aio: fix race in ring buffer page lookup introduced by page migration support (d6c355c7) · Commits · 戴 / test

Prior to the introduction of page migration support in "fs/aio: Add support to aio ring pages migration" / , mapping of the ring buffer pages was done via get_user_pages() while retaining mmap_sem held for write. This avoided possible races with userland racing an munmap() or mremap(). The page migration patch, however, switched to using mm_populate() to prime the page mapping. mm_populate() cannot be called with mmap_sem held. Instead of dropping the mmap_sem, revert to the old behaviour and simply drop the use of mm_populate() since get_user_pages() will cause the pages to get mapped anyways. Thanks to Al Viro for spotting this issue. Signed-off-by:

Benjamin LaHaise <bcrl@kvack.org>

fs/aio.c

+12 −3

Original line number	Diff line number	Diff line
		@@ -307,16 +307,25 @@ static int aio_setup_ring(struct kioctx *ctx)
		aio_free_ring(ctx);
		return -EAGAIN;
		}
		up_write(&mm->mmap_sem);

		mm_populate(ctx->mmap_base, populate);

		pr_debug("mmap address: 0x%08lx\n", ctx->mmap_base);

		/* We must do this while still holding mmap_sem for write, as we
		* need to be protected against userspace attempting to mremap()
		* or munmap() the ring buffer.
		*/
		ctx->nr_pages = get_user_pages(current, mm, ctx->mmap_base, nr_pages,
		1, 0, ctx->ring_pages, NULL);

		/* Dropping the reference here is safe as the page cache will hold
		* onto the pages for us. It is also required so that page migration
		* can unmap the pages and get the right reference count.
		*/
		for (i = 0; i < ctx->nr_pages; i++)
		put_page(ctx->ring_pages[i]);

		up_write(&mm->mmap_sem);

		if (unlikely(ctx->nr_pages != nr_pages)) {
		aio_free_ring(ctx);
		return -EAGAIN;

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