Merge tag 'nativebhi' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 mitigations from Thomas Gleixner:
 "Mitigations for the native BHI hardware vulnerabilty:

  Branch History Injection (BHI) attacks may allow a malicious
  application to influence indirect branch prediction in kernel by
  poisoning the branch history. eIBRS isolates indirect branch targets
  in ring0. The BHB can still influence the choice of indirect branch
  predictor entry, and although branch predictor entries are isolated
  between modes when eIBRS is enabled, the BHB itself is not isolated
  between modes.

  Add mitigations against it either with the help of microcode or with
  software sequences for the affected CPUs"

[ This also ends up enabling the full mitigation by default despite the
  system call hardening, because apparently there are other indirect
  calls that are still sufficiently reachable, and the 'auto' case just
  isn't hardened enough.

  We'll have some more inevitable tweaking in the future    - Linus ]

* tag 'nativebhi' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  KVM: x86: Add BHI_NO
  x86/bhi: Mitigate KVM by default
  x86/bhi: Add BHI mitigation knob
  x86/bhi: Enumerate Branch History Injection (BHI) bug
  x86/bhi: Define SPEC_CTRL_BHI_DIS_S
  x86/bhi: Add support for clearing branch history at syscall entry
  x86/syscall: Don't force use of indirect calls for system calls
  x86/bugs: Change commas to semicolons in 'spectre_v2' sysfs file

Author: torvalds

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

@@ -138,11 +138,10 @@ associated with the source address of the indirect branch. Specifically,
 the BHB might be shared across privilege levels even in the presence of
 Enhanced IBRS.
 
-Currently the only known real-world BHB attack vector is via
-unprivileged eBPF. Therefore, it's highly recommended to not enable
-unprivileged eBPF, especially when eIBRS is used (without retpolines).
-For a full mitigation against BHB attacks, it's recommended to use
-retpolines (or eIBRS combined with retpolines).
+Previously the only known real-world BHB attack vector was via unprivileged
+eBPF. Further research has found attacks that don't require unprivileged eBPF.
+For a full mitigation against BHB attacks it is recommended to set BHI_DIS_S or
+use the BHB clearing sequence.
 
 Attack scenarios
 ----------------
@@ -430,6 +429,23 @@ The possible values in this file are:
   'PBRSB-eIBRS: Not affected'  CPU is not affected by PBRSB
   ===========================  =======================================================
 
+  - Branch History Injection (BHI) protection status:
+
+.. list-table::
+
+ * - BHI: Not affected
+   - System is not affected
+ * - BHI: Retpoline
+   - System is protected by retpoline
+ * - BHI: BHI_DIS_S
+   - System is protected by BHI_DIS_S
+ * - BHI: SW loop; KVM SW loop
+   - System is protected by software clearing sequence
+ * - BHI: Syscall hardening
+   - Syscalls are hardened against BHI
+ * - BHI: Syscall hardening; KVM: SW loop
+   - System is protected from userspace attacks by syscall hardening; KVM is protected by software clearing sequence
+
 Full mitigation might require a microcode update from the CPU
 vendor. When the necessary microcode is not available, the kernel will
 report vulnerability.
@@ -484,7 +500,11 @@ Spectre variant 2
 
    Systems which support enhanced IBRS (eIBRS) enable IBRS protection once at
    boot, by setting the IBRS bit, and they're automatically protected against
-   Spectre v2 variant attacks.
+   some Spectre v2 variant attacks. The BHB can still influence the choice of
+   indirect branch predictor entry, and although branch predictor entries are
+   isolated between modes when eIBRS is enabled, the BHB itself is not isolated
+   between modes. Systems which support BHI_DIS_S will set it to protect against
+   BHI attacks.
 
    On Intel's enhanced IBRS systems, this includes cross-thread branch target
    injections on SMT systems (STIBP). In other words, Intel eIBRS enables
@@ -638,6 +658,22 @@ kernel command line.
 		spectre_v2=off. Spectre variant 1 mitigations
 		cannot be disabled.
 
+	spectre_bhi=
+
+		[X86] Control mitigation of Branch History Injection
+		(BHI) vulnerability. Syscalls are hardened against BHI
+		regardless of this setting. This setting affects the deployment
+		of the HW BHI control and the SW BHB clearing sequence.
+
+		on
+			unconditionally enable.
+		off
+			unconditionally disable.
+		auto
+			enable if hardware mitigation
+			control(BHI_DIS_S) is available, otherwise
+			enable alternate mitigation in KVM.
+
 For spectre_v2_user see Documentation/admin-guide/kernel-parameters.txt
 
 Mitigation selection guide

Documentation/admin-guide/kernel-parameters.txt

@@ -6063,6 +6063,18 @@
 	sonypi.*=	[HW] Sony Programmable I/O Control Device driver
 			See Documentation/admin-guide/laptops/sonypi.rst
 
+	spectre_bhi=	[X86] Control mitigation of Branch History Injection
+			(BHI) vulnerability. Syscalls are hardened against BHI
+			reglardless of this setting. This setting affects the
+			deployment of the HW BHI control and the SW BHB
+			clearing sequence.
+
+			on   - unconditionally enable.
+			off  - unconditionally disable.
+			auto - (default) enable hardware mitigation
+			       (BHI_DIS_S) if available, otherwise enable
+			       alternate mitigation in KVM.
+
 	spectre_v2=	[X86,EARLY] Control mitigation of Spectre variant 2
 			(indirect branch speculation) vulnerability.
 			The default operation protects the kernel from

arch/x86/Kconfig

@@ -2633,6 +2633,32 @@ config MITIGATION_RFDS
 	  stored in floating point, vector and integer registers.
 	  See also <file:Documentation/admin-guide/hw-vuln/reg-file-data-sampling.rst>
 
+choice
+	prompt "Clear branch history"
+	depends on CPU_SUP_INTEL
+	default SPECTRE_BHI_ON
+	help
+	  Enable BHI mitigations. BHI attacks are a form of Spectre V2 attacks
+	  where the branch history buffer is poisoned to speculatively steer
+	  indirect branches.
+	  See <file:Documentation/admin-guide/hw-vuln/spectre.rst>
+
+config SPECTRE_BHI_ON
+	bool "on"
+	help
+	  Equivalent to setting spectre_bhi=on command line parameter.
+config SPECTRE_BHI_OFF
+	bool "off"
+	help
+	  Equivalent to setting spectre_bhi=off command line parameter.
+config SPECTRE_BHI_AUTO
+	bool "auto"
+	depends on BROKEN
+	help
+	  Equivalent to setting spectre_bhi=auto command line parameter.
+
+endchoice
+
 endif
 
 config ARCH_HAS_ADD_PAGES

arch/x86/entry/common.c

@@ -49,7 +49,7 @@ static __always_inline bool do_syscall_x64(struct pt_regs *regs, int nr)
 
 	if (likely(unr < NR_syscalls)) {
 		unr = array_index_nospec(unr, NR_syscalls);
-		regs->ax = sys_call_table[unr](regs);
+		regs->ax = x64_sys_call(regs, unr);
 		return true;
 	}
 	return false;
@@ -66,7 +66,7 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
 
 	if (IS_ENABLED(CONFIG_X86_X32_ABI) && likely(xnr < X32_NR_syscalls)) {
 		xnr = array_index_nospec(xnr, X32_NR_syscalls);
-		regs->ax = x32_sys_call_table[xnr](regs);
+		regs->ax = x32_sys_call(regs, xnr);
 		return true;
 	}
 	return false;
@@ -162,7 +162,7 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs, int nr)
 
 	if (likely(unr < IA32_NR_syscalls)) {
 		unr = array_index_nospec(unr, IA32_NR_syscalls);
-		regs->ax = ia32_sys_call_table[unr](regs);
+		regs->ax = ia32_sys_call(regs, unr);
 	} else if (nr != -1) {
 		regs->ax = __ia32_sys_ni_syscall(regs);
 	}
@@ -189,7 +189,7 @@ static __always_inline bool int80_is_external(void)
 }
 
 /**
- * int80_emulation - 32-bit legacy syscall entry
+ * do_int80_emulation - 32-bit legacy syscall C entry from asm
  *
  * This entry point can be used by 32-bit and 64-bit programs to perform
  * 32-bit system calls.  Instances of INT $0x80 can be found inline in
@@ -207,7 +207,7 @@ static __always_inline bool int80_is_external(void)
  *   eax:				system call number
  *   ebx, ecx, edx, esi, edi, ebp:	arg1 - arg 6
  */
-DEFINE_IDTENTRY_RAW(int80_emulation)
+__visible noinstr void do_int80_emulation(struct pt_regs *regs)
 {
 	int nr;
 

arch/x86/entry/entry_64.S

@@ -116,6 +116,7 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
 	/* clobbers %rax, make sure it is after saving the syscall nr */
 	IBRS_ENTER
 	UNTRAIN_RET
+	CLEAR_BRANCH_HISTORY
 
 	call	do_syscall_64		/* returns with IRQs disabled */
 
@@ -1491,3 +1492,63 @@ SYM_CODE_START_NOALIGN(rewind_stack_and_make_dead)
 	call	make_task_dead
 SYM_CODE_END(rewind_stack_and_make_dead)
 .popsection
+
+/*
+ * This sequence executes branches in order to remove user branch information
+ * from the branch history tracker in the Branch Predictor, therefore removing
+ * user influence on subsequent BTB lookups.
+ *
+ * It should be used on parts prior to Alder Lake. Newer parts should use the
+ * BHI_DIS_S hardware control instead. If a pre-Alder Lake part is being
+ * virtualized on newer hardware the VMM should protect against BHI attacks by
+ * setting BHI_DIS_S for the guests.
+ *
+ * CALLs/RETs are necessary to prevent Loop Stream Detector(LSD) from engaging
+ * and not clearing the branch history. The call tree looks like:
+ *
+ * call 1
+ *    call 2
+ *      call 2
+ *        call 2
+ *          call 2
+ * 	      call 2
+ * 	      ret
+ * 	    ret
+ *        ret
+ *      ret
+ *    ret
+ * ret
+ *
+ * This means that the stack is non-constant and ORC can't unwind it with %rsp
+ * alone.  Therefore we unconditionally set up the frame pointer, which allows
+ * ORC to unwind properly.
+ *
+ * The alignment is for performance and not for safety, and may be safely
+ * refactored in the future if needed.
+ */
+SYM_FUNC_START(clear_bhb_loop)
+	push	%rbp
+	mov	%rsp, %rbp
+	movl	$5, %ecx
+	ANNOTATE_INTRA_FUNCTION_CALL
+	call	1f
+	jmp	5f
+	.align 64, 0xcc
+	ANNOTATE_INTRA_FUNCTION_CALL
+1:	call	2f
+	RET
+	.align 64, 0xcc
+2:	movl	$5, %eax
+3:	jmp	4f
+	nop
+4:	sub	$1, %eax
+	jnz	3b
+	sub	$1, %ecx
+	jnz	1b
+	RET
+5:	lfence
+	pop	%rbp
+	RET
+SYM_FUNC_END(clear_bhb_loop)
+EXPORT_SYMBOL_GPL(clear_bhb_loop)
+STACK_FRAME_NON_STANDARD(clear_bhb_loop)

arch/x86/entry/entry_64_compat.S

@@ -92,6 +92,7 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
 
 	IBRS_ENTER
 	UNTRAIN_RET
+	CLEAR_BRANCH_HISTORY
 
 	/*
 	 * SYSENTER doesn't filter flags, so we need to clear NT and AC
@@ -206,6 +207,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_after_hwframe, SYM_L_GLOBAL)
 
 	IBRS_ENTER
 	UNTRAIN_RET
+	CLEAR_BRANCH_HISTORY
 
 	movq	%rsp, %rdi
 	call	do_fast_syscall_32
@@ -276,3 +278,17 @@ SYM_INNER_LABEL(entry_SYSRETL_compat_end, SYM_L_GLOBAL)
 	ANNOTATE_NOENDBR
 	int3
 SYM_CODE_END(entry_SYSCALL_compat)
+
+/*
+ * int 0x80 is used by 32 bit mode as a system call entry. Normally idt entries
+ * point to C routines, however since this is a system call interface the branch
+ * history needs to be scrubbed to protect against BHI attacks, and that
+ * scrubbing needs to take place in assembly code prior to entering any C
+ * routines.
+ */
+SYM_CODE_START(int80_emulation)
+	ANNOTATE_NOENDBR
+	UNWIND_HINT_FUNC
+	CLEAR_BRANCH_HISTORY
+	jmp do_int80_emulation
+SYM_CODE_END(int80_emulation)

arch/x86/entry/syscall_32.c

@@ -18,8 +18,25 @@
 #include <asm/syscalls_32.h>
 #undef __SYSCALL
 
+/*
+ * The sys_call_table[] is no longer used for system calls, but
+ * kernel/trace/trace_syscalls.c still wants to know the system
+ * call address.
+ */
+#ifdef CONFIG_X86_32
 #define __SYSCALL(nr, sym) __ia32_##sym,
-
-__visible const sys_call_ptr_t ia32_sys_call_table[] = {
+const sys_call_ptr_t sys_call_table[] = {
 #include <asm/syscalls_32.h>
 };
+#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) {
+	#include <asm/syscalls_32.h>
+	default: return __ia32_sys_ni_syscall(regs);
+	}
+};

arch/x86/entry/syscall_64.c

@@ -11,8 +11,23 @@
 #include <asm/syscalls_64.h>
 #undef __SYSCALL
 
+/*
+ * The sys_call_table[] is no longer used for system calls, but
+ * kernel/trace/trace_syscalls.c still wants to know the system
+ * call address.
+ */
 #define __SYSCALL(nr, sym) __x64_##sym,
-
-asmlinkage const sys_call_ptr_t sys_call_table[] = {
+const sys_call_ptr_t sys_call_table[] = {
 #include <asm/syscalls_64.h>
 };
+#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) {
+	#include <asm/syscalls_64.h>
+	default: return __x64_sys_ni_syscall(regs);
+	}
+};

arch/x86/entry/syscall_x32.c

@@ -11,8 +11,12 @@
 #include <asm/syscalls_x32.h>
 #undef __SYSCALL
 
-#define __SYSCALL(nr, sym) __x64_##sym,
+#define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs);
 
-asmlinkage const sys_call_ptr_t x32_sys_call_table[] = {
-#include <asm/syscalls_x32.h>
+long x32_sys_call(const struct pt_regs *regs, unsigned int nr)
+{
+	switch (nr) {
+	#include <asm/syscalls_x32.h>
+	default: return __x64_sys_ni_syscall(regs);
+	}
 };

arch/x86/include/asm/cpufeatures.h

@@ -461,11 +461,15 @@
 
 /*
  * Extended auxiliary flags: Linux defined - for features scattered in various
- * CPUID levels like 0x80000022, etc.
+ * CPUID levels like 0x80000022, etc and Linux defined features.
  *
  * Reuse free bits when adding new feature flags!
  */
 #define X86_FEATURE_AMD_LBR_PMC_FREEZE	(21*32+ 0) /* AMD LBR and PMC Freeze */
+#define X86_FEATURE_CLEAR_BHB_LOOP	(21*32+ 1) /* "" Clear branch history at syscall entry using SW loop */
+#define X86_FEATURE_BHI_CTRL		(21*32+ 2) /* "" BHI_DIS_S HW control available */
+#define X86_FEATURE_CLEAR_BHB_HW	(21*32+ 3) /* "" BHI_DIS_S HW control enabled */
+#define X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT (21*32+ 4) /* "" Clear branch history at vmexit using SW loop */
 
 /*
  * BUG word(s)
@@ -515,4 +519,5 @@
 #define X86_BUG_SRSO			X86_BUG(1*32 + 0) /* AMD SRSO bug */
 #define X86_BUG_DIV0			X86_BUG(1*32 + 1) /* AMD DIV0 speculation bug */
 #define X86_BUG_RFDS			X86_BUG(1*32 + 2) /* CPU is vulnerable to Register File Data Sampling */
+#define X86_BUG_BHI			X86_BUG(1*32 + 3) /* CPU is affected by Branch History Injection */
 #endif /* _ASM_X86_CPUFEATURES_H */

arch/x86/include/asm/msr-index.h

@@ -61,10 +61,13 @@
 #define SPEC_CTRL_SSBD			BIT(SPEC_CTRL_SSBD_SHIFT)	/* Speculative Store Bypass Disable */
 #define SPEC_CTRL_RRSBA_DIS_S_SHIFT	6	   /* Disable RRSBA behavior */
 #define SPEC_CTRL_RRSBA_DIS_S		BIT(SPEC_CTRL_RRSBA_DIS_S_SHIFT)
+#define SPEC_CTRL_BHI_DIS_S_SHIFT	10	   /* Disable Branch History Injection behavior */
+#define SPEC_CTRL_BHI_DIS_S		BIT(SPEC_CTRL_BHI_DIS_S_SHIFT)
 
 /* A mask for bits which the kernel toggles when controlling mitigations */
 #define SPEC_CTRL_MITIGATIONS_MASK	(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD \
-							| SPEC_CTRL_RRSBA_DIS_S)
+							| SPEC_CTRL_RRSBA_DIS_S \
+							| SPEC_CTRL_BHI_DIS_S)
 
 #define MSR_IA32_PRED_CMD		0x00000049 /* Prediction Command */
 #define PRED_CMD_IBPB			BIT(0)	   /* Indirect Branch Prediction Barrier */
@@ -163,6 +166,10 @@
 						 * are restricted to targets in
 						 * kernel.
 						 */
+#define ARCH_CAP_BHI_NO			BIT(20)	/*
+						 * CPU is not affected by Branch
+						 * History Injection.
+						 */
 #define ARCH_CAP_PBRSB_NO		BIT(24)	/*
 						 * Not susceptible to Post-Barrier
 						 * Return Stack Buffer Predictions.