FAILED: patch "[PATCH] io_uring: reexpand under-reexpanded iters" failed to apply to 5.14-stable tree

The patch below does not apply to the 5.14-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 89c2b3b74918200e46699338d7bcc19b1ea12110 Mon Sep 17 00:00:00 2001
From: Pavel Begunkov <[email protected]>
Date: Mon, 23 Aug 2021 11:18:45 +0100
Subject: [PATCH] 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.

index d94fb5835a20..71639d39caf9 100644
@@ -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 @@ static int io_write(struct io_kiocb *req, unsigned int issue_flags)
 	} 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;