Merge tag 'x86_urgent_for_v6.5_rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fixes from Borislav Petkov:

 - AMD's automatic IBRS doesn't enable cross-thread branch target
   injection protection (STIBP) for user processes. Enable STIBP on such
   systems.

 - Do not delete (but put the ref instead) of AMD MCE error thresholding
   sysfs kobjects when destroying them in order not to delete the kernfs
   pointer prematurely

 - Restore annotation in ret_from_fork_asm() in order to fix kthread
   stack unwinding from being marked as unreliable and thus breaking
   livepatching

* tag 'x86_urgent_for_v6.5_rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/cpu: Enable STIBP on AMD if Automatic IBRS is enabled
  x86/MCE/AMD: Decrement threshold_bank refcount when removing threshold blocks
  x86: Fix kthread unwind

3 files changed, 26 insertions, 9 deletions

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 91f6818884fa..43606de22511 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -285,7 +285,15 @@ SYM_FUNC_END(__switch_to_asm)
  */
 .pushsection .text, "ax"
 SYM_CODE_START(ret_from_fork_asm)
-	UNWIND_HINT_REGS
+	/*
+	 * This is the start of the kernel stack; even through there's a
+	 * register set at the top, the regset isn't necessarily coherent
+	 * (consider kthreads) and one cannot unwind further.
+	 *
+	 * This ensures stack unwinds of kernel threads terminate in a known
+	 * good state.
+	 */
+	UNWIND_HINT_END_OF_STACK
 	ANNOTATE_NOENDBR // copy_thread
 	CALL_DEPTH_ACCOUNT
 
@@ -295,6 +303,12 @@ SYM_CODE_START(ret_from_fork_asm)
 	movq	%r12, %rcx		/* fn_arg */
 	call	ret_from_fork
 
+	/*
+	 * Set the stack state to what is expected for the target function
+	 * -- at this point the register set should be a valid user set
+	 * and unwind should work normally.
+	 */
+	UNWIND_HINT_REGS
 	jmp	swapgs_restore_regs_and_return_to_usermode
 SYM_CODE_END(ret_from_fork_asm)
 .popsection
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9e2a91830f72..95507448e781 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1150,19 +1150,21 @@ spectre_v2_user_select_mitigation(void)
 	}
 
 	/*
-	 * If no STIBP, enhanced IBRS is enabled, or SMT impossible, STIBP
+	 * If no STIBP, Intel enhanced IBRS is enabled, or SMT impossible, STIBP
 	 * is not required.
 	 *
-	 * Enhanced IBRS also protects against cross-thread branch target
+	 * Intel's Enhanced IBRS also protects against cross-thread branch target
 	 * injection in user-mode as the IBRS bit remains always set which
 	 * implicitly enables cross-thread protections.  However, in legacy IBRS
 	 * mode, the IBRS bit is set only on kernel entry and cleared on return
-	 * to userspace. This disables the implicit cross-thread protection,
-	 * so allow for STIBP to be selected in that case.
+	 * to userspace.  AMD Automatic IBRS also does not protect userspace.
+	 * These modes therefore disable the implicit cross-thread protection,
+	 * so allow for STIBP to be selected in those cases.
 	 */
 	if (!boot_cpu_has(X86_FEATURE_STIBP) ||
 	    !smt_possible ||
-	    spectre_v2_in_eibrs_mode(spectre_v2_enabled))
+	    (spectre_v2_in_eibrs_mode(spectre_v2_enabled) &&
+	     !boot_cpu_has(X86_FEATURE_AUTOIBRS)))
 		return;
 
 	/*
@@ -2294,7 +2296,8 @@ static ssize_t mmio_stale_data_show_state(char *buf)
 
 static char *stibp_state(void)
 {
-	if (spectre_v2_in_eibrs_mode(spectre_v2_enabled))
+	if (spectre_v2_in_eibrs_mode(spectre_v2_enabled) &&
+	    !boot_cpu_has(X86_FEATURE_AUTOIBRS))
 		return "";
 
 	switch (spectre_v2_user_stibp) {
diff --git a/arch/x86/kernel/cpu/mce/amd.c b/arch/x86/kernel/cpu/mce/amd.c
index 5e74610b39e7..c4ec4ca47e11 100644
--- a/arch/x86/kernel/cpu/mce/amd.c
+++ b/arch/x86/kernel/cpu/mce/amd.c
@@ -1261,10 +1261,10 @@ static void __threshold_remove_blocks(struct threshold_bank *b)
 	struct threshold_block *pos = NULL;
 	struct threshold_block *tmp = NULL;
 
-	kobject_del(b->kobj);
+	kobject_put(b->kobj);
 
 	list_for_each_entry_safe(pos, tmp, &b->blocks->miscj, miscj)
-		kobject_del(&pos->kobj);
+		kobject_put(b->kobj);
 }
 
 static void threshold_remove_bank(struct threshold_bank *bank)