[PATCH v6 2/5] x86/kexec: do unconditional WBINVD for bare-metal in relocate_kernel()

From: Kai Huang
Date: Mon Sep 09 2024 - 04:08:56 EST

Both SME and TDX can leave caches in incoherent state due to memory
encryption. During kexec, the caches must be flushed before jumping to
the second kernel to avoid silent memory corruption to the second kernel.

During kexec, the WBINVD in stop_this_cpu() flushes caches for all
remote cpus when they are being stopped. For SME, the WBINVD in
relocate_kernel() flushes the cache for the last running cpu (which is
executing the kexec).

Similarly, to support kexec for TDX host, after stopping all remote cpus
with cache flushed, the kernel needs to flush cache for the last running
cpu.

Use the existing WBINVD in relocate_kernel() to cover TDX host as well.

However, instead of sprinkling around vendor-specific checks, just do
unconditional WBINVD to cover both SME and TDX. Kexec is not a fast path
so having one additional WBINVD for platforms w/o SME/TDX is acceptable.

But only do WBINVD for bare-metal because TDX guests and SEV-ES/SEV-SNP
guests will get unexpected (and yet unnecessary) exception (#VE or #VC)
which the kernel is unable to handle at this stage.

Note commit 93c1800b3799 ("x86/kexec: Fix bug with call depth tracking")
moved calling 'cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT)' as an argument
of relocate_kernel() to an earlier place before load_segments() by
adding a variable 'host_mem_enc_active'. The reason was the call to
cc_platform_has() after load_segments() caused a fault and system crash
when call depth tracking is active because load_segments() resets GS to
0 but call depth tracking uses per-CPU variable to operate.

Use !cpu_feature_enabled(X86_FEATURE_HYPERVISOR) to check whether the
kernel runs on bare-metal. cpu_feature_enabled() is always inline but
not a function call, thus it is safe to use it after load_segments()
when call depth tracking is enabled. Remove the 'host_mem_enc_active'
variable and use cpu_feature_enabled() directly as the argument when
calling relocate_kernel().

Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Dave Young <dyoung@xxxxxxxxxx>
Cc: David Kaplan <david.kaplan@xxxxxxx>
Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---

v5 - >v6:
- Use cpu_feature_enabled() instead of boot_cpu_has() - Boris
- Resolve rebase conflict with commit 93c1800b3799 ("x86/kexec: Fix bug
with call depth tracking")

v4 -> v5:
- Add Tom's tag

v3 -> v4:
- Use "exception (#VE or #VC)" for TDX and SEV-ES/SEV-SNP in changelog
and comments. (Kirill, Tom)
- "Save the bare_metal" -> "Save the bare_metal flag" (Tom)
- Point out "WBINVD is not necessary for TDX and SEV-ES/SEV-SNP guests"
in the comment. (Tom)

v2 -> v3:
- Change to only do WBINVD for bare metal


---
arch/x86/include/asm/kexec.h | 2 +-
arch/x86/kernel/machine_kexec_64.c | 9 +--------
arch/x86/kernel/relocate_kernel_64.S | 19 +++++++++++++++----
3 files changed, 17 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
index ae5482a2f0ca..b3429c70847d 100644
--- a/arch/x86/include/asm/kexec.h
+++ b/arch/x86/include/asm/kexec.h
@@ -128,7 +128,7 @@ relocate_kernel(unsigned long indirection_page,
unsigned long page_list,
unsigned long start_address,
unsigned int preserve_context,
- unsigned int host_mem_enc_active);
+ unsigned int bare_metal);
#endif

#define ARCH_HAS_KIMAGE_ARCH
diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index 9c9ac606893e..225242840467 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -322,16 +322,9 @@ void machine_kexec_cleanup(struct kimage *image)
void machine_kexec(struct kimage *image)
{
unsigned long page_list[PAGES_NR];
- unsigned int host_mem_enc_active;
int save_ftrace_enabled;
void *control_page;

- /*
- * This must be done before load_segments() since if call depth tracking
- * is used then GS must be valid to make any function calls.
- */
- host_mem_enc_active = cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT);
-
#ifdef CONFIG_KEXEC_JUMP
if (image->preserve_context)
save_processor_state();
@@ -392,7 +385,7 @@ void machine_kexec(struct kimage *image)
(unsigned long)page_list,
image->start,
image->preserve_context,
- host_mem_enc_active);
+ !cpu_feature_enabled(X86_FEATURE_HYPERVISOR));

#ifdef CONFIG_KEXEC_JUMP
if (image->preserve_context)
diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index e9e88c342f75..19821c3fbc46 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -52,7 +52,7 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
* %rsi page_list
* %rdx start address
* %rcx preserve_context
- * %r8 host_mem_enc_active
+ * %r8 bare_metal
*/

/* Save the CPU context, used for jumping back */
@@ -80,7 +80,7 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
pushq $0
popfq

- /* Save SME active flag */
+ /* Save the bare_metal flag */
movq %r8, %r12

/*
@@ -161,9 +161,20 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
movq %r9, %cr3

/*
- * If SME is active, there could be old encrypted cache line
+ * The kernel could leave caches in incoherent state on SME/TDX
+ * capable platforms. Just do unconditional WBINVD to avoid
+ * silent memory corruption to the new kernel for these platforms.
+ *
+ * For SME, need to flush cache here before copying the kernel.
+ * When it is active, there could be old encrypted cache line
* entries that will conflict with the now unencrypted memory
- * used by kexec. Flush the caches before copying the kernel.
+ * used by kexec.
+ *
+ * Do WBINVD for bare-metal only to cover both SME and TDX. It
+ * isn't necessary to perform a WBINVD in a guest and performing
+ * one could result in an exception (#VE or #VC) for a TDX or
+ * SEV-ES/SEV-SNP guest that can crash the guest since, at this
+ * stage, the kernel has torn down the IDT.
*/
testq %r12, %r12
jz .Lsme_off
--
2.46.0