futex: Don't include process MM in futex 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.

Fix 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@hefring.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: André Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/r/20231019204548.1236437-2-ben.wolsieffer@hefring.com

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index ade7c731972dd5c05fb69c8c7182a62e81b7cc01..52695c59d04114350d29f1bf0403c19b873663d6 100644 (file)
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -252,7 +252,17 @@ int get_futex_key(u32 __user *uaddr, unsigned int flags, 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;
        }