Re: [PATCH] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

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On 25.08.22 09:38, Jan Beulich wrote:
On 24.08.2022 16:26, Juergen Gross wrote:
The error exit of privcmd_ioctl_dm_op() is calling unlock_pages()
potentially with pages being NULL, leading to a NULL dereference.

Fix that by calling unlock_pages only if lock_pages() was at least
partially successful.

Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: ab520be8cd5d ("xen/privcmd: Add IOCTL_PRIVCMD_DM_OP")
Reported-by: Rustam Subkhankulov <subkhankulov@xxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit I wonder whether you did consider the variant actually
reducing code size (and avoiding the need for yet another label),
...

--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -679,7 +679,7 @@ static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
  	rc = lock_pages(kbufs, kdata.num, pages, nr_pages, &pinned);
  	if (rc < 0) {
  		nr_pages = pinned;

... dropping this line and ...

-		goto out;
+		goto unlock;
  	}
for (i = 0; i < kdata.num; i++) {
@@ -691,8 +691,9 @@ static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
  	rc = HYPERVISOR_dm_op(kdata.dom, kdata.num, xbufs);
  	xen_preemptible_hcall_end();
-out:
+ unlock:
  	unlock_pages(pages, nr_pages);

... passing "pinned" here.

Looking into this I found another problem: NOT using pinned is wrong, as
lock_pages() doesn't guarantee that all pages were really locked. I think
lock_pages() should return an error, in case pin_user_pages_fast() didn't
lock as many pages es expected.


Juergen

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