x86/entry_32: Clear CPU buffers after register restore in NMI return

CPU buffers are currently cleared after call to exc_nmi, but before
register state is restored. This may be okay for MDS mitigation but not for
RDFS. Because RDFS mitigation requires CPU buffers to be cleared when
registers don't have any sensitive data.

Move CLEAR_CPU_BUFFERS after RESTORE_ALL_NMI.

Fixes: a0e2dab ("x86/entry_32: Add VERW just before userspace transition")
Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc:stable@vger.kernel.org
Link: https://lore.kernel.org/all/20240925-fix-dosemu-vm86-v7-2-1de0daca2d42%40linux.intel.com

Author: pa1gupta

arch/x86/entry/entry_32.S

@@ -1145,7 +1145,6 @@ SYM_CODE_START(asm_exc_nmi)
 
 	/* Not on SYSENTER stack. */
 	call	exc_nmi
-	CLEAR_CPU_BUFFERS
 	jmp	.Lnmi_return
 
 .Lnmi_from_sysenter_stack:
@@ -1166,6 +1165,7 @@ SYM_CODE_START(asm_exc_nmi)
 
 	CHECK_AND_APPLY_ESPFIX
 	RESTORE_ALL_NMI cr3_reg=%edi pop=4
+	CLEAR_CPU_BUFFERS
 	jmp	.Lirq_return
 
 #ifdef CONFIG_X86_ESPFIX32
@@ -1207,6 +1207,7 @@ SYM_CODE_START(asm_exc_nmi)
 	 *  1 - orig_ax
 	 */
 	lss	(1+5+6)*4(%esp), %esp			# back to espfix stack
+	CLEAR_CPU_BUFFERS
 	jmp	.Lirq_return
 #endif
 SYM_CODE_END(asm_exc_nmi)