linux-kernel - [PATCH v4 16/26] x86/boot: Reduce lower limit of physical KASLR

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Date:   Thu, 15 Dec 2022 15:38:07 +0300
From:   Evgeniy Baskov <baskov@...ras.ru>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Evgeniy Baskov <baskov@...ras.ru>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Peter Jones <pjones@...hat.com>,
        "Limonciello, Mario" <mario.limonciello@....com>,
        joeyli <jlee@...e.com>, lvc-project@...uxtesting.org,
        x86@...nel.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v4 16/26] x86/boot: Reduce lower limit of physical KASLR

Set lower limit of physical KASLR to 64M.

Previously is was set to 512M when kernel is loaded higher than that.
That prevented physical KASLR from being performed on x86_32, where
upper limit is also set to 512M. The limit is pretty arbitrary, and the
most important is to set it above the ISA hole, i.e. higher than 16M.

It was not that important before, but now kernel is not getting
relocated to the lower address when booting via EFI, exposing the
KASLR failures.

Tested-by: Mario Limonciello <mario.limonciello@....com>
Tested-by: Peter Jones <pjones@...hat.com>
Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
---
 arch/x86/boot/compressed/kaslr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index c0ee116c4fa2..74d1327adbba 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -852,10 +852,10 @@ void choose_random_location(unsigned long input,
 
 	/*
 	 * Low end of the randomization range should be the
-	 * smaller of 512M or the initial kernel image
+	 * smaller of 64M or the initial kernel image
 	 * location:
 	 */
-	min_addr = min(*output, 512UL << 20);
+	min_addr = min(*output, 64UL << 20);
 	/* Make sure minimum is aligned. */
 	min_addr = ALIGN(min_addr, CONFIG_PHYSICAL_ALIGN);
 
-- 
2.37.4