[PATCH] x86/mm: fix overflow in __cpa_addr

[Rik van Riel <riel@surriel.com>]

On Sun, 05 Oct 2025 16:30:24 -0700
syzbot <syzbot+afec6555eef563c66c97@syzkaller.appspotmail.com> wrote:


> ==================================================================
> 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 should fix it, unless syzbot finds an unexpected new way to
torture things.

---8<---
From e32cd734444d9a95bb2d1067d934f33ab66f050b Mon Sep 17 00:00:00 2001
From: Rik van Riel <riel@surriel.com>
Date: Sun, 5 Oct 2025 23:32:48 -0400
Subject: [PATCH] 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.

Signed-off-by: Rik van Riel <riel@surriel.com>
Cc: stable@kernel.org
Reported-by: syzbot+afec6555eef563c66c97@syzkaller.appspotmail.com
Fixes: 86e6815b316e ("x86/mm: Change cpa_flush() to call flush_kernel_range() directly")
---
 arch/x86/mm/pat/set_memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index d2d54b8c4dbb..970981893c9b 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/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;
 
-- 
2.51.0