[PATCH v2] io_uring: fix dead-hung for non-iter fixed rw

[Pavel Begunkov <asml.silence@gmail.com>]

Read/write requests to devices without implemented read/write_iter
using fixed buffers causes general protection fault, which totally
hangs a machine.

io_import_fixed() initialises iov_iter with bvec, but loop_rw_iter()
accesses it as iovec, so dereferencing random address.

kmap() page by page in this case

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
v2: use kmap

P.S. this one passes all tests well

 fs/io_uring.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 8119cbae4fb6..1a9f34645586 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1613,9 +1613,19 @@ static ssize_t loop_rw_iter(int rw, struct file *file, struct kiocb *kiocb,
 		return -EAGAIN;
 
 	while (iov_iter_count(iter)) {
-		struct iovec iovec = iov_iter_iovec(iter);
+		struct iovec iovec;
 		ssize_t nr;
 
+		if (!iov_iter_is_bvec(iter)) {
+			iovec = iov_iter_iovec(iter);
+		} else {
+			/* fixed buffers import bvec */
+			iovec.iov_base = kmap(iter->bvec->bv_page)
+						+ iter->iov_offset;
+			iovec.iov_len = min(iter->count,
+					iter->bvec->bv_len - iter->iov_offset);
+		}
+
 		if (rw == READ) {
 			nr = file->f_op->read(file, iovec.iov_base,
 					      iovec.iov_len, &kiocb->ki_pos);
@@ -1624,6 +1634,9 @@ static ssize_t loop_rw_iter(int rw, struct file *file, struct kiocb *kiocb,
 					       iovec.iov_len, &kiocb->ki_pos);
 		}
 
+		if (iov_iter_is_bvec(iter))
+			kunmap(iter->bvec->bv_page);
+
 		if (nr < 0) {
 			if (!ret)
 				ret = nr;
-- 
2.24.0