linux-kernel - [PATCH] KASLR: Properly injest nokaslr on x86 to avoid passing it to init process

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Message-ID: <20241023231647.GA860895@lichtman.org>
Date: Wed, 23 Oct 2024 23:16:47 +0000
From: Nir Lichtman <nir@...htman.org>
To: dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	linux-kernel@...r.kernel.org, m.younesbadr@...il.com
Subject: [PATCH] KASLR: Properly injest nokaslr on x86 to avoid passing it to
 init process

Problem: Currently booting the kernel with "nokaslr" on x86 incorrectly
doesn't recognize this parameter and passes it on to the init process

Reason: On x86, this parameter is parsed by the early loader, and so the
main kernel itself doesn't do anything with it.

Example: I have encountered this issue when booting the kernel with QEMU
using -kernel and -initrd with a simple initrd I have built containing
bash as the init executable. Upon running init, the kernel passed the
"unrecognized parameter" to bash causing bash to exit with a failure and
the kernel to panic.

Solution: Ingest this parameter as part of the kernel logic in x86 so the
kernel will recognize this as a valid parameter and not pass it to user
mode.
This is similar to the logic that already exists in the case of ARM64
which can be found in arch/arm64/kernel/kaslr.c:43

Crediting also Mahmoud since he came up with a similar patch a couple of
months ago, the patch was left with unaddressed CR comments by Borislav
which I have addressed in this patch,
link to thread: https://lore.kernel.org/all/20240331200546.869343-1-m.younesbadr@gmail.com/

Signed-off-by: Mahmoud Younes <m.younesbadr@...il.com>
Signed-off-by: Nir Lichtman <nir@...htman.org>
---
 arch/x86/mm/kaslr.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 230f1dee4f09..4d159aa6910c 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -71,6 +71,21 @@ static inline unsigned long get_padding(struct kaslr_memory_region *region)
 	return (region->size_tb << TB_SHIFT);
 }
 
+/*
+ * nokaslr param handling is done by the loader which treats the
+ * boot parameters as read only so this is a hack to ingest this
+ * to keep it from passing to user mode
+ */
+static int __init parse_nokaslr(char *p)
+{
+	if (!!(boot_params.hdr.loadflags & KASLR_FLAG)) {
+		pr_warn("the loader has not parsed the nokaslr flag");
+	}
+
+	return 0;
+}
+early_param("nokaslr", parse_nokaslr);
+
 /* Initialize base and padding for each memory region randomized with KASLR */
 void __init kernel_randomize_memory(void)
 {
-- 
2.39.2