[PATCH] vhost-vdpa: fix use-after-free of v->config_ctx

[Tim Gardner]

From: Stefano Garzarella <sgarzare at redhat.com>

CVE-2021-29266

When the 'v->config_ctx' eventfd_ctx reference is released we didn't
set it to NULL. So if the same character device (e.g. /dev/vhost-vdpa-0)
is re-opened, the 'v->config_ctx' is invalid and calling again
vhost_vdpa_config_put() causes use-after-free issues like the
following refcount_t underflow:

    refcount_t: underflow; use-after-free.
    WARNING: CPU: 2 PID: 872 at lib/refcount.c:28 refcount_warn_saturate+0xae/0xf0
    RIP: 0010:refcount_warn_saturate+0xae/0xf0
    Call Trace:
     eventfd_ctx_put+0x5b/0x70
     vhost_vdpa_release+0xcd/0x150 [vhost_vdpa]
     __fput+0x8e/0x240
     ____fput+0xe/0x10
     task_work_run+0x66/0xa0
     exit_to_user_mode_prepare+0x118/0x120
     syscall_exit_to_user_mode+0x21/0x50
     ? __x64_sys_close+0x12/0x40
     do_syscall_64+0x45/0x50
     entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: 776f395004d8 ("vhost_vdpa: Support config interrupt in vdpa")
Cc: lingshan.zhu at intel.com
Cc: stable at vger.kernel.org
Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
Link: https://lore.kernel.org/r/20210311135257.109460-2-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Reviewed-by: Zhu Lingshan <lingshan.zhu at intel.com>
Acked-by: Jason Wang <jasowang at redhat.com>
(cherry picked from commit f6bbf0010ba004f5e90c7aefdebc0ee4bd3283b9)
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
---
 drivers/vhost/vdpa.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index 3b6d4e4fd295..d3221169d938 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -304,8 +304,10 @@ static long vhost_vdpa_get_vring_num(struct vhost_vdpa *v, u16 __user *argp)
 
 static void vhost_vdpa_config_put(struct vhost_vdpa *v)
 {
-	if (v->config_ctx)
+	if (v->config_ctx) {
 		eventfd_ctx_put(v->config_ctx);
+		v->config_ctx = NULL;
+	}
 }
 
 static long vhost_vdpa_set_config_call(struct vhost_vdpa *v, u32 __user *argp)
-- 
2.17.1