Re: [PATCH 8/8] xen/hvc: replace BUG_ON() with negative return value

On 13.05.21 12:25, Greg Kroah-Hartman wrote:

On Thu, May 13, 2021 at 12:03:02PM +0200, Juergen Gross wrote:

quoted

Xen frontends shouldn't BUG() in case of illegal data received from
their backends. So replace the BUG_ON()s when reading illegal data from
the ring page with negative return values.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
  drivers/tty/hvc/hvc_xen.c | 15 +++++++++++++--
  1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
index 92c9a476defc..30d7ffb1e04c 100644
--- a/drivers/tty/hvc/hvc_xen.c
+++ b/drivers/tty/hvc/hvc_xen.c

@@ -86,6 +86,11 @@ static int __write_console(struct xencons_info *xencons,
  	cons = intf->out_cons;
  	prod = intf->out_prod;
  	mb();			/* update queue values before going on */
+
+	if (WARN_ONCE((prod - cons) > sizeof(intf->out),
+		      "Illegal ring page indices"))
+		return -EINVAL;

How nice, you just rebooted on panic-on-warn systems :(

quoted

+
  	BUG_ON((prod - cons) > sizeof(intf->out));

Why keep this line?

Failed to delete it, sorry.

Please just fix this up properly, if userspace can trigger this, then
both the WARN_ON() and BUG_ON() are not correct and need to be correctly
handled.

It can be triggered by the console backend, but I agree a WARN isn't the
way to go here.


Juergen