x86/entry/32: Clear the CS high bits

jan-kiszka · Ingo Molnar · commit 04f4f954b695 · 2018-10-17T12:30:20.000+02:00
Even if not on an entry stack, the CS's high bits must be initialized because they are unconditionally evaluated in PARANOID_EXIT_TO_KERNEL_MODE. Failing to do so broke the boot on Galileo Gen2 and IOT2000 boards. [ bp: Make the commit message tone passive and impartial. ] Fixes: b92a165 ("x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack") Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com> Signed-off-by: Borislav Petkov <bp@suse.de> Reviewed-by: Joerg Roedel <jroedel@suse.de> Acked-by: Joerg Roedel <jroedel@suse.de> CC: "H. Peter Anvin" <hpa@zytor.com> CC: Andrea Arcangeli <aarcange@redhat.com> CC: Andy Lutomirski <luto@kernel.org> CC: Boris Ostrovsky <boris.ostrovsky@oracle.com> CC: Brian Gerst <brgerst@gmail.com> CC: Dave Hansen <dave.hansen@intel.com> CC: David Laight <David.Laight@aculab.com> CC: Denys Vlasenko <dvlasenk@redhat.com> CC: Eduardo Valentin <eduval@amazon.com> CC: Greg KH <gregkh@linuxfoundation.org> CC: Ingo Molnar <mingo@kernel.org> CC: Jiri Kosina <jkosina@suse.cz> CC: Josh Poimboeuf <jpoimboe@redhat.com> CC: Juergen Gross <jgross@suse.com> CC: Linus Torvalds <torvalds@linux-foundation.org> CC: Peter Zijlstra <peterz@infradead.org> CC: Thomas Gleixner <tglx@linutronix.de> CC: Will Deacon <will.deacon@arm.com> CC: aliguori@amazon.com CC: daniel.gruss@iaik.tugraz.at CC: hughd@google.com CC: keescook@google.com CC: linux-mm <linux-mm@kvack.org> CC: x86-ml <x86@kernel.org> Link: http://lkml.kernel.org/r/f271c747-1714-5a5b-a71f-ae189a093b8d@siemens.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
@@ -389,6 +389,13 @@
 	 * that register for the time this macro runs
 	 */
 
+	/*
+	 * The high bits of the CS dword (__csh) are used for
+	 * CS_FROM_ENTRY_STACK and CS_FROM_USER_CR3. Clear them in case
+	 * hardware didn't do this for us.
+	 */
+	andl	$(0x0000ffff), PT_CS(%esp)
+
 	/* Are we on the entry stack? Bail out if not! */
 	movl	PER_CPU_VAR(cpu_entry_area), %ecx
 	addl	$CPU_ENTRY_AREA_entry_stack + SIZEOF_entry_stack, %ecx
@@ -407,12 +414,6 @@
 	/* Load top of task-stack into %edi */
 	movl	TSS_entry2task_stack(%edi), %edi
 
-	/*
-	 * Clear unused upper bits of the dword containing the word-sized CS
-	 * slot in pt_regs in case hardware didn't clear it for us.
-	 */
-	andl	$(0x0000ffff), PT_CS(%esp)
-
 	/* Special case - entry from kernel mode via entry stack */
 #ifdef CONFIG_VM86
 	movl	PT_EFLAGS(%esp), %ecx		# mix EFLAGS and CS
