[PATCH v14 01/12] splice: Fix O_DIRECT file read splice to avoid reversion of ITER_PIPE

[David Howells]

Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:

> Please don't use alloc_pages_bulk_list().
...
> If you have that array, you can then use release_pages() ...

Done.  See attached replacement patch.

David
---
splice: Fix O_DIRECT file read splice to avoid reversion of ITER_PIPE

With the upcoming iov_iter_extract_pages() function, pages extracted from a
non-user-backed iterator such as ITER_PIPE aren't pinned.
__iomap_dio_rw(), however, calls iov_iter_revert() to shorten the iterator
to just the bufferage it is going to use - which has the side-effect of
freeing the excess pipe buffers, even though they're attached to a bio and
may get written to by DMA (thanks to Hillf Danton for spotting this[1]).

This then causes memory corruption that is particularly noticable when the
syzbot test[2] is run.  The test boils down to:

        out = creat(argv[1], 0666);
        ftruncate(out, 0x800);
        lseek(out, 0x200, SEEK_SET);
        in = open(argv[1], O_RDONLY | O_DIRECT | O_NOFOLLOW);
        sendfile(out, in, NULL, 0x1dd00);

run repeatedly in parallel.  What I think is happening is that ftruncate()
occasionally shortens the DIO read that's about to be made by sendfile's
splice core by reducing i_size.

Fix this by splitting the handling of a splice from an O_DIRECT file fd off
from that of non-DIO and in this case, replacing the use of an ITER_PIPE
iterator with an ITER_BVEC iterator for which reversion won't free the
buffers.  The DIO-specific code bulk allocates all the buffers it thinks it
is going to use in advance, does the read synchronously and only then trims
the buffer down.  The pages we did use get pushed into the pipe.

This should be more efficient for DIO read by virtue of doing a bulk page
allocation, but slightly less efficient by ignoring any partial page in the
pipe.

Fixes: 920756a3306a ("block: Convert bio_iov_iter_get_pages to use iov_iter_extract_pages")
Reported-by: syzbot+a440341a59e3b7142895@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Jens Axboe <axboe@xxxxxxxxx>
cc: Christoph Hellwig <hch@xxxxxx>
cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
cc: David Hildenbrand <david@xxxxxxxxxx>
cc: John Hubbard <jhubbard@xxxxxxxxxx>
cc: linux-mm@xxxxxxxxx
cc: linux-block@xxxxxxxxxxxxxxx
cc: linux-fsdevel@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20230207094731.1390-1-hdanton@xxxxxxxx/ [1]
Link: https://lore.kernel.org/r/000000000000b0b3c005f3a09383@xxxxxxxxxx/ [2]
---
Notes:
    ver #14)
     - Use alloc_pages_bulk_array() rather than alloc_pages_bulk_list().
     - Use release_pages() rather than a loop calling __free_page().
    
    ver #13)
     - Don't completely replace generic_file_splice_read(), but rather only use
       this if we're doing a splicing from an O_DIRECT file fd.

 fs/splice.c |   98 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 98 insertions(+)

diff --git a/fs/splice.c b/fs/splice.c
index 5969b7a1d353..91244270b36e 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -282,6 +282,101 @@ void splice_shrink_spd(struct splice_pipe_desc *spd)
 	kfree(spd->partial);
 }
 
+/*
+ * Splice data from an O_DIRECT file into pages and then add them to the output
+ * pipe.
+ */
+static ssize_t generic_file_direct_splice_read(struct file *in, loff_t *ppos,
+					       struct pipe_inode_info *pipe,
+					       size_t len, unsigned int flags)
+{
+	struct iov_iter to;
+	struct bio_vec *bv;
+	struct kiocb kiocb;
+	struct page **pages;
+	unsigned int head;
+	ssize_t ret;
+	size_t used, npages, chunk, remain, reclaim;
+	int i;
+
+	/* Work out how much data we can actually add into the pipe */
+	used = pipe_occupancy(pipe->head, pipe->tail);
+	npages = max_t(ssize_t, pipe->max_usage - used, 0);
+	len = min_t(size_t, len, npages * PAGE_SIZE);
+	npages = DIV_ROUND_UP(len, PAGE_SIZE);
+
+	bv = kzalloc(array_size(npages, sizeof(bv[0])) +
+		     array_size(npages, sizeof(struct page *)), GFP_KERNEL);
+	if (!bv)
+		return -ENOMEM;
+
+	pages = (void *)(bv + npages);
+	npages = alloc_pages_bulk_array(GFP_USER, npages, pages);
+	if (!npages) {
+		kfree(bv);
+		return -ENOMEM;
+	}
+
+	remain = len = min_t(size_t, len, npages * PAGE_SIZE);
+
+	for (i = 0; i < npages; i++) {
+		chunk = min_t(size_t, PAGE_SIZE, remain);
+		bv[i].bv_page = pages[i];
+		bv[i].bv_offset = 0;
+		bv[i].bv_len = chunk;
+		remain -= chunk;
+	}
+
+	/* Do the I/O */
+	iov_iter_bvec(&to, ITER_DEST, bv, npages, len);
+	init_sync_kiocb(&kiocb, in);
+	kiocb.ki_pos = *ppos;
+	ret = call_read_iter(in, &kiocb, &to);
+
+	reclaim = npages * PAGE_SIZE;
+	remain = 0;
+	if (ret > 0) {
+		reclaim -= ret;
+		remain = ret;
+		*ppos = kiocb.ki_pos;
+		file_accessed(in);
+	} else if (ret < 0) {
+		/*
+		 * callers of ->splice_read() expect -EAGAIN on
+		 * "can't put anything in there", rather than -EFAULT.
+		 */
+		if (ret == -EFAULT)
+			ret = -EAGAIN;
+	}
+
+	/* Free any pages that didn't get touched at all. */
+	reclaim /= PAGE_SIZE;
+	if (reclaim) {
+		npages -= reclaim;
+		release_pages(pages + npages, reclaim);
+	}
+
+	/* Push the remaining pages into the pipe. */
+	head = pipe->head;
+	for (i = 0; i < npages; i++) {
+		struct pipe_buffer *buf = &pipe->bufs[head & (pipe->ring_size - 1)];
+
+		chunk = min_t(size_t, remain, PAGE_SIZE);
+		*buf = (struct pipe_buffer) {
+			.ops	= &default_pipe_buf_ops,
+			.page	= bv[i].bv_page,
+			.offset	= 0,
+			.len	= chunk,
+		};
+		head++;
+		remain -= chunk;
+	}
+	pipe->head = head;
+
+	kfree(bv);
+	return ret;
+}
+
 /**
  * generic_file_splice_read - splice data from file to a pipe
  * @in:		file to splice from
@@ -303,6 +398,9 @@ ssize_t generic_file_splice_read(struct file *in, loff_t *ppos,
 	struct kiocb kiocb;
 	int ret;
 
+	if (in->f_flags & O_DIRECT)
+		return generic_file_direct_splice_read(in, ppos, pipe, len, flags);
+
 	iov_iter_pipe(&to, ITER_DEST, pipe, len);
 	init_sync_kiocb(&kiocb, in);
 	kiocb.ki_pos = *ppos;