[PATCH v2 1/3] brd: protect page with rcu

[Yu Kuai <yukuai1@xxxxxxxxxxxxxxx>]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

Currently, after fetching the page by xa_load() in IO path, there is no
protection and page can be freed concurrently by discard:

cpu0
brd_submit_bio
 brd_do_bvec
  page = brd_lookup_page
                          cpu1
                          brd_submit_bio
                           brd_do_discard
                            page = __xa_erase()
                            __free_page()
  // page UAF

Fix the problem by protecting page with rcu.

Meanwhile, if page is already freed, also prevent BUG_ON() by skipping
the write, and user will get zero data later if there is no page.

Fixes: 9ead7efc6f3f ("brd: implement discard support")
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 drivers/block/brd.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index 79e96221f887..98c9297beafe 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -132,23 +132,32 @@ static bool brd_rw_bvec(struct brd_device *brd, struct bio *bio)
 		}
 	}
 
+	rcu_read_lock();
 	page = brd_lookup_page(brd, sector);
 
 	kaddr = bvec_kmap_local(&bv);
 	if (op_is_write(opf)) {
-		BUG_ON(!page);
-		memcpy_to_page(page, offset, kaddr, bv.bv_len);
+		if (page)
+			memcpy_to_page(page, offset, kaddr, bv.bv_len);
 	} else if (page) {
 		memcpy_from_page(kaddr, page, offset, bv.bv_len);
 	} else {
 		memset(kaddr, 0, bv.bv_len);
 	}
 	kunmap_local(kaddr);
+	rcu_read_unlock();
 
 	bio_advance_iter_single(bio, &bio->bi_iter, bv.bv_len);
 	return true;
 }
 
+static void brd_free_one_page(struct rcu_head *head)
+{
+	struct page *page = container_of(head, struct page, rcu_head);
+
+	__free_page(page);
+}
+
 static void brd_do_discard(struct brd_device *brd, sector_t sector, u32 size)
 {
 	sector_t aligned_sector = (sector + PAGE_SECTORS) & ~PAGE_SECTORS;
@@ -159,7 +168,7 @@ static void brd_do_discard(struct brd_device *brd, sector_t sector, u32 size)
 	while (size >= PAGE_SIZE && aligned_sector < rd_size * 2) {
 		page = __xa_erase(&brd->brd_pages, aligned_sector >> PAGE_SECTORS_SHIFT);
 		if (page) {
-			__free_page(page);
+			call_rcu(&page->rcu_head, brd_free_one_page);
 			brd->brd_nr_pages--;
 		}
 		aligned_sector += PAGE_SECTORS;
-- 
2.39.2