linux-kernel - Re: [RFC PATCH] x86/boot/compressed: Disable physical KASLR when memmap= appears

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Message-ID: <20251107205020.GEaQ5bjLIU1Y7ePsJY@fat_crate.local>
Date: Fri, 7 Nov 2025 21:50:20 +0100
From: Borislav Petkov <bp@...en8.de>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, kees@...nel.org,
	Ard Biesheuvel <ardb@...nel.org>,
	Michal Clapinski <mclapinski@...gle.com>,
	Chris Li <chrisl@...nel.org>
Subject: Re: [RFC PATCH] x86/boot/compressed: Disable physical KASLR when
 memmap= appears

On Thu, Nov 06, 2025 at 06:30:20PM +0100, Ard Biesheuvel wrote:
> Reported-by: Michal Clapinski <mclapinski@...gle.com>
> Reported-by: Chris Li <chrisl@...nel.org>
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/x86/boot/compressed/kaslr.c | 100 ++-----------------------------
>  1 file changed, 6 insertions(+), 94 deletions(-)

I love diffstats like that.

How about this simplification ontop of yours:

---

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index 96805d6c10a4..7875ae2d514e 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -67,8 +67,6 @@ static unsigned long get_boot_seed(void)
 #define KASLR_COMPRESSED_BOOT
 #include "../../lib/kaslr.c"
 
-static bool memmap_found;
-
 /*
  * Store memory limit: MAXMEM on 64-bit and KERNEL_IMAGE_SIZE on 32-bit.
  * It may be reduced by "mem=nn[KMG]" command line options.
@@ -165,9 +163,7 @@ static void handle_mem_options(void)
 		if (!val && strcmp(param, "--") == 0)
 			break;
 
-		if (!strcmp(param, "memmap")) {
-			memmap_found = true;
-		} else if (IS_ENABLED(CONFIG_X86_64) && strstr(param, "hugepages")) {
+		if (IS_ENABLED(CONFIG_X86_64) && strstr(param, "hugepages")) {
 			parse_gb_huge_pages(param, val);
 		} else if (!strcmp(param, "mem")) {
 			char *p = val;
@@ -724,12 +720,6 @@ static unsigned long find_random_phys_addr(unsigned long minimum,
 	if (minimum + image_size > mem_limit)
 		return 0;
 
-	/* Check if memmap= appears on the command line */
-	if (memmap_found) {
-		debug_putstr("memmap= found on the command line, disabling physical KASLR\n");
-		return 0;
-	}
-
 	/*
 	 * During kexec handover only process KHO scratch areas that are known
 	 * not to contain any data that must be preserved.
@@ -783,6 +773,11 @@ void choose_random_location(unsigned long input,
 		return;
 	}
 
+	if (cmdline_find_option_bool("memmap")) {
+		warn("KASLR disabled: 'memmap' on cmdline.");
+		return;
+	}
+
 	boot_params_ptr->hdr.loadflags |= KASLR_FLAG;
 
 	if (IS_ENABLED(CONFIG_X86_32))


-- 
Regards/Gruss,
    Boris.

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