io_uring: reexpand under-reexpanded iters

[   74.211232] BUG: KASAN: stack-out-of-bounds in iov_iter_revert+0x809/0x900
[   74.212778] Read of size 8 at addr ffff888025dc78b8 by task
syz-executor.0/828
[   74.214756] CPU: 0 PID: 828 Comm: syz-executor.0 Not tainted
5.14.0-rc3-next-20210730 #1
[   74.216525] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[   74.219033] Call Trace:
[   74.219683]  dump_stack_lvl+0x8b/0xb3
[   74.220706]  print_address_description.constprop.0+0x1f/0x140
[   74.224226]  kasan_report.cold+0x7f/0x11b
[   74.226085]  iov_iter_revert+0x809/0x900
[   74.227960]  io_write+0x57d/0xe40
[   74.232647]  io_issue_sqe+0x4da/0x6a80
[   74.242578]  __io_queue_sqe+0x1ac/0xe60
[   74.245358]  io_submit_sqes+0x3f6e/0x76a0
[   74.248207]  __do_sys_io_uring_enter+0x90c/0x1a20
[   74.257167]  do_syscall_64+0x3b/0x90
[   74.257984]  entry_SYSCALL_64_after_hwframe+0x44/0xae

old_size = iov_iter_count();
...
iov_iter_revert(old_size - iov_iter_count());

If iov_iter_revert() is done base on the initial size as above, and the
iter is truncated and not reexpanded in the middle, it miscalculates
borders causing problems. This trace is due to no one reexpanding after
generic_write_checks().

Now iters store how many bytes has been truncated, so reexpand them to
the initial state right before reverting.

Cc: stable@vger.kernel.org
Reported-by: Palash Oswal <oswalpalash@gmail.com>
Reported-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Reported-and-tested-by: syzbot+9671693590ef5aad8953@syzkaller.appspotmail.com
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index d94fb5835a20e8320e0af56c9fb120425bb7e614..71639d39caf983ad62d17114b193b675ce9bf324 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3281,6 +3281,7 @@ static int io_read(struct io_kiocb *req, unsigned int issue_flags)
                if (req->flags & REQ_F_NOWAIT)
                        goto done;
                /* some cases will consume bytes even on error returns */
+               iov_iter_reexpand(iter, iter->count + iter->truncated);
                iov_iter_revert(iter, io_size - iov_iter_count(iter));
                ret = 0;
        } else if (ret == -EIOCBQUEUED) {
@@ -3420,6 +3421,7 @@ done:
        } else {
 copy_iov:
                /* some cases will consume bytes even on error returns */
+               iov_iter_reexpand(iter, iter->count + iter->truncated);
                iov_iter_revert(iter, io_size - iov_iter_count(iter));
                ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
                return ret ?: -EAGAIN;