[PATCH] x86/CPU/AMD: Make sure EFER[AIBRSE] is set

[Borislav Petkov]

On Fri, Feb 24, 2023 at 04:09:31PM -0800, Josh Poimboeuf wrote:
> Ah, I had to stare it that for a bit to figure out how it works.

Yeah, it is a bit "hidden". :)

> setup_real_mode() reads MSR_EFER from the boot CPU and stores it in
> trampoline_header->efer.  Then the other CPUs read that stored value in
> startup_32() and write it into their MSR.

Exactly.

> Yeah, I think that would be good.  Otherwise it's rather magical.

Yap, see below.

> That EFER MSR is a surprising place to put that bit.

That MSR is very important on AMD. Consider it AMD's CR4. :-)

Thx.

---
From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>
Date: Sat, 25 Feb 2023 01:11:31 +0100
Subject: [PATCH] x86/CPU/AMD: Make sure EFER[AIBRSE] is set

The AutoIBRS bit gets set only on the BSP as part of determining which
mitigation to enable on AMD. Setting on the APs relies on the
circumstance that the APs get booted through the trampoline and EFER
- the MSR which contains that bit - gets replicated on every AP from the
BSP.

However, this can change in the future and considering the security
implications of this bit not being set on every CPU, make sure it is set
by verifying EFER later in the boot process and on every AP.

Reported-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Link: https://lore.kernel.org/r/20230224185257.o3mcmloei5zqu7wa@treble
---
 arch/x86/kernel/cpu/amd.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 380753b14cab..de624c1442c2 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -996,6 +996,16 @@ static void init_amd(struct cpuinfo_x86 *c)
 		msr_set_bit(MSR_K7_HWCR, MSR_K7_HWCR_IRPERF_EN_BIT);
 
 	check_null_seg_clears_base(c);
+
+	/*
+	 * Make sure EFER[AIBRSE - Automatic IBRS Enable] is set. The APs are brought up
+	 * using the trampoline code and as part of it, EFER gets prepared there in order
+	 * to be replicated onto them. Regardless, set it here again, if not set, to protect
+	 * against any future refactoring/code reorganization which might miss setting
+	 * this important bit.
+	 */
+	if (cpu_has(c, X86_FEATURE_AUTOIBRS))
+		msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
 }
 
 #ifdef CONFIG_X86_32
-- 
2.35.1



-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette