linux-kernel - [tip: x86/microcode] x86/boot/32: Disable stackprotector and tracing for mk_early_pgtbl

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Date:   Mon, 09 Oct 2023 12:29:56 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Borislav Petkov (AMD)" <bp@...en8.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/microcode] x86/boot/32: Disable stackprotector and tracing
 for mk_early_pgtbl_32()

The following commit has been merged into the x86/microcode branch of tip:

Commit-ID:     b6bedf0825b8a74edef6fa7100cb078e879bbce7
Gitweb:        https://git.kernel.org/tip/b6bedf0825b8a74edef6fa7100cb078e879bbce7
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Mon, 02 Oct 2023 13:59:36 +02:00
Committer:     Borislav Petkov (AMD) <bp@...en8.de>
CommitterDate: Mon, 02 Oct 2023 18:25:49 +02:00

x86/boot/32: Disable stackprotector and tracing for mk_early_pgtbl_32()

Stackprotector cannot work before paging is enabled. The read from the per
CPU variable __stack_chk_guard is always accessing the virtual address
either directly on UP or via FS on SMP. In physical address mode this
results in an access to memory above 3GB.

So this works by chance as the hardware returns the same value when there
is no RAM at this physical address. When there is RAM populated above 3G
then the read is by chance the same as nothing changes that memory during
the very early boot stage.

Stop relying on pure luck and disable the stack protector for the only C
function which is called during early boot before paging is enabled.

Remove function tracing from the whole source file as there is no way to
trace this at all, but in case of CONFIG_DYNAMIC_FTRACE=n
mk_early_pgtbl_32() would access global function tracer variables in
physical address mode which again might work by chance.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Link: https://lore.kernel.org/r/20231002115902.156063939@linutronix.de
---
 arch/x86/kernel/Makefile | 1 +
 arch/x86/kernel/head32.c | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 3269a0e..0000325 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -16,6 +16,7 @@ CFLAGS_REMOVE_kvmclock.o = -pg
 CFLAGS_REMOVE_ftrace.o = -pg
 CFLAGS_REMOVE_early_printk.o = -pg
 CFLAGS_REMOVE_head64.o = -pg
+CFLAGS_REMOVE_head32.o = -pg
 CFLAGS_REMOVE_sev.o = -pg
 CFLAGS_REMOVE_rethook.o = -pg
 endif
diff --git a/arch/x86/kernel/head32.c b/arch/x86/kernel/head32.c
index bde27a3..f34b15c 100644
--- a/arch/x86/kernel/head32.c
+++ b/arch/x86/kernel/head32.c
@@ -73,7 +73,8 @@ asmlinkage __visible void __init __noreturn i386_start_kernel(void)
  * always zero at this stage.
  */
 void __init mk_early_pgtbl_32(void);
-void __init mk_early_pgtbl_32(void)
+
+void __init __no_stack_protector mk_early_pgtbl_32(void)
 {
 #ifdef __pa
 #undef __pa