linux-kernel - [PATCH] futex: Don't include process in key on no-MMU

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Message-ID: <20231019204548.1236437-2-ben.wolsieffer@hefring.com>
Date:   Thu, 19 Oct 2023 16:45:49 -0400
From:   Ben Wolsieffer <ben.wolsieffer@...ring.com>
To:     linux-kernel@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        André Almeida <andrealmeid@...lia.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ben Wolsieffer <ben.wolsieffer@...ring.com>
Subject: [PATCH] futex: Don't include process in key on no-MMU

On no-MMU, all futexes are treated as private because there is no need
to map a virtual address to physical to match the futex across
processes. This doesn't quite work though, because private futexes
include the current process's mm_struct as part of their key. This makes
it impossible for one process to wake up a shared futex being waited on
in another process.

This patch fixes this bug by excluding the mm_struct from the key. With
a single address space, the futex address is already a unique key.

Fixes: 784bdf3bb694 ("futex: Assume all mappings are private on !MMU systems")
Signed-off-by: Ben Wolsieffer <ben.wolsieffer@...ring.com>
---
 kernel/futex/core.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 514e4582b863..d4141b054718 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -248,7 +248,17 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
 	 *        but access_ok() should be faster than find_vma()
 	 */
 	if (!fshared) {
-		key->private.mm = mm;
+		/*
+		 * On no-MMU, shared futexes are treated as private, therefore
+		 * we must not include the current process in the key. Since
+		 * there is only one address space, the address is a unique key
+		 * on its own.
+		 */
+		if (IS_ENABLED(CONFIG_MMU))
+			key->private.mm = mm;
+		else
+			key->private.mm = NULL;
+
 		key->private.address = address;
 		return 0;
 	}
-- 
2.42.0