From: Andy Lutomirski <luto@kernel.org>

When decoding an instruction or handling a perf event that references an
LDT segment, if we don't have a valid user context, trying to access the
LDT by any means other than SLDT is racy.  Certainly, using
current->active_mm is wrong, as active_mm can point to a real user mm when
CR3 and LDTR no longer reference that mm.

Clean up the code.  If nmi_uaccess_okay() says we don't have a valid
context, just fail.  Otherwise use current->mm.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/d456e7da9dbd271aacd14812d4b9b74e7d7edd52.1641659630.git.luto@kernel.org
---
 arch/x86/events/core.c   |    9 ++++++++-
 arch/x86/lib/insn-eval.c |   13 ++++++++++---
 2 files changed, 18 insertions(+), 4 deletions(-)

--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2798,8 +2798,15 @@ static unsigned long get_segment_base(un
 #ifdef CONFIG_MODIFY_LDT_SYSCALL
 		struct ldt_struct *ldt;
 
+		/*
+		 * If we're not in a valid context with a real (not just lazy)
+		 * user mm, then don't even try.
+		 */
+		if (!nmi_uaccess_okay())
+			return 0;
+
 		/* IRQs are off, so this synchronizes with smp_store_release */
-		ldt = READ_ONCE(current->active_mm->context.ldt);
+		ldt = smp_load_acquire(&current->mm->context.ldt);
 		if (!ldt || idx >= ldt->nr_entries)
 			return 0;
 
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -631,14 +631,21 @@ static bool get_desc(struct desc_struct
 		/* Bits [15:3] contain the index of the desired entry. */
 		sel >>= 3;
 
-		mutex_lock(&current->active_mm->context.lock);
-		ldt = current->active_mm->context.ldt;
+		/*
+		 * If we're not in a valid context with a real (not just lazy)
+		 * user mm, then don't even try.
+		 */
+		if (!nmi_uaccess_okay())
+			return false;
+
+		mutex_lock(&current->mm->context.lock);
+		ldt = current->mm->context.ldt;
 		if (ldt && sel < ldt->nr_entries) {
 			*out = ldt->entries[sel];
 			success = true;
 		}
 
-		mutex_unlock(&current->active_mm->context.lock);
+		mutex_unlock(&current->mm->context.lock);
 
 		return success;
 	}