VMCI: fix race between vmci_host_setup_notify and vmci_ctx_unset_notify

During our test, it is found that a warning can be trigger in try_grab_folio
as follow:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 1678 at mm/gup.c:147 try_grab_folio+0x106/0x130
Modules linked in:
CPU: 0 UID: 0 PID: 1678 Comm: syz.3.31 Not tainted 6.15.0-rc5 #163 PREEMPT(undef)
RIP: 0010:try_grab_folio+0x106/0x130
Call Trace:
<TASK>
follow_huge_pmd+0x240/0x8e0
follow_pmd_mask.constprop.0.isra.0+0x40b/0x5c0
follow_pud_mask.constprop.0.isra.0+0x14a/0x170
follow_page_mask+0x1c2/0x1f0
__get_user_pages+0x176/0x950
__gup_longterm_locked+0x15b/0x1060
? gup_fast+0x120/0x1f0
gup_fast_fallback+0x17e/0x230
get_user_pages_fast+0x5f/0x80
vmci_host_unlocked_ioctl+0x21c/0xf80
RIP: 0033:0x54d2cd
---[ end trace 0000000000000000 ]---

Digging into the source, context->notify_page may init by get_user_pages_fast
and can be seen in vmci_ctx_unset_notify which will try to put_page. However
get_user_pages_fast is not finished here and lead to following
try_grab_folio warning. The race condition is shown as follow:

cpu0 cpu1
vmci_host_do_set_notify
vmci_host_setup_notify
get_user_pages_fast(uva, 1, FOLL_WRITE, &context->notify_page);
lockless_pages_from_mm
gup_pgd_range
gup_huge_pmd // update &context->notify_page
vmci_host_do_set_notify
vmci_ctx_unset_notify
notify_page = context->notify_page;
if (notify_page)
put_page(notify_page); // page is freed
__gup_longterm_locked
__get_user_pages
follow_trans_huge_pmd
try_grab_folio // warn here

To slove this, use local variable page to make notify_page can be seen
after finish get_user_pages_fast.

Fixes: a1d88436d53a ("VMCI: Fix two UVA mapping bugs")
Cc: stable <[e-mail]>
Closes: https://lore.kernel.org/all/[e-mail]/
Signed-off-by: Wupeng Ma <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/vmw_vmci/vmci_host.c

···
 static int vmci_host_setup_notify(struct vmci_ctx *context,
 				  unsigned long uva)
 {
+	struct page *page;
 	int retval;
 
 	if (context->notify_page) {
···
 	/*
 	 * Lock physical page backing a given user VA.
 	 */
-	retval = get_user_pages_fast(uva, 1, FOLL_WRITE, &context->notify_page);
-	if (retval != 1) {
-		context->notify_page = NULL;
+	retval = get_user_pages_fast(uva, 1, FOLL_WRITE, &page);
+	if (retval != 1)
 		return VMCI_ERROR_GENERIC;
-	}
-	if (context->notify_page == NULL)
-		return VMCI_ERROR_UNAVAILABLE;
+
+	context->notify_page = page;
 
 	/*
 	 * Map the locked page and set up notify pointer.