linux-hardening - [PATCH v2 1/6] LoongArch: Simplified randomization layout after jump new kernel processing

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Date:   Tue,  4 Apr 2023 16:41:43 +0800
From:   Qing Zhang <zhangqing@...ngson.cn>
To:     Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Huacai Chen <chenhuacai@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        WANG Xuerui <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        kasan-dev@...glegroups.com, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, loongarch@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v2 1/6] LoongArch: Simplified randomization layout after jump new kernel processing

Modified relocate_kernel is not returned directly new kernel's entry point，
instead, we share start_kernel processing with the normal kernel, which avoids
calling 'jr a0' directly and we can do other operations(eg: kasan_early_init)
before start_kernel when CONFIG_RANDOMIZE_BASE is turned on.

Signed-off-by: Qing Zhang <zhangqing@...ngson.cn>
---
 arch/loongarch/include/asm/setup.h |  2 +-
 arch/loongarch/kernel/head.S       | 10 +++++-----
 arch/loongarch/kernel/relocate.c   |  8 ++------
 3 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/arch/loongarch/include/asm/setup.h b/arch/loongarch/include/asm/setup.h
index be05c0e706a2..2dca0d1dd90a 100644
--- a/arch/loongarch/include/asm/setup.h
+++ b/arch/loongarch/include/asm/setup.h
@@ -33,7 +33,7 @@ extern long __la_abs_end;
 extern long __rela_dyn_begin;
 extern long __rela_dyn_end;
 
-extern void * __init relocate_kernel(void);
+extern unsigned long __init relocate_kernel(void);
 
 #endif
 
diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
index aa64b179744f..35c4a78614c3 100644
--- a/arch/loongarch/kernel/head.S
+++ b/arch/loongarch/kernel/head.S
@@ -95,12 +95,12 @@ SYM_CODE_START(kernel_entry)			# kernel entry point
 	PTR_LI		sp, (_THREAD_SIZE - PT_SIZE)
 	PTR_ADD		sp, sp, tp
 	set_saved_sp	sp, t0, t1
-#endif
-
-	/* relocate_kernel() returns the new kernel entry point */
-	jr		a0
-	ASM_BUG()
 
+	/* Jump to new kernel: new_pc = current_pc + random_offset */
+	pcaddi		t0, 0
+	add.d		t0, t0, a0
+	jirl		zero, t0, 0xc
+#endif
 #endif
 
 	bl		start_kernel
diff --git a/arch/loongarch/kernel/relocate.c b/arch/loongarch/kernel/relocate.c
index 01f94d1e3edf..6c3eff9af9fb 100644
--- a/arch/loongarch/kernel/relocate.c
+++ b/arch/loongarch/kernel/relocate.c
@@ -157,12 +157,11 @@ static inline void __init update_reloc_offset(unsigned long *addr, long random_o
 	*new_addr = (unsigned long)reloc_offset;
 }
 
-void * __init relocate_kernel(void)
+unsigned long __init relocate_kernel(void)
 {
 	unsigned long kernel_length;
 	unsigned long random_offset = 0;
 	void *location_new = _text; /* Default to original kernel start */
-	void *kernel_entry = start_kernel; /* Default to original kernel entry point */
 	char *cmdline = early_ioremap(fw_arg1, COMMAND_LINE_SIZE); /* Boot command line is passed in fw_arg1 */
 
 	strscpy(boot_command_line, cmdline, COMMAND_LINE_SIZE);
@@ -190,9 +189,6 @@ void * __init relocate_kernel(void)
 
 		reloc_offset += random_offset;
 
-		/* Return the new kernel's entry point */
-		kernel_entry = RELOCATED_KASLR(start_kernel);
-
 		/* The current thread is now within the relocated kernel */
 		__current_thread_info = RELOCATED_KASLR(__current_thread_info);
 
@@ -204,7 +200,7 @@ void * __init relocate_kernel(void)
 
 	relocate_absolute(random_offset);
 
-	return kernel_entry;
+	return random_offset;
 }
 
 /*
-- 
2.20.1