Commit c73801ae authored Oct 19, 2023 by Ben Wolsieffer Committed by Ingo Molnar Oct 27, 2023

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: 784bdf3b ("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

parent 184fdf9f

kernel/futex/core.c

+11 −1

Original line number	Diff line number	Diff line
		@@ -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) {
		/*
		* 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;
		}