io_uring/memmap: move vm_flags_set() outside of ctx->mmap_lock

Commit Message

Jens Axboe March 13, 2025, 11:44 a.m. UTC

syzbot complaints that sometimes the ordering between ctx->mmap_lock
and vma->vm_lock->lock aren't the same, which obviously makes lockdep
unhappy.

We'd normally expect the ctx->mmap_lock to nest inside the vma lock,
but vm_flags_set() -> vma_start_write() from the ->mmap() patch can
happen in the opposite order.

Move the vm_flags_set() to before ctx->mmap_lock is grabbed. This does
potentially leak the VM_DONT_EXPAND set for that vma, and while that
could get cleared, doesn't look like it's necessary to do so. Hence just
keep it simple.

Reported-by: [e-mail]
Link: https://lore.kernel.org/io-uring/[e-mail]/
Signed-off-by: Jens Axboe <[e-mail]>

---

Patch

diff --git a/io_uring/memmap.c b/io_uring/memmap.c
index 361134544427..d325b6ab6b99 100644
--- a/io_uring/memmap.c
+++ b/io_uring/memmap.c
@@ -309,7 +309,6 @@  static int io_region_mmap(struct io_ring_ctx *ctx,
 {
 	unsigned long nr_pages = min(mr->nr_pages, max_pages);
 
-	vm_flags_set(vma, VM_DONTEXPAND);
 	return vm_insert_pages(vma, vma->vm_start, mr->pages, &nr_pages);
 }
 
@@ -322,6 +321,8 @@  __cold int io_uring_mmap(struct file *file, struct vm_area_struct *vma)
 	struct io_mapped_region *region;
 	void *ptr;
 
+	vm_flags_set(vma, VM_DONTEXPAND);
+
 	guard(mutex)(&ctx->mmap_lock);
 
 	ptr = io_uring_validate_mmap_request(file, vma->vm_pgoff, sz);