linux-kernel - [PATCH V2 3/3] x86/mm/pti: warn and stop when pti_clone

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Date:   Thu, 10 Dec 2020 22:35:26 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH V2 3/3] x86/mm/pti: warn and stop when pti_clone_pagetable() is on 1G page

From: Lai Jiangshan <laijs@...ux.alibaba.com>

Right now, we only clone parts of the kernel image and the cpu entry area.
The cpu entry area would be insane to map with 1G pages since it maps so
many different kinds of pages and has *small* mappings.

For the kernel image to have a 1GB area with uniform permissions seems
pretty far away to practice.  It would be en even more remote possibility
that a large swath of it would need to be cloned for PTI.  Kernel text
with a non-PCID system is probably as close as we would get.  I'm also
not even sure we have the code to create 1GB mappings for parts of the
image.

In other words, no caller calls pti_clone_pagetable() on range that
includes pud large page(1G) by now. If it were called in such case,
there would be bugs in the caller side or other places, so it worths
a warning for robustness.

We also add check for pgd_large() & p4d_large() with the same reason,
and pgd_large() & p4d_large() are constant 0 which just acts as the
self-comment in code without any overhead.

[ Many thanks to Dave Hansen for more elaborated changelog ]

Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
---
 arch/x86/mm/pti.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index cd6da1d42ba9..e8d2df072c5c 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -321,10 +321,10 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
 			break;

 		pgd = pgd_offset_k(addr);
-		if (WARN_ON(pgd_none(*pgd)))
+		if (WARN_ON(pgd_none(*pgd) || pgd_large(*pgd)))
 			return;
 		p4d = p4d_offset(pgd, addr);
-		if (WARN_ON(p4d_none(*p4d)))
+		if (WARN_ON(p4d_none(*p4d) || p4d_large(*p4d)))
 			return;

 		pud = pud_offset(p4d, addr);
@@ -333,6 +333,8 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
 			addr = round_up(addr + 1, PUD_SIZE);
 			continue;
 		}
+		if (WARN_ON(pud_large(*pud)))
+			return;

 		pmd = pmd_offset(pud, addr);
 		if (pmd_none(*pmd)) {
-- 
2.19.1.6.gb485710b