[PATCH v2] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

[Juergen Gross <jgross@suse.com>]

The error exit of privcmd_ioctl_dm_op() is calling unlock_pages()
potentially with pages being NULL, leading to a NULL dereference.

Additionally lock_pages() doesn't check for pin_user_pages_fast()
having been completely successful, resulting in potentially not
locking all pages into memory. This could result in sporadic failures
when using the related memory in user mode.

Fix all of that by calling unlock_pages() always with the real number
of pinned pages, which will be zero in case pages being NULL, and by
checking the number of patches pinned by pin_user_pages_fast()
matching the expected number of pages.

Cc: <stable@vger.kernel.org>
Fixes: ab520be8cd5d ("xen/privcmd: Add IOCTL_PRIVCMD_DM_OP")
Reported-by: Rustam Subkhankulov <subkhankulov@ispras.ru>
Signed-off-by: Juergen Gross <jgross@suse.com>
---
V2:
- use "pinned" as parameter for unlock_pages() (Jan Beulich)
- drop label "unlock" again (Jan Beulich)
- add check for complete success of pin_user_pages_fast()
---
 drivers/xen/privcmd.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index 3369734108af..7dc62510635e 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -602,6 +602,10 @@ static int lock_pages(
 		*pinned += page_count;
 		nr_pages -= page_count;
 		pages += page_count;
+
+		/* Exact reason isn't known, EFAULT is one possibility. */
+		if (page_count < requested)
+			return -EFAULT;
 	}
 
 	return 0;
@@ -677,10 +681,8 @@ 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;
+	if (rc < 0)
 		goto out;
-	}
 
 	for (i = 0; i < kdata.num; i++) {
 		set_xen_guest_handle(xbufs[i].h, kbufs[i].uptr);
@@ -692,7 +694,7 @@ static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
 	xen_preemptible_hcall_end();
 
 out:
-	unlock_pages(pages, nr_pages);
+	unlock_pages(pages, pinned);
 	kfree(xbufs);
 	kfree(pages);
 	kfree(kbufs);
-- 
2.35.3