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futex: Don't include process MM in futex key on no-MMU

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[ Upstream commit c73801ae4f ]

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: 784bdf3bb6 ("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
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Ben Wolsieffer 2023-10-19 16:45:49 -04:00 committed by Greg Kroah-Hartman
parent 3c1a20c122
commit dce53a017c
1 changed files with 11 additions and 1 deletions
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12
kernel/futex/core.c
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@ -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;
}