linux-kernel - Re: Linux 6.11-rc1

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Message-ID: <20240731163105.GG33588@noisy.programming.kicks-ass.net>
Date: Wed, 31 Jul 2024 18:31:05 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Guenter Roeck <linux@...ck-us.net>, Jens Axboe <axboe@...nel.dk>,
	Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Peter Anvin <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: Linux 6.11-rc1

On Wed, Jul 31, 2024 at 09:17:44AM -0700, Linus Torvalds wrote:
> On Wed, 31 Jul 2024 at 08:55, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Right, so Thomas found that i386-pti fails to map the entire entry text.
> > Specifically pti_clone_pgtable() hard relies -- and does not verify --
> > that the start address is aligned to the given granularity.
> >
> > Now, i386 does not align __entry_text_start, and so the termination
> > condition goes sideways and pte_clone_entry() does not always work right
> > and it becomes a games of code layout roulette.
> 
> Lovely.

:-)

This fixes the alignment assumptions and makes it all go again.

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 2e69abf4f852..bfdf5f45b137 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -374,14 +374,14 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
 			 */
 			*target_pmd = *pmd;
 
-			addr += PMD_SIZE;
+			addr = round_up(addr + 1, PMD_SIZE);
 
 		} else if (level == PTI_CLONE_PTE) {
 
 			/* Walk the page-table down to the pte level */
 			pte = pte_offset_kernel(pmd, addr);
 			if (pte_none(*pte)) {
-				addr += PAGE_SIZE;
+				addr = round_up(addr + 1, PAGE_SIZE);
 				continue;
 			}
 
@@ -401,7 +401,7 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
 			/* Clone the PTE */
 			*target_pte = *pte;
 
-			addr += PAGE_SIZE;
+			addr = round_up(addr + 1, PAGE_SIZE);
 
 		} else {
 			BUG();