Merge tag 'x86_bugs_saferet' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

 - Add a mitigation for the attack vector of interrupting the saferet
   sequence used in the SRSO mitigation and still poisoning the RSB.

   Do that by emulating the saferet sequence and thus avoiding executing
   a RET instruction.

* tag 'x86_bugs_saferet' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/bugs: Make Safe-RET robust against interrupt injection

4 files changed, 123 insertions, 1 deletions

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index c6d996593f32..253f0a585927 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -937,6 +937,8 @@ SYM_CODE_START(paranoid_entry)
 	IBRS_ENTER save_reg=%r15
 	UNTRAIN_RET_FROM_CALL
 
+	HANDLE_INTR_SAFERET 8(%rsp)
+
 	RET
 SYM_CODE_END(paranoid_entry)
 
@@ -1039,6 +1041,11 @@ SYM_CODE_START(error_entry)
 	movl	%ecx, %eax			/* zero extend */
 	cmpq	%rax, RIP+8(%rsp)
 	je	.Lbstep_iret
+
+	VALIDATE_UNRET_END
+
+	HANDLE_INTR_SAFERET 8(%rsp)
+
 	cmpq	$.Lgs_change, RIP+8(%rsp)
 	jne	.Lerror_entry_done_lfence
 
@@ -1057,7 +1064,6 @@ SYM_CODE_START(error_entry)
 	FENCE_SWAPGS_KERNEL_ENTRY
 	CALL_DEPTH_ACCOUNT
 	leaq	8(%rsp), %rax			/* return pt_regs pointer */
-	VALIDATE_UNRET_END
 	RET
 
 .Lbstep_iret:
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index b68892e6d58c..2ea6591bc7b9 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -12,6 +12,7 @@
 #include <asm/msr-index.h>
 #include <asm/unwind_hints.h>
 #include <asm/percpu.h>
+#include <asm/ptrace-abi.h>
 
 /*
  * Call depth tracking for Intel SKL CPUs to address the RSB underflow
@@ -176,6 +177,50 @@
 	add	$(BITS_PER_LONG/8), %_ASM_SP;		\
 	lfence;
 
+/*
+ * Helper for detecting if an interrupt occurred at an unsafe location within
+ * Safe-RET.  If Safe-RET is interrupted after the CALL or LEA the RSB may get
+ * poisoned by the interrupt handler.
+ *
+ * The Safe-RET sequence is:
+ *
+ * CALL
+ * LEA 8(%RSP), %RSP
+ * RET
+ *
+ * The two CMPs below check whether RIP points to after the CALL or after the
+ * LEA.
+ *
+ * The LFENCE below is to address this particular speculation case:
+ *
+ * 1. Userspace runs and poisons the BTB around the safe-RET routine
+ *
+ * 2. Userspace triggers some kind of exception
+ *
+ * 3. Kernel executes error_entry() and mis-speculates the branch into thinking
+ *    it actually came from kernel space
+ *
+ * 4. The kernel then further mis-speculates that the exception occurred due
+ *    to an interrupted safe-RET
+ *
+ * 5. The handle_interrupted_saferet() routine speculatively executes and
+ *    speculatively does a safe-RET. But this is unsafe since it was never
+ *    untrained.
+ *
+ * The LFENCE fixes this by ensuring step 5 is never reached speculatively.
+ * Note that this LFENCE only occurs if safe-RET was actually interrupted (so
+ * it's outside of the normal path).
+ */
+#define __HANDLE_INTR_SAFERET(name, pt_regs)		\
+	cmpq	$(name), RIP+pt_regs;			\
+	jb	1f;					\
+	cmpq	$(name)+5, RIP+pt_regs;			\
+	ja	1f;					\
+	lfence;						\
+	leaq	pt_regs, %rdi;				\
+	call	handle_interrupted_saferet;		\
+	1:
+
 #ifdef __ASSEMBLER__
 
 /*
@@ -293,6 +338,14 @@
 #define UNTRAIN_RET_FROM_CALL \
 	__UNTRAIN_RET X86_FEATURE_ENTRY_IBPB, __stringify(RESET_CALL_DEPTH_FROM_CALL)
 
+.macro HANDLE_INTR_SAFERET pt_regs
+#ifdef CONFIG_MITIGATION_SRSO
+	ALTERNATIVE_2 "", \
+	__stringify(__HANDLE_INTR_SAFERET(srso_safe_ret, \pt_regs)), X86_FEATURE_SRSO, \
+	__stringify(__HANDLE_INTR_SAFERET(srso_alias_safe_ret, \pt_regs)), X86_FEATURE_SRSO_ALIAS
+
+#endif
+.endm
 
 .macro CALL_DEPTH_ACCOUNT
 #ifdef CONFIG_MITIGATION_CALL_DEPTH_TRACKING
@@ -625,6 +678,10 @@ static __always_inline void x86_idle_clear_cpu_buffers(void)
 		x86_clear_cpu_buffers();
 }
 
+void srso_safe_ret(void);
+void srso_alias_safe_ret(void);
+void handle_interrupted_saferet(struct pt_regs *regs);
+
 #endif /* __ASSEMBLER__ */
 
 #endif /* _ASM_X86_NOSPEC_BRANCH_H_ */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index d9af230c0512..82436b3534fa 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -3775,3 +3775,42 @@ void __warn_thunk(void)
 {
 	WARN_ONCE(1, "Unpatched return thunk in use. This should not happen!\n");
 }
+
+#ifdef CONFIG_MITIGATION_SRSO
+/*
+ * Called during exception/interrupt entry if interrupted during the
+ * safe-RET sequence.  The safe-RET sequence consists of 3 instructions:
+ *
+ *	CALL
+ *	LEA 8(%RSP), %RSP
+ *	RET
+ *
+ * An interrupt after the CALL or after the LEA could potentially lead
+ * to branch predictor poisoning and results in the sequence not being
+ * able to be safely resumed.
+ *
+ * Therefore, modify the regs state as if the remaining part of the
+ * safe-RET sequence executed so the interrupt returns back to the
+ * desired return target, instead of the to the safe-RET sequence.
+ */
+void noinstr handle_interrupted_saferet(struct pt_regs *regs)
+{
+	unsigned long rip = regs->ip;
+
+	if (rip == (unsigned long) srso_safe_ret ||
+	    rip == (unsigned long) srso_alias_safe_ret) {
+	    /* Modify stack pointer as if LEA executed: */
+	    regs->sp += 8;
+	}
+
+	/*
+	 * Adjust registers as if RET executed:
+	 *
+	 * 1. Read the return address off the stack and into rIP:
+	 */
+	regs->ip = *(unsigned long *)(regs->sp);
+
+	/* 2. Pop rIP off the stack: */
+	regs->sp += 8;
+}
+#endif /* CONFIG_MITIGATION_SRSO */
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 8f1fed0c3b83..f9ca1d8eabca 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -207,10 +207,24 @@ __EXPORT_THUNK(srso_alias_untrain_ret)
 
 	.pushsection .text..__x86.rethunk_safe
 SYM_CODE_START_NOALIGN(srso_alias_safe_ret)
+
+	/*
+	 * Tell objtool that those are not function pointers referenced by
+	 * __HANDLE_INTR_SAFERET(). Below too.
+	 */
+	ANNOTATE_NOENDBR
+
+	/*
+	 * Safe-RET sequence. If you need to change it, adjust
+	 * handle_interrupted_saferet() too.
+	 */
 	lea 8(%_ASM_SP), %_ASM_SP
 	UNWIND_HINT_FUNC
+
+	ANNOTATE_NOENDBR
 	ANNOTATE_UNRET_SAFE
 	ret
+	/* End of Safe-RET sequence */
 	int3
 SYM_FUNC_END(srso_alias_safe_ret)
 
@@ -245,8 +259,14 @@ SYM_CODE_START_LOCAL_NOALIGN(srso_untrain_ret)
  * the stack.
  */
 SYM_INNER_LABEL(srso_safe_ret, SYM_L_GLOBAL)
+	/*
+	 * Safe-RET sequence. If you need to change it, adjust
+	 * handle_interrupted_saferet() too.
+	 */
 	lea 8(%_ASM_SP), %_ASM_SP
 	ret
+	/* End of Safe-RET sequence */
+
 	int3
 	int3
 	/* end of movabs */