[PATCH 2/7] x86/events, x86/insn-eval: Remove incorrect active_mm references
From: Peter Zijlstra
Date: Tue Nov 19 2024 - 11:33:52 EST
From: 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.
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/d456e7da9dbd271aacd14812d4b9b74e7d7edd52.1641659630.git.luto@xxxxxxxxxx
---
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(¤t->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(¤t->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(¤t->mm->context.lock);
+ ldt = current->mm->context.ldt;
if (ldt && sel < ldt->nr_entries) {
*out = ldt->entries[sel];
success = true;
}
- mutex_unlock(¤t->active_mm->context.lock);
+ mutex_unlock(¤t->mm->context.lock);
return success;
}