linux-kernel - [tip: x86/mm] x86/mm: Ensure forced page table splitting

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Date:   Sat, 17 Dec 2022 18:55:28 -0000
From:   "tip-bot2 for Dave Hansen" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Marco Elver <elver@...gle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/mm] x86/mm: Ensure forced page table splitting

The following commit has been merged into the x86/mm branch of tip:

Commit-ID:     3e844d842d49cdbe61a4b338bdd512654179488a
Gitweb:        https://git.kernel.org/tip/3e844d842d49cdbe61a4b338bdd512654179488a
Author:        Dave Hansen <dave.hansen@...ux.intel.com>
AuthorDate:    Fri, 18 Nov 2022 07:16:16 -08:00
Committer:     Dave Hansen <dave.hansen@...ux.intel.com>
CommitterDate: Thu, 15 Dec 2022 10:37:28 -08:00

x86/mm: Ensure forced page table splitting

There are a few kernel users like kfence that require 4k pages to work
correctly and do not support large mappings.  They use set_memory_4k()
to break down those large mappings.

That, in turn relies on cpa_data->force_split option to indicate to
set_memory code that it should split page tables regardless of whether
the need to be.

But, a recent change added an optimization which would return early
if a set_memory request came in that did not change permissions.  It
did not consult ->force_split and would mistakenly optimize away the
splitting that set_memory_4k() needs.  This broke kfence.

Skip the same-permission optimization when ->force_split is set.

Fixes: 127960a05548 ("x86/mm: Inhibit _PAGE_NX changes from cpa_process_alias()")
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Tested-by: Marco Elver <elver@...gle.com>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lore.kernel.org/all/CA+G9fYuFxZTxkeS35VTZMXwQvohu73W3xbZ5NtjebsVvH6hCuA@mail.gmail.com/
---
 arch/x86/mm/pat/set_memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index 220361c..0db6951 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -1727,7 +1727,8 @@ static int __change_page_attr_set_clr(struct cpa_data *cpa, int primary)
 	/*
 	 * No changes, easy!
 	 */
-	if (!(pgprot_val(cpa->mask_set) | pgprot_val(cpa->mask_clr)))
+	if (!(pgprot_val(cpa->mask_set) | pgprot_val(cpa->mask_clr)) &&
+	    !cpa->force_split)
 		return ret;
 
 	while (rempages) {