linux-kernel - [tip: x86/alternatives] x86/events, x86/insn-eval: Remove incorrect current->active

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Message-ID: <174448361789.31282.10406016673269368609.tip-bot2@tip-bot2>
Date: Sat, 12 Apr 2025 18:46:57 -0000
From: "tip-bot2 for Andy Lutomirski" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Andy Lutomirski <luto@...nel.org>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...riel.com>,
 "H. Peter Anvin" <hpa@...or.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: x86/alternatives] x86/events, x86/insn-eval: Remove incorrect
 current->active_mm references

The following commit has been merged into the x86/alternatives branch of tip:

Commit-ID:     81e3cbdef230fd9adfa8569044b07290afd66708
Gitweb:        https://git.kernel.org/tip/81e3cbdef230fd9adfa8569044b07290afd66708
Author:        Andy Lutomirski <luto@...nel.org>
AuthorDate:    Wed, 02 Apr 2025 11:45:35 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Sat, 12 Apr 2025 10:05:46 +02:00

x86/events, x86/insn-eval: Remove incorrect current->active_mm references

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@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Rik van Riel <riel@...riel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Link: https://lore.kernel.org/r/20250402094540.3586683-3-mingo@kernel.org
---
 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 6866cc5..95118b5 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2803,8 +2803,15 @@ 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 98631c0..f786401 100644
--- 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 *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;
 	}