5.10-stable review patch. If anyone has any objections, please let me know.
------------------
From: Ben Wolsieffer <ben.wolsieffer@xxxxxxxxxxx>
[ 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@xxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Darren Hart <dvhart@xxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: André Almeida <andrealmeid@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20231019204548.1236437-2-ben.wolsieffer@xxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
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 8dd0bc50ac36d..cde0ca876b935 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -514,7 +514,17 @@ static 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
