Merge tag 'x86_bugs_for_v6.11_rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 cpu mitigation updates from Borislav Petkov:

 - Add a spectre_bhi=vmexit mitigation option aimed at cloud
   environments

 - Remove duplicated Spectre cmdline option documentation

 - Add separate macro definitions for syscall handlers which do not
   return in order to address objtool warnings

* tag 'x86_bugs_for_v6.11_rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/bugs: Add 'spectre_bhi=vmexit' cmdline option
  x86/bugs: Remove duplicate Spectre cmdline option descriptions
  x86/syscall: Mark exit[_group] syscall handlers __noreturn

Author: torvalds

Documentation/admin-guide/hw-vuln/spectre.rst

@@ -592,85 +592,19 @@ Spectre variant 2
 Mitigation control on the kernel command line
 ---------------------------------------------
 
-Spectre variant 2 mitigation can be disabled or force enabled at the
-kernel command line.
+In general the kernel selects reasonable default mitigations for the
+current CPU.
 
-	nospectre_v1
+Spectre default mitigations can be disabled or changed at the kernel
+command line with the following options:
 
-		[X86,PPC] Disable mitigations for Spectre Variant 1
-		(bounds check bypass). With this option data leaks are
-		possible in the system.
+	- nospectre_v1
+	- nospectre_v2
+	- spectre_v2={option}
+	- spectre_v2_user={option}
+	- spectre_bhi={option}
 
-	nospectre_v2
-
-		[X86] Disable all mitigations for the Spectre variant 2
-		(indirect branch prediction) vulnerability. System may
-		allow data leaks with this option, which is equivalent
-		to spectre_v2=off.
-
-
-        spectre_v2=
-
-		[X86] Control mitigation of Spectre variant 2
-		(indirect branch speculation) vulnerability.
-		The default operation protects the kernel from
-		user space attacks.
-
-		on
-			unconditionally enable, implies
-			spectre_v2_user=on
-		off
-			unconditionally disable, implies
-		        spectre_v2_user=off
-		auto
-			kernel detects whether your CPU model is
-		        vulnerable
-
-		Selecting 'on' will, and 'auto' may, choose a
-		mitigation method at run time according to the
-		CPU, the available microcode, the setting of the
-		CONFIG_MITIGATION_RETPOLINE configuration option,
-		and the compiler with which the kernel was built.
-
-		Selecting 'on' will also enable the mitigation
-		against user space to user space task attacks.
-
-		Selecting 'off' will disable both the kernel and
-		the user space protections.
-
-		Specific mitigations can also be selected manually:
-
-                retpoline               auto pick between generic,lfence
-                retpoline,generic       Retpolines
-                retpoline,lfence        LFENCE; indirect branch
-                retpoline,amd           alias for retpoline,lfence
-                eibrs                   Enhanced/Auto IBRS
-                eibrs,retpoline         Enhanced/Auto IBRS + Retpolines
-                eibrs,lfence            Enhanced/Auto IBRS + LFENCE
-                ibrs                    use IBRS to protect kernel
-
-		Not specifying this option is equivalent to
-		spectre_v2=auto.
-
-		In general the kernel by default selects
-		reasonable mitigations for the current CPU. To
-		disable Spectre variant 2 mitigations, boot with
-		spectre_v2=off. Spectre variant 1 mitigations
-		cannot be disabled.
-
-	spectre_bhi=
-
-		[X86] Control mitigation of Branch History Injection
-		(BHI) vulnerability.  This setting affects the deployment
-		of the HW BHI control and the SW BHB clearing sequence.
-
-		on
-			(default) Enable the HW or SW mitigation as
-			needed.
-		off
-			Disable the mitigation.
-
-For spectre_v2_user see Documentation/admin-guide/kernel-parameters.txt
+For more details on the available options, refer to Documentation/admin-guide/kernel-parameters.txt
 
 Mitigation selection guide
 --------------------------

Documentation/admin-guide/kernel-parameters.txt

@@ -6125,9 +6125,15 @@
 			deployment of the HW BHI control and the SW BHB
 			clearing sequence.
 
-			on   - (default) Enable the HW or SW mitigation
-			       as needed.
-			off  - Disable the mitigation.
+			on     - (default) Enable the HW or SW mitigation as
+				 needed.  This protects the kernel from
+				 both syscalls and VMs.
+			vmexit - On systems which don't have the HW mitigation
+				 available, enable the SW mitigation on vmexit
+				 ONLY.  On such systems, the host kernel is
+				 protected from VM-originated BHI attacks, but
+				 may still be vulnerable to syscall attacks.
+			off    - Disable the mitigation.
 
 	spectre_v2=	[X86,EARLY] Control mitigation of Spectre variant 2
 			(indirect branch speculation) vulnerability.

arch/x86/entry/syscall_32.c

@@ -14,9 +14,12 @@
 #endif
 
 #define __SYSCALL(nr, sym) extern long __ia32_##sym(const struct pt_regs *);
-
+#define __SYSCALL_NORETURN(nr, sym) extern long __noreturn __ia32_##sym(const struct pt_regs *);
 #include <asm/syscalls_32.h>
-#undef __SYSCALL
+#undef  __SYSCALL
+
+#undef  __SYSCALL_NORETURN
+#define __SYSCALL_NORETURN __SYSCALL
 
 /*
  * The sys_call_table[] is no longer used for system calls, but
@@ -28,11 +31,10 @@
 const sys_call_ptr_t sys_call_table[] = {
 #include <asm/syscalls_32.h>
 };
-#undef __SYSCALL
+#undef  __SYSCALL
 #endif
 
 #define __SYSCALL(nr, sym) case nr: return __ia32_##sym(regs);
-
 long ia32_sys_call(const struct pt_regs *regs, unsigned int nr)
 {
 	switch (nr) {

arch/x86/entry/syscall_64.c

@@ -8,8 +8,12 @@
 #include <asm/syscall.h>
 
 #define __SYSCALL(nr, sym) extern long __x64_##sym(const struct pt_regs *);
+#define __SYSCALL_NORETURN(nr, sym) extern long __noreturn __x64_##sym(const struct pt_regs *);
 #include <asm/syscalls_64.h>
-#undef __SYSCALL
+#undef  __SYSCALL
+
+#undef  __SYSCALL_NORETURN
+#define __SYSCALL_NORETURN __SYSCALL
 
 /*
  * The sys_call_table[] is no longer used for system calls, but
@@ -20,10 +24,9 @@
 const sys_call_ptr_t sys_call_table[] = {
 #include <asm/syscalls_64.h>
 };
-#undef __SYSCALL
+#undef  __SYSCALL
 
 #define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs);
-
 long x64_sys_call(const struct pt_regs *regs, unsigned int nr)
 {
 	switch (nr) {

arch/x86/entry/syscall_x32.c

@@ -8,11 +8,14 @@
 #include <asm/syscall.h>
 
 #define __SYSCALL(nr, sym) extern long __x64_##sym(const struct pt_regs *);
+#define __SYSCALL_NORETURN(nr, sym) extern long __noreturn __x64_##sym(const struct pt_regs *);
 #include <asm/syscalls_x32.h>
-#undef __SYSCALL
+#undef  __SYSCALL
 
-#define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs);
+#undef  __SYSCALL_NORETURN
+#define __SYSCALL_NORETURN __SYSCALL
 
+#define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs);
 long x32_sys_call(const struct pt_regs *regs, unsigned int nr)
 {
 	switch (nr) {

arch/x86/entry/syscalls/syscall_32.tbl

@@ -3,7 +3,7 @@
 # 32-bit system call numbers and entry vectors
 #
 # The format is:
-# <number> <abi> <name> <entry point> <compat entry point>
+# <number> <abi> <name> <entry point> [<compat entry point> [noreturn]]
 #
 # The __ia32_sys and __ia32_compat_sys stubs are created on-the-fly for
 # sys_*() system calls and compat_sys_*() compat system calls if
@@ -13,7 +13,7 @@
 # The abi is always "i386" for this file.
 #
 0	i386	restart_syscall		sys_restart_syscall
-1	i386	exit			sys_exit
+1	i386	exit			sys_exit			-			noreturn
 2	i386	fork			sys_fork
 3	i386	read			sys_read
 4	i386	write			sys_write
@@ -264,7 +264,7 @@
 249	i386	io_cancel		sys_io_cancel
 250	i386	fadvise64		sys_ia32_fadvise64
 # 251 is available for reuse (was briefly sys_set_zone_reclaim)
-252	i386	exit_group		sys_exit_group
+252	i386	exit_group		sys_exit_group			-			noreturn
 253	i386	lookup_dcookie
 254	i386	epoll_create		sys_epoll_create
 255	i386	epoll_ctl		sys_epoll_ctl

arch/x86/entry/syscalls/syscall_64.tbl

@@ -3,7 +3,7 @@
 # 64-bit system call numbers and entry vectors
 #
 # The format is:
-# <number> <abi> <name> <entry point>
+# <number> <abi> <name> <entry point> [<compat entry point> [noreturn]]
 #
 # The __x64_sys_*() stubs are created on-the-fly for sys_*() system calls
 #
@@ -69,7 +69,7 @@
 57	common	fork			sys_fork
 58	common	vfork			sys_vfork
 59	64	execve			sys_execve
-60	common	exit			sys_exit
+60	common	exit			sys_exit			-			noreturn
 61	common	wait4			sys_wait4
 62	common	kill			sys_kill
 63	common	uname			sys_newuname
@@ -240,7 +240,7 @@
 228	common	clock_gettime		sys_clock_gettime
 229	common	clock_getres		sys_clock_getres
 230	common	clock_nanosleep		sys_clock_nanosleep
-231	common	exit_group		sys_exit_group
+231	common	exit_group		sys_exit_group			-			noreturn
 232	common	epoll_wait		sys_epoll_wait
 233	common	epoll_ctl		sys_epoll_ctl
 234	common	tgkill			sys_tgkill

arch/x86/kernel/cpu/bugs.c

@@ -1625,6 +1625,7 @@ static bool __init spec_ctrl_bhi_dis(void)
 enum bhi_mitigations {
 	BHI_MITIGATION_OFF,
 	BHI_MITIGATION_ON,
+	BHI_MITIGATION_VMEXIT_ONLY,
 };
 
 static enum bhi_mitigations bhi_mitigation __ro_after_init =
@@ -1639,6 +1640,8 @@ static int __init spectre_bhi_parse_cmdline(char *str)
 		bhi_mitigation = BHI_MITIGATION_OFF;
 	else if (!strcmp(str, "on"))
 		bhi_mitigation = BHI_MITIGATION_ON;
+	else if (!strcmp(str, "vmexit"))
+		bhi_mitigation = BHI_MITIGATION_VMEXIT_ONLY;
 	else
 		pr_err("Ignoring unknown spectre_bhi option (%s)", str);
 
@@ -1659,19 +1662,22 @@ static void __init bhi_select_mitigation(void)
 			return;
 	}
 
+	/* Mitigate in hardware if supported */
 	if (spec_ctrl_bhi_dis())
 		return;
 
 	if (!IS_ENABLED(CONFIG_X86_64))
 		return;
 
-	/* Mitigate KVM by default */
-	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
-	pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit\n");
+	if (bhi_mitigation == BHI_MITIGATION_VMEXIT_ONLY) {
+		pr_info("Spectre BHI mitigation: SW BHB clearing on VM exit only\n");
+		setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
+		return;
+	}
 
-	/* Mitigate syscalls when the mitigation is forced =on */
+	pr_info("Spectre BHI mitigation: SW BHB clearing on syscall and VM exit\n");
 	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);
-	pr_info("Spectre BHI mitigation: SW BHB clearing on syscall\n");
+	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
 }
 
 static void __init spectre_v2_select_mitigation(void)

arch/x86/um/sys_call_table_32.c

@@ -9,6 +9,10 @@
 #include <linux/cache.h>
 #include <asm/syscall.h>
 
+extern asmlinkage long sys_ni_syscall(unsigned long, unsigned long,
+				      unsigned long, unsigned long,
+				      unsigned long, unsigned long);
+
 /*
  * Below you can see, in terms of #define's, the differences between the x86-64
  * and the UML syscall table.
@@ -22,15 +26,13 @@
 #define sys_vm86 sys_ni_syscall
 
 #define __SYSCALL_WITH_COMPAT(nr, native, compat)	__SYSCALL(nr, native)
+#define __SYSCALL_NORETURN __SYSCALL
 
 #define __SYSCALL(nr, sym) extern asmlinkage long sym(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);
 #include <asm/syscalls_32.h>
+#undef  __SYSCALL
 
-#undef __SYSCALL
 #define __SYSCALL(nr, sym) sym,
-
-extern asmlinkage long sys_ni_syscall(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);
-
 const sys_call_ptr_t sys_call_table[] ____cacheline_aligned = {
 #include <asm/syscalls_32.h>
 };

arch/x86/um/sys_call_table_64.c

@@ -9,6 +9,10 @@
 #include <linux/cache.h>
 #include <asm/syscall.h>
 
+extern asmlinkage long sys_ni_syscall(unsigned long, unsigned long,
+				      unsigned long, unsigned long,
+				      unsigned long, unsigned long);
+
 /*
  * Below you can see, in terms of #define's, the differences between the x86-64
  * and the UML syscall table.
@@ -18,14 +22,13 @@
 #define sys_iopl sys_ni_syscall
 #define sys_ioperm sys_ni_syscall
 
+#define __SYSCALL_NORETURN __SYSCALL
+
 #define __SYSCALL(nr, sym) extern asmlinkage long sym(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);
 #include <asm/syscalls_64.h>
+#undef  __SYSCALL
 
-#undef __SYSCALL
 #define __SYSCALL(nr, sym) sym,
-
-extern asmlinkage long sys_ni_syscall(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);
-
 const sys_call_ptr_t sys_call_table[] ____cacheline_aligned = {
 #include <asm/syscalls_64.h>
 };