[PATCH v4 1/3] x86/mce: Use is_copy_from_user() to determine copy-from-user context

[Shuai Xue]

Commit 4c132d1d844a ("x86/futex: Remove .fixup usage") introduced a new
extable fixup type, EX_TYPE_EFAULT_REG, and commit 4c132d1d844a
("x86/futex: Remove .fixup usage") updated the extable fixup type for
copy-from-user operations, changing it from EX_TYPE_UACCESS to
EX_TYPE_EFAULT_REG. The error context for copy-from-user operations no
longer functions as an in-kernel recovery context. Consequently, the error
context for copy-from-user operations no longer functions as an in-kernel
recovery context, resulting in kernel panics with the message: "Machine
check: Data load in unrecoverable area of kernel."

The critical aspect is identifying whether the error context involves a
read from user memory. We do not care about the ex-type if we know its a
MOV reading from userspace. is_copy_from_user() return true when both of
the following conditions are met:

    - the current instruction is copy
    - source address is user memory

So, use is_copy_from_user() to determin if a context is copy user directly.

Fixes: 4c132d1d844a ("x86/futex: Remove .fixup usage")
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Shuai Xue <xueshuai@xxxxxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/mce/severity.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/mce/severity.c b/arch/x86/kernel/cpu/mce/severity.c
index dac4d64dfb2a..2235a7477436 100644
--- a/arch/x86/kernel/cpu/mce/severity.c
+++ b/arch/x86/kernel/cpu/mce/severity.c
@@ -300,13 +300,12 @@ static noinstr int error_context(struct mce *m, struct pt_regs *regs)
 	copy_user  = is_copy_from_user(regs);
 	instrumentation_end();
 
-	switch (fixup_type) {
-	case EX_TYPE_UACCESS:
-		if (!copy_user)
-			return IN_KERNEL;
-		m->kflags |= MCE_IN_KERNEL_COPYIN;
-		fallthrough;
+	if (copy_user) {
+		m->kflags |= MCE_IN_KERNEL_COPYIN | MCE_IN_KERNEL_RECOV;
+		return IN_KERNEL_RECOV;
+	}
 
+	switch (fixup_type) {
 	case EX_TYPE_FAULT_MCE_SAFE:
 	case EX_TYPE_DEFAULT_MCE_SAFE:
 		m->kflags |= MCE_IN_KERNEL_RECOV;
-- 
2.39.3