x86/mitigations: Clear CPU buffers on the SYSCALL fast path

BugLink: https://bugs.launchpad.net/bugs/1875905

The fast SYSCALL exit path returns with SYSRET to userspace after
verifying that there's no pending work. MDS mitigation mandates that CPU
buffers must be cleared on transition from kernel to userspace so do
that here too.
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/x86/entry/entry_64.S

@@ -222,6 +222,8 @@ entry_SYSCALL_64_fastpath:
 	testl	$_TIF_ALLWORK_MASK, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
 	jnz	int_ret_from_sys_call_irqs_off	/* Go to the slow path */
 
+	call	mds_user_clear_buffers
+
 	movq	RIP(%rsp), %rcx
 	movq	EFLAGS(%rsp), %r11
 	RESTORE_C_REGS_EXCEPT_RCX_R11

arch/x86/include/asm/spec-ctrl.h

@@ -85,4 +85,6 @@ static inline void speculative_store_bypass_ht_init(void) { }
 extern void speculation_ctrl_update(unsigned long tif);
 extern void speculation_ctrl_update_current(void);
 
+extern void mds_user_clear_buffers(void);
+
 #endif

arch/x86/kernel/cpu/bugs.c

@@ -307,6 +307,11 @@ static int __init mds_cmdline(char *str)
 }
 early_param("mds", mds_cmdline);
 
+void mds_user_clear_buffers(void)
+{
+	mds_user_clear_cpu_buffers();
+}
+
 #undef pr_fmt
 #define pr_fmt(fmt)	"TAA: " fmt