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SubjectRe: [PATCH] fix unbalanced page refcounting in bio_map_user_iov
On Sat, Sep 23, 2017 at 05:55:37PM +0100, Al Viro wrote:

> IOW, the loop on failure exit should go through the bio, like __bio_unmap_user()
> does. We *also* need to put everything left unused in pages[], but only from the
> last iteration through iov_for_each().
>
> Frankly, I would prefer to reuse the pages[], rather than append to it on each
> iteration. Used iov_iter_get_pages_alloc(), actually.

Something like completely untested diff below, perhaps...

diff --git a/block/bio.c b/block/bio.c
index b38e962fa83e..b5fe23597b41 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1323,94 +1323,60 @@ struct bio *bio_map_user_iov(struct request_queue *q,
const struct iov_iter *iter,
gfp_t gfp_mask)
{
- int j;
- int nr_pages = 0;
- struct page **pages;
struct bio *bio;
- int cur_page = 0;
- int ret, offset;
+ struct bio_vec *bvec;
struct iov_iter i;
- struct iovec iov;
-
- iov_for_each(iov, i, *iter) {
- unsigned long uaddr = (unsigned long) iov.iov_base;
- unsigned long len = iov.iov_len;
- unsigned long end = (uaddr + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
- unsigned long start = uaddr >> PAGE_SHIFT;
-
- /*
- * Overflow, abort
- */
- if (end < start)
- return ERR_PTR(-EINVAL);
-
- nr_pages += end - start;
- /*
- * buffer must be aligned to at least logical block size for now
- */
- if (uaddr & queue_dma_alignment(q))
- return ERR_PTR(-EINVAL);
- }
+ int ret, j;

- if (!nr_pages)
+ if (!iov_iter_count(iter))
return ERR_PTR(-EINVAL);

- bio = bio_kmalloc(gfp_mask, nr_pages);
+ bio = bio_kmalloc(gfp_mask, iov_iter_npages(iter, BIO_MAX_PAGES));
if (!bio)
return ERR_PTR(-ENOMEM);

- ret = -ENOMEM;
- pages = kcalloc(nr_pages, sizeof(struct page *), gfp_mask);
- if (!pages)
- goto out;
+ i = *iter;
+ while (iov_iter_count(&i)) {
+ struct page **pages;
+ size_t offs;
+ ssize_t bytes;
+ int npages, j;

- iov_for_each(iov, i, *iter) {
- unsigned long uaddr = (unsigned long) iov.iov_base;
- unsigned long len = iov.iov_len;
- unsigned long end = (uaddr + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
- unsigned long start = uaddr >> PAGE_SHIFT;
- const int local_nr_pages = end - start;
- const int page_limit = cur_page + local_nr_pages;
-
- ret = get_user_pages_fast(uaddr, local_nr_pages,
- (iter->type & WRITE) != WRITE,
- &pages[cur_page]);
- if (ret < local_nr_pages) {
- ret = -EFAULT;
+ bytes = iov_iter_get_pages_alloc(&i, &pages, LONG_MAX, &offs);
+ if (bytes <= 0) {
+ ret = bytes ? bytes : -EFAULT;
goto out_unmap;
}

- offset = offset_in_page(uaddr);
- for (j = cur_page; j < page_limit; j++) {
- unsigned int bytes = PAGE_SIZE - offset;
+ npages = DIV_ROUND_UP(offs + bytes, PAGE_SIZE);

- if (len <= 0)
- break;
+ if (unlikely(offs & queue_dma_alignment(q))) {
+ ret = -EINVAL;
+ j = 0;
+ } else {
+ for (j = 0; j < npages; j++) {
+ struct page *page = pages[j];
+ unsigned n = PAGE_SIZE - offs;
+ unsigned prev_bi_vcnt = bio->bi_vcnt;
+
+ if (!bio_add_pc_page(q, bio, page, n, offs)) {
+ iov_iter_truncate(&i, 0);
+ break;
+ }
+
+ if (bio->bi_vcnt == prev_bi_vcnt)
+ put_page(page);

- if (bytes > len)
- bytes = len;
-
- /*
- * sorry...
- */
- if (bio_add_pc_page(q, bio, pages[j], bytes, offset) <
- bytes)
- break;
-
- len -= bytes;
- offset = 0;
+ iov_iter_advance(&i, n);
+ bytes -= n;
+ offs = 0;
+ }
}
-
- cur_page = j;
- /*
- * release the pages we didn't map into the bio, if any
- */
- while (j < page_limit)
+ while (j < npages)
put_page(pages[j++]);
+ kvfree(pages);
}

- kfree(pages);
-
bio_set_flag(bio, BIO_USER_MAPPED);

/*
@@ -1423,13 +1389,9 @@ struct bio *bio_map_user_iov(struct request_queue *q,
return bio;

out_unmap:
- for (j = 0; j < nr_pages; j++) {
- if (!pages[j])
- break;
- put_page(pages[j]);
+ bio_for_each_segment_all(bvec, bio, j) {
+ put_page(bvec->bv_page);
}
- out:
- kfree(pages);
bio_put(bio);
return ERR_PTR(ret);
}
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