6.5-stable review patch. If anyone has any objections, please let me know.
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From: Ben Wolsieffer ben.wolsieffer@hefring.com
[ Upstream commit c73801ae4f22b390228ebf471d55668e824198b6 ]
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.co... Signed-off-by: Sasha Levin sashal@kernel.org --- 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 514e4582b8634..d4141b0547187 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; }