[PATCH 10/23] x86/events, x86/insn-eval: Remove incorrect active_mm references

[Andy Lutomirski <luto@xxxxxxxxxx>]

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.

Cc: Joerg Roedel <jroedel@xxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/events/core.c   |  9 ++++++++-
 arch/x86/lib/insn-eval.c | 13 ++++++++++---
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 6dfa8ddaa60f..930082f0eba5 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2800,8 +2800,15 @@ static unsigned long get_segment_base(unsigned int segment)
 #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;
 
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index a1d24fdc07cf..87a85a9dcdc4 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -609,14 +609,21 @@ static bool get_desc(struct desc_struct *out, unsigned short sel)
 		/* 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;
 	}
-- 
2.33.1