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futex: Move futex_hash_free() back to __mmput()

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To avoid a memory leak via mm_alloc() + mmdrop() the futex cleanup code
has been moved to __mmdrop(). This resulted in a warnings if the futex
hash table has been allocated via vmalloc() the mmdrop() was invoked
from atomic context.
The free path must stay in __mmput() to ensure it is invoked from
preemptible context.

In order to avoid the memory leak, delay the allocation of
mm_struct::mm->futex_ref to futex_hash_allocate(). This works because
neither the per-CPU counter nor the private hash has been allocated and
therefore
- futex_private_hash() callers (such as exit_pi_state_list()) don't
  acquire reference if there is no private hash yet. There is also no
  reference put.

- Regular callers (futex_hash()) fallback to global hash. No reference
  counting here.

The futex_ref member can be allocated in futex_hash_allocate() before
the private hash itself is allocated. This happens either while the
first thread is created or on request. In both cases the process has
just a single thread so there can be either futex operation in progress
or the request to create a private hash.

Move futex_hash_free() back to __mmput();
Move the allocation of mm_struct::futex_ref to futex_hash_allocate().

  [ bp: Fold a follow-up fix to prevent a use-after-free:
    https://lore.kernel.org/r/20250830213806.sEKuuGSm@linutronix.de ]

Fixes:  e703b7e247 ("futex: Move futex cleanup to __mmdrop()")
Closes: https://lore.kernel.org/all/20250821102721.6deae493@kernel.org/
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lkml.kernel.org/r/20250822141238.PfnkTjFb@linutronix.de
This commit is contained in:
Sebastian Andrzej Siewior 2025-08-22 16:12:38 +02:00 • committed by Borislav Petkov (AMD)
parent 1b237f190e
commit d9b05321e2
2 changed files with 13 additions and 5 deletions
Download patch file Download diff file Expand all files Collapse all files

2
kernel/fork.c
View file

@ -689,7 +689,6 @@ void __mmdrop(struct mm_struct *mm)
mm_pasid_drop(mm);
mm_destroy_cid(mm);
percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
futex_hash_free(mm);
free_mm(mm);
}
@ -1138,6 +1137,7 @@ static inline void __mmput(struct mm_struct *mm)
if (mm->binfmt)
module_put(mm->binfmt->module);
lru_gen_del_mm(mm);
futex_hash_free(mm);
mmdrop(mm);
}

16
kernel/futex/core.c
View file

@ -1722,12 +1722,9 @@ int futex_mm_init(struct mm_struct *mm)
RCU_INIT_POINTER(mm->futex_phash, NULL);
mm->futex_phash_new = NULL;
/* futex-ref */
mm->futex_ref = NULL;
atomic_long_set(&mm->futex_atomic, 0);
mm->futex_batches = get_state_synchronize_rcu();
mm->futex_ref = alloc_percpu(unsigned int);
if (!mm->futex_ref)
return -ENOMEM;
this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
return 0;
}
@ -1801,6 +1798,17 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
}
}
if (!mm->futex_ref) {
/*
* This will always be allocated by the first thread and
* therefore requires no locking.
*/
mm->futex_ref = alloc_percpu(unsigned int);
if (!mm->futex_ref)
return -ENOMEM;
this_cpu_inc(*mm->futex_ref); /* 0 -> 1 */
}
fph = kvzalloc(struct_size(fph, queues, hash_slots),
GFP_KERNEL_ACCOUNT | __GFP_NOWARN);
if (!fph)