linux-kernel - [PATCH v10 23/45] x86/head/64: Re-enable stack protection

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Message-ID: <20220209181039.1262882-24-brijesh.singh@amd.com>
Date:   Wed, 9 Feb 2022 12:10:17 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <kvm@...r.kernel.org>, <linux-efi@...r.kernel.org>,
        <platform-driver-x86@...r.kernel.org>,
        <linux-coco@...ts.linux.dev>, <linux-mm@...ck.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        "Vitaly Kuznetsov" <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        <brijesh.ksingh@...il.com>, <tony.luck@...el.com>,
        <marcorr@...gle.com>, <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: [PATCH v10 23/45] x86/head/64: Re-enable stack protection

From: Michael Roth <michael.roth@....com>

Due to commit 103a4908ad4d ("x86/head/64: Disable stack protection for
head$(BITS).o"), kernel/head{32,64}.c are compiled with
-fno-stack-protector to allow a call to set_bringup_idt_handler(), which
would otherwise have stack protection enabled with
CONFIG_STACKPROTECTOR_STRONG.

While sufficient for that case, there may still be issues with calls to
any external functions that were compiled with stack protection enabled
that in-turn make stack-protected calls, or if the exception handlers
set up by set_bringup_idt_handler() make calls to stack-protected
functions.

Subsequent patches for SEV-SNP CPUID validation support will introduce
both such cases. Attempting to disable stack protection for everything
in scope to address that is prohibitive since much of the code, like
SEV-ES #VC handler, is shared code that remains in use after boot and
could benefit from having stack protection enabled. Attempting to inline
calls is brittle and can quickly balloon out to library/helper code
where that's not really an option.

Instead, re-enable stack protection for head32.c/head64.c, and make the
appropriate changes to ensure the segment used for the stack canary is
initialized in advance of any stack-protected C calls.

For head64.c:

- The BSP will enter from startup_64() and call into C code
  (startup_64_setup_env()) shortly after setting up the stack, which
  may result in calls to stack-protected code. Set up %gs early to allow
  for this safely.
- APs will enter from secondary_startup_64*(), and %gs will be set up
  soon after. There is one call to C code prior to %gs being setup
  (__startup_secondary_64()), but it is only to fetch 'sme_me_mask'
  global, so just load 'sme_me_mask' directly instead, and remove the
  now-unused __startup_secondary_64() function.

For head32.c:

- BSPs/APs will set %fs to __BOOT_DS prior to any C calls. In recent
  kernels, the compiler is configured to access the stack canary at
  %fs:__stack_chk_guard [1], which overlaps with the initial per-cpu
  '__stack_chk_guard' variable in the initial/"master" .data..percpu
  area. This is sufficient to allow access to the canary for use
  during initial startup, so no changes are needed there.

[1] commit 3fb0fdb3bbe7 ("x86/stackprotector/32: Make the canary into a regular percpu variable")

Suggested-by: Joerg Roedel <jroedel@...e.de> #for 64-bit %gs set up
Signed-off-by: Michael Roth <michael.roth@....com>
Signed-off-by: Brijesh Singh <brijesh.singh@....com>
---
 arch/x86/include/asm/setup.h |  1 -
 arch/x86/kernel/Makefile     |  1 -
 arch/x86/kernel/head64.c     |  9 ---------
 arch/x86/kernel/head_64.S    | 24 +++++++++++++++++++++---
 4 files changed, 21 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index a12458a7a8d4..72ede9159951 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -49,7 +49,6 @@ extern unsigned long saved_video_mode;
 extern void reserve_standard_io_resources(void);
 extern void i386_reserve_resources(void);
 extern unsigned long __startup_64(unsigned long physaddr, struct boot_params *bp);
-extern unsigned long __startup_secondary_64(void);
 extern void startup_64_setup_env(unsigned long physbase);
 extern void early_setup_idt(void);
 extern void __init do_early_exception(struct pt_regs *regs, int trapnr);
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 6aef9ee28a39..bd45e5ee6fe3 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -48,7 +48,6 @@ endif
 # non-deterministic coverage.
 KCOV_INSTRUMENT		:= n
 
-CFLAGS_head$(BITS).o	+= -fno-stack-protector
 CFLAGS_cc_platform.o	+= -fno-stack-protector
 
 CFLAGS_irq.o := -I $(srctree)/$(src)/../include/asm/trace
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 4a24b121a2ba..e94e32ba9636 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -318,15 +318,6 @@ unsigned long __head __startup_64(unsigned long physaddr,
 	return sme_postprocess_startup(bp, pmd);
 }
 
-unsigned long __startup_secondary_64(void)
-{
-	/*
-	 * Return the SME encryption mask (if SME is active) to be used as a
-	 * modifier for the initial pgdir entry programmed into CR3.
-	 */
-	return sme_get_me_mask();
-}
-
 /* Wipe all early page tables except for the kernel symbol map */
 static void __init reset_early_page_tables(void)
 {
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 9c2c3aff5ee4..9e84263bcb94 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -65,6 +65,22 @@ SYM_CODE_START_NOALIGN(startup_64)
 	leaq	(__end_init_task - FRAME_SIZE)(%rip), %rsp
 
 	leaq	_text(%rip), %rdi
+
+	/*
+	 * initial_gs points to initial fixed_percpu_data struct with storage for
+	 * the stack protector canary. Global pointer fixups are needed at this
+	 * stage, so apply them as is done in fixup_pointer(), and initialize %gs
+	 * such that the canary can be accessed at %gs:40 for subsequent C calls.
+	 */
+	movl	$MSR_GS_BASE, %ecx
+	movq	initial_gs(%rip), %rax
+	movq	$_text, %rdx
+	subq	%rdx, %rax
+	addq	%rdi, %rax
+	movq	%rax, %rdx
+	shrq	$32,  %rdx
+	wrmsr
+
 	pushq	%rsi
 	call	startup_64_setup_env
 	popq	%rsi
@@ -145,9 +161,11 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	 * Retrieve the modifier (SME encryption mask if SME is active) to be
 	 * added to the initial pgdir entry that will be programmed into CR3.
 	 */
-	pushq	%rsi
-	call	__startup_secondary_64
-	popq	%rsi
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+	movq	sme_me_mask, %rax
+#else
+	xorq	%rax, %rax
+#endif
 
 	/* Form the CR3 value being sure to include the CR3 modifier */
 	addq	$(init_top_pgt - __START_KERNEL_map), %rax
-- 
2.25.1