Patch "x86/mm: Rework wbinvd, hlt operation in stop_this_cpu()" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

x86/mm: Rework wbinvd, hlt operation in stop_this_cpu()

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-mm-rework-wbinvd-hlt-operation-in-stop_this_cpu.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tom Lendacky <[email protected]>
Date: Wed, 17 Jan 2018 17:41:41 -0600
Subject: x86/mm: Rework wbinvd, hlt operation in stop_this_cpu()

From: Tom Lendacky <[email protected]>

commit f23d74f6c66c3697e032550eeef3f640391a3a7d upstream.

Some issues have been reported with the for loop in stop_this_cpu() that issues the 'wbinvd; hlt' sequence. Reverting this sequence to halt() has been shown to resolve the issue.

However, the wbinvd is needed when running with SME. The reason for the wbinvd is to prevent cache flush races between encrypted and non-encrypted entries that have the same physical address. This can occur when kexec'ing from memory encryption active to inactive or vice-versa. The important thing is to not have outside of kernel text memory references (such as stack usage), so the usage of the native_*() functions is needed since these expand as inline asm sequences. So instead of reverting the change, rework the sequence.

Move the wbinvd instruction outside of the for loop as native_wbinvd() and make its execution conditional on X86_FEATURE_SME. In the for loop, change the asm 'wbinvd; hlt' sequence back to a halt sequence but use the native_halt() call.

---
 arch/x86/kernel/process.c |   25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

@@ -380,19 +380,24 @@ void stop_this_cpu(void *dummy)
 	disable_local_APIC();
 	mcheck_cpu_clear(this_cpu_ptr(&cpu_info));
 
+	/*
+	 * Use wbinvd on processors that support SME. This provides support
+	 * for performing a successful kexec when going from SME inactive
+	 * to SME active (or vice-versa). The cache must be cleared so that
+	 * if there are entries with the same physical address, both with and
+	 * without the encryption bit, they don't race each other when flushed
+	 * and potentially end up with the wrong entry being committed to
+	 * memory.
+	 */
+	if (boot_cpu_has(X86_FEATURE_SME))
+		native_wbinvd();
 	for (;;) {
 		/*
-		 * Use wbinvd followed by hlt to stop the processor. This
-		 * provides support for kexec on a processor that supports
-		 * SME. With kexec, going from SME inactive to SME active
-		 * requires clearing cache entries so that addresses without
-		 * the encryption bit set don't corrupt the same physical
-		 * address that has the encryption bit set when caches are
-		 * flushed. To achieve this a wbinvd is performed followed by
-		 * a hlt. Even if the processor is not in the kexec/SME
-		 * scenario this only adds a wbinvd to a halting processor.
+		 * Use native_halt() so that memory contents don't change
+		 * (stack usage and variables) after possibly issuing the
+		 * native_wbinvd() above.
 		 */
-		asm volatile("wbinvd; hlt" : : : "memory");
+		native_halt();
 	}
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/x86-mm-clean-up-register-saving-in-the-__enc_copy-assembly-code.patch queue-4.14/x86-use-__nostackprotect-for-sme_encrypt_kernel.patch queue-4.14/x86-mm-use-a-struct-to-reduce-parameters-for-sme-pgd-mapping.patch queue-4.14/x86-mm-centralize-pmd-flags-in-sme_encrypt_kernel.patch queue-4.14/x86-retpoline-fill-rsb-on-context-switch-for-affected-cpus.patch queue-4.14/x86-mm-prepare-sme_encrypt_kernel-for-page-aligned-encryption.patch queue-4.14/x86-retpoline-add-lfence-to-the-retpoline-rsb-filling-rsb-macros.patch queue-4.14/x86-mm-rework-wbinvd-hlt-operation-in-stop_this_cpu.patch queue-4.14/x86-mm-encrypt-the-initrd-earlier-for-bsp-microcode-update.patch