);
+		bpf_map_memcg_exit(old_memcg, new_memcg);
+		if (!new_page)
+			return VM_FAULT_OOM;
+	}
+
+	if (raw_res_spin_lock_irqsave(&arena->spinlock, flags)) {
 		/*
 		 * A failed lock means a possible deadlock was detected. Don't
 		 * return VM_FAULT_RETRY: this handler never took mmap_lock, but
 		 * the fault path would re-take it on retry and deadlock. Fail.
 		 */
+		if (new_page)
+			free_pages_nolock(new_page, 0);
 		return VM_FAULT_SIGBUS;
+	}
 
 	page = vmalloc_to_page((void *)kaddr);
 	if (page) {
-		if (page == arena->scratch_page)
-			/* BPF triggered scratch here; don't lazy-alloc over it */
-			goto out_sigsegv;
+		if (page == arena->scratch_page) {
+			/*
+			 * A scratch page marks a hole. Segfault only if the user
+			 * asked for it; otherwise we could lazy-allocate but
+			 * choose not to over a hole, so report a bus error.
+			 */
+			fault_ret = (arena->map.map_flags & BPF_F_SEGV_ON_FAULT) ?
+				    VM_FAULT_SIGSEGV : VM_FAULT_SIGBUS;
+			goto out_err_locked;
+		}
 		/* already have a page vmap-ed */
 		goto out;
 	}
 
+	if (arena->map.map_flags & BPF_F_SEGV_ON_FAULT) {
+		/* User space requested to segfault when page is not allocated by bpf prog */
+		fault_ret = VM_FAULT_SIGSEGV;
+		goto out_err_locked;
+	}
+
 	bpf_map_memcg_enter(&arena->map, &old_memcg, &new_memcg);
 
-	if (arena->map.map_flags & BPF_F_SEGV_ON_FAULT)
-		/* User space requested to segfault when page is not allocated by bpf prog */
-		goto out_sigsegv_memcg;
+	if (!new_page) {
+		/*
+		 * Very rare race: the bpf program had allocated a page here, so
+		 * the lockless probe saw it and we skipped preallocation, but it
+		 * freed the page before we took the lock. Now we do need one;
+		 * sleeping is not allowed here, so fall back to the non-blocking
+		 * allocator and give up if it fails.
+		 */
+		ret = bpf_map_alloc_pages(map, map->numa_node, 1, &new_page);
+		if (ret) {
+			fault_ret = VM_FAULT_SIGBUS;
+			goto out_err_locked_memcg;
+		}
+	}
 
 	ret = range_tree_clear(&arena->rt, vmf->pgoff, 1);
-	if (ret)
-		goto out_sigsegv_memcg;
-
-	struct apply_range_data data = { .arena = arena, .pages = &page, .i = 0 };
-	/* Account into memcg of the process that created bpf_arena */
-	ret = bpf_map_alloc_pages(map, NUMA_NO_NODE, 1, &page);
 	if (ret) {
-		range_tree_set(&arena->rt, vmf->pgoff, 1);
-		goto out_sigsegv_memcg;
+		fault_ret = VM_FAULT_SIGBUS;
+		goto out_err_locked_memcg;
 	}
+	struct apply_range_data data = { .arena = arena, .pages = &new_page, .i = 0 };
 
 	ret = apply_to_page_range(&init_mm, kaddr, PAGE_SIZE, apply_range_set_cb, &data);
 	if (ret) {
 		range_tree_set(&arena->rt, vmf->pgoff, 1);
-		free_pages_nolock(page, 0);
-		goto out_sigsegv_memcg;
+		fault_ret = VM_FAULT_SIGBUS;
+		goto out_err_locked_memcg;
 	}
 	flush_vmap_cache(kaddr, PAGE_SIZE);
 	bpf_map_memcg_exit(old_memcg, new_memcg);
+	/* new_page was consumed */
+	page = new_page;
+	new_page = NULL;
 out:
 	page_ref_add(page, 1);
 	raw_res_spin_unlock_irqrestore(&arena->spinlock, flags);
+	if (new_page)
+		free_pages_nolock(new_page, 0);
 	vmf->page = page;
 	return 0;
-out_sigsegv_memcg:
+
+out_err_locked_memcg:
 	bpf_map_memcg_exit(old_memcg, new_memcg);
-out_sigsegv:
+out_err_locked:
 	raw_res_spin_unlock_irqrestore(&arena->spinlock, flags);
-	return VM_FAULT_SIGSEGV;
+	if (new_page)
+		free_pages_nolock(new_page, 0);
+	return fault_ret;
 }
 
 static const struct vm_operations_struct arena_vm_ops = {
-- 
2.43.0