x86/mm: Fix overflow in __cpa_addr()

The change to have cpa_flush() call flush_kernel_pages() introduced
a bug where __cpa_addr() can access an address one larger than the
largest one in the cpa->pages array.

KASAN reports the issue like this:

BUG: KASAN: slab-out-of-bounds in __cpa_addr arch/x86/mm/pat/set_memory.c:309 [inline]
BUG: KASAN: slab-out-of-bounds in __cpa_addr+0x1d3/0x220 arch/x86/mm/pat/set_memory.c:306
Read of size 8 at addr ffff88801f75e8f8 by task syz.0.17/5978

This bug could cause cpa_flush() to not properly flush memory,
which somehow never showed any symptoms in my tests, possibly
because cpa_flush() is called so rarely, but could potentially
cause issues for other people.

Fix the issue by directly calculating the flush end address
from the start address.

Fixes: 86e6815 ("x86/mm: Change cpa_flush() to call flush_kernel_range() directly")
Reported-by: syzbot+afec6555eef563c66c97@syzkaller.appspotmail.com
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kiryl Shutsemau <kas@kernel.org>
Link: https://lore.kernel.org/all/68e2ff90.050a0220.2c17c1.0038.GAE@google.com/

Author: rikvanriel

arch/x86/mm/pat/set_memory.c

@@ -446,7 +446,7 @@ static void cpa_flush(struct cpa_data *cpa, int cache)
 	}
 
 	start = fix_addr(__cpa_addr(cpa, 0));
-	end =   fix_addr(__cpa_addr(cpa, cpa->numpages));
+	end =   start + cpa->numpages * PAGE_SIZE;
 	if (cpa->force_flush_all)
 		end = TLB_FLUSH_ALL;