[PATCH v1 3/4] x86/mce: add EX_TYPE_EFAULT_REG as in-kernel recovery context to fix copy-from-user operations regression

[Shuai Xue <xueshuai@xxxxxxxxxxxxxxxxx>]

Commit 4c132d1d844a ("x86/futex: Remove .fixup usage") introduced a new
extable fixup type, EX_TYPE_EFAULT_REG, and later patches updated the
extable fixup type for copy-from-user operations, changing it from
EX_TYPE_UACCESS to EX_TYPE_EFAULT_REG.

Specifically, commit 99641e094d6c ("x86/uaccess: Remove .fixup usage")
altered the extable fixup type for the get_user family, while commit
4c132d1d844a ("x86/futex: Remove .fixup usage") addressed the futex
operations. This change inadvertently caused a regression where the error
context for some copy-from-user operations no longer functions as an
in-kernel recovery context, leading to kernel panics with the message:
"Machine check: Data load in unrecoverable area of kernel."

To fix the regression, add EX_TYPE_EFAULT_REG as a in-kernel recovery
context for copy-from-user operations.

Signed-off-by: Shuai Xue <xueshuai@xxxxxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/mce/severity.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cpu/mce/severity.c b/arch/x86/kernel/cpu/mce/severity.c
index dac4d64dfb2a..14c2d71c3ce1 100644
--- a/arch/x86/kernel/cpu/mce/severity.c
+++ b/arch/x86/kernel/cpu/mce/severity.c
@@ -16,6 +16,7 @@
 #include <asm/traps.h>
 #include <asm/insn.h>
 #include <asm/insn-eval.h>
+#include <linux/extable.h>
 
 #include "internal.h"
 
@@ -285,7 +286,8 @@ static bool is_copy_from_user(struct pt_regs *regs)
  */
 static noinstr int error_context(struct mce *m, struct pt_regs *regs)
 {
-	int fixup_type;
+	const struct exception_table_entry *e;
+	int fixup_type, imm;
 	bool copy_user;
 
 	if ((m->cs & 3) == 3)
@@ -294,9 +296,14 @@ static noinstr int error_context(struct mce *m, struct pt_regs *regs)
 	if (!mc_recoverable(m->mcgstatus))
 		return IN_KERNEL;
 
+	e = search_exception_tables(m->ip);
+	if (!e)
+		return IN_KERNEL;
+
 	/* Allow instrumentation around external facilities usage. */
 	instrumentation_begin();
-	fixup_type = ex_get_fixup_type(m->ip);
+	fixup_type = FIELD_GET(EX_DATA_TYPE_MASK, e->data);
+	imm  = FIELD_GET(EX_DATA_IMM_MASK,  e->data);
 	copy_user  = is_copy_from_user(regs);
 	instrumentation_end();
 
@@ -304,9 +311,13 @@ static noinstr int error_context(struct mce *m, struct pt_regs *regs)
 	case EX_TYPE_UACCESS:
 		if (!copy_user)
 			return IN_KERNEL;
-		m->kflags |= MCE_IN_KERNEL_COPYIN;
-		fallthrough;
-
+		m->kflags |= MCE_IN_KERNEL_COPYIN | MCE_IN_KERNEL_RECOV;
+		return IN_KERNEL_RECOV;
+	case EX_TYPE_IMM_REG:
+		if (!copy_user || imm != -EFAULT)
+			return IN_KERNEL;
+		m->kflags |= MCE_IN_KERNEL_COPYIN | MCE_IN_KERNEL_RECOV;
+		return IN_KERNEL_RECOV;
 	case EX_TYPE_FAULT_MCE_SAFE:
 	case EX_TYPE_DEFAULT_MCE_SAFE:
 		m->kflags |= MCE_IN_KERNEL_RECOV;
-- 
2.39.3