linux-kernel - [RFC PATCH v3 01/20] x86/kexec: Ensure control_code

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Message-ID: <20241125100815.2512-2-dwmw2@infradead.org>
Date: Mon, 25 Nov 2024 09:54:31 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: kexec@...ts.infradead.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Kai Huang <kai.huang@...el.com>,
	Nikolay Borisov <nik.borisov@...e.com>,
	linux-kernel@...r.kernel.org,
	Simon Horman <horms@...nel.org>,
	Dave Young <dyoung@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	jpoimboe@...nel.org,
	bsz@...zon.de
Subject: [RFC PATCH v3 01/20] x86/kexec: Ensure control_code_page is mapped in kexec page tables

From: David Woodhouse <dwmw@...zon.co.uk>

The control_code_page should be explicitly mapped into the identity
mapped page tables for the relocate_kernel environment. This only seems
to have worked by luck before, because it tended to be within the same
2MiB or 1GiB large page already mapped for another reason.

A subsequent commit will reduce the control_code_page to a single 4KiB
page instead of a higher-order allocation, and seems to make it much
*less* likely that we get lucky with its placement. This leads to a
fault when relocate_kernel() first tries to access the page through its
identity-mapped virtual address.

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
Cc: stable@...r.kernel.org
---
 arch/x86/kernel/machine_kexec_64.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index 9c9ac606893e..b9b6243ee223 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -240,6 +240,12 @@ static int init_pgtable(struct kimage *image, unsigned long start_pgtable)
 	if (direct_gbpages)
 		info.direct_gbpages = true;
 
+	/* Ensure the control code page itself is in the direct map */
+	result = kernel_ident_mapping_init(&info, level4p, start_pgtable + PAGE_SIZE,
+					   start_pgtable + KEXEC_CONTROL_CODE_MAX_SIZE);
+	if (result)
+		return result;
+
 	for (i = 0; i < nr_pfn_mapped; i++) {
 		mstart = pfn_mapped[i].start << PAGE_SHIFT;
 		mend   = pfn_mapped[i].end << PAGE_SHIFT;
-- 
2.47.0