linux-kernel - [PATCH 2/2] x86/mm/64: free the gap between BSS_MAIN and BSS

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Message-Id: <1752092219-16248-2-git-send-email-hamzamahfooz@linux.microsoft.com>
Date: Wed,  9 Jul 2025 13:16:59 -0700
From: Hamza Mahfooz <hamzamahfooz@...ux.microsoft.com>
To: linux-kernel@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Guenter Roeck <linux@...ck-us.net>,
	Hamza Mahfooz <hamzamahfooz@...ux.microsoft.com>,
	Jared White <jaredwhite@...rosoft.com>
Subject: [PATCH 2/2] x86/mm/64: free the gap between BSS_MAIN and BSS_DECRYPTED

This region is unused after mark_rodata_ro() and takes up, up to 2 MiB
of memory. So, free it in mark_rodata_ro().

Co-developed-by: Jared White <jaredwhite@...rosoft.com>
Signed-off-by: Jared White <jaredwhite@...rosoft.com>
Signed-off-by: Hamza Mahfooz <hamzamahfooz@...ux.microsoft.com>
---
 arch/x86/include/asm/sections.h | 1 +
 arch/x86/kernel/vmlinux.lds.S   | 2 ++
 arch/x86/mm/init_64.c           | 4 ++++
 3 files changed, 7 insertions(+)

diff --git a/arch/x86/include/asm/sections.h b/arch/x86/include/asm/sections.h
index 30e8ee7006f9..1d6589bc0f68 100644
--- a/arch/x86/include/asm/sections.h
+++ b/arch/x86/include/asm/sections.h
@@ -8,6 +8,7 @@
 extern char __relocate_kernel_start[], __relocate_kernel_end[];
 extern char __brk_base[], __brk_limit[];
 extern char __end_rodata_aligned[];
+extern char __start_bss_decrypted_gap[];
 
 #if defined(CONFIG_X86_64)
 extern char __end_rodata_hpage_align[];
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 522297cebe7c..f74a7a0d7a01 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -82,6 +82,8 @@ const_cpu_current_top_of_stack = cpu_current_top_of_stack;
  * decrypted to avoid exposing more than we wish.
  */
 #define BSS_DECRYPTED						\
+	. = ALIGN(PAGE_SIZE);					\
+	__start_bss_decrypted_gap = .;				\
 	. = ALIGN(PMD_SIZE);					\
 	__start_bss_decrypted = .;				\
 	__pi___start_bss_decrypted = .;				\
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index fdb6cab524f0..ff7ff5c12eb4 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1385,6 +1385,8 @@ void mark_rodata_ro(void)
 	unsigned long end = (unsigned long)__end_rodata_hpage_align;
 	unsigned long text_end = PFN_ALIGN(_etext);
 	unsigned long rodata_end = PFN_ALIGN(__end_rodata);
+	unsigned long bss_decrypted_gap_start = PFN_ALIGN(__start_bss_decrypted_gap);
+	unsigned long bss_decrypted_gap_stop = PFN_ALIGN(__start_bss_decrypted);
 	unsigned long all_end;
 
 	printk(KERN_INFO "Write protecting the kernel read-only data: %luk\n",
@@ -1422,6 +1424,8 @@ void mark_rodata_ro(void)
 				(void *)text_end, (void *)rodata_start);
 	free_kernel_image_pages("unused kernel image (rodata/data gap)",
 				(void *)rodata_end, (void *)_sdata);
+	free_kernel_image_pages("unused kernel image (bss_decrypted gap)",
+				(void *)bss_decrypted_gap_start, (void *)bss_decrypted_gap_stop);
 }
 
 /*
-- 
2.49.0