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brd: protect page with rcu

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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: 9ead7efc6f ("brd: implement discard support")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250506061756.2970934-2-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Yu Kuai
2025-05-06 07:42:27 -06:00
committed by Jens Axboe
parent e96ee7e1de
commit 0e8acffc1b
1 changed files with 17 additions and 3 deletions
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drivers/block/brd.c
+17 -3
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@@ -132,12 +132,18 @@ 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);
/*
* Page can be removed by concurrent discard, it's fine to skip
* the write and user will read zero data if page does not
* exist.
*/
if (page)
memcpy_to_page(page, offset, kaddr, bv.bv_len);
} else {
if (page)
memcpy_from_page(kaddr, page, offset, bv.bv_len);
@@ -145,11 +151,19 @@ static bool brd_rw_bvec(struct brd_device *brd, struct bio *bio)
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;
@@ -160,7 +174,7 @@ 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;