[PATCH v5 28/34] x86/fred: fixup fault on ERETU by jumping to fred_entrypoint_user

[Xin Li]

If the stack frame contains an invalid user context (e.g. due to invalid SS,
a non-canonical RIP, etc.) the ERETU instruction will trap (#SS or #GP).

>From a Linux point of view, this really should be considered a user space
failure, so use the standard fault fixup mechanism to intercept the fault,
fix up the exception frame, and redirect execution to fred_entrypoint_user.
The end result is that it appears just as if the hardware had taken the
exception immediately after completing the transition to user space.

Suggested-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
---
 arch/x86/entry/entry_64_fred.S             |  8 +++++--
 arch/x86/include/asm/extable_fixup_types.h |  4 +++-
 arch/x86/mm/extable.c                      | 28 ++++++++++++++++++++++
 3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
index 1fb765fd3871..027ef8f1e600 100644
--- a/arch/x86/entry/entry_64_fred.S
+++ b/arch/x86/entry/entry_64_fred.S
@@ -5,8 +5,10 @@
  * The actual FRED entry points.
  */
 #include <linux/linkage.h>
-#include <asm/errno.h>
+#include <asm/asm.h>
 #include <asm/asm-offsets.h>
+#include <asm/errno.h>
+#include <asm/export.h>
 #include <asm/fred.h>
 
 #include "calling.h"
@@ -38,7 +40,9 @@ SYM_CODE_START_NOALIGN(fred_entrypoint_user)
 	call	fred_entry_from_user
 SYM_INNER_LABEL(fred_exit_user, SYM_L_GLOBAL)
 	FRED_EXIT
-	ERETU
+1:	ERETU
+
+	_ASM_EXTABLE_TYPE(1b, fred_entrypoint_user, EX_TYPE_ERETU)
 SYM_CODE_END(fred_entrypoint_user)
 
 /*
diff --git a/arch/x86/include/asm/extable_fixup_types.h b/arch/x86/include/asm/extable_fixup_types.h
index 991e31cfde94..1585c798a02f 100644
--- a/arch/x86/include/asm/extable_fixup_types.h
+++ b/arch/x86/include/asm/extable_fixup_types.h
@@ -64,6 +64,8 @@
 #define	EX_TYPE_UCOPY_LEN4		(EX_TYPE_UCOPY_LEN | EX_DATA_IMM(4))
 #define	EX_TYPE_UCOPY_LEN8		(EX_TYPE_UCOPY_LEN | EX_DATA_IMM(8))
 
-#define EX_TYPE_ZEROPAD			20 /* longword load with zeropad on fault */
+#define	EX_TYPE_ZEROPAD			20 /* longword load with zeropad on fault */
+
+#define	EX_TYPE_ERETU			21
 
 #endif
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 60814e110a54..88a2c419ce8b 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -6,6 +6,7 @@
 #include <xen/xen.h>
 
 #include <asm/fpu/api.h>
+#include <asm/fred.h>
 #include <asm/sev.h>
 #include <asm/traps.h>
 #include <asm/kdebug.h>
@@ -195,6 +196,29 @@ static bool ex_handler_ucopy_len(const struct exception_table_entry *fixup,
 	return ex_handler_uaccess(fixup, regs, trapnr);
 }
 
+#ifdef CONFIG_X86_FRED
+static bool ex_handler_eretu(const struct exception_table_entry *fixup,
+			     struct pt_regs *regs, unsigned long error_code)
+{
+	struct pt_regs *uregs = (struct pt_regs *)(regs->sp - offsetof(struct pt_regs, ip));
+	unsigned short ss = uregs->ss;
+	unsigned short cs = uregs->cs;
+
+	fred_info(uregs)->edata = fred_event_data(regs);
+	uregs->ssx = regs->ssx;
+	uregs->ss = ss;
+	uregs->csx = regs->csx;
+	uregs->current_stack_level = 0;
+	uregs->cs = cs;
+
+	/* Copy error code to uregs and adjust stack pointer accordingly */
+	uregs->orig_ax = error_code;
+	regs->sp -= 8;
+
+	return ex_handler_default(fixup, regs);
+}
+#endif
+
 int ex_get_fixup_type(unsigned long ip)
 {
 	const struct exception_table_entry *e = search_exception_tables(ip);
@@ -272,6 +296,10 @@ int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 		return ex_handler_ucopy_len(e, regs, trapnr, reg, imm);
 	case EX_TYPE_ZEROPAD:
 		return ex_handler_zeropad(e, regs, fault_addr);
+#ifdef CONFIG_X86_FRED
+	case EX_TYPE_ERETU:
+		return ex_handler_eretu(e, regs, error_code);
+#endif
 	}
 	BUG();
 }
-- 
2.34.1