[PATCH v4] padata: Fix refcnt handling in padata_free_shell()

From: WangJinchao
Date: Sun Oct 15 2023 - 21:16:03 EST


In a high-load arm64 environment, the pcrypt_aead01 test in LTP can lead
to system UAF (Use-After-Free) issues. Due to the lengthy analysis of
the pcrypt_aead01 function call, I'll describe the problem scenario
using a simplified model:

Suppose there's a user of padata named `user_function` that adheres to
the padata requirement of calling `padata_free_shell` after `serial()`
has been invoked, as demonstrated in the following code:

```c
struct request {
struct padata_priv padata;
struct completion *done;
};

void parallel(struct padata_priv *padata) {
do_something();
}

void serial(struct padata_priv *padata) {
struct request *request = container_of(padata,
struct request,
padata);
complete(request->done);
}

void user_function() {
DECLARE_COMPLETION(done)
padata->parallel = parallel;
padata->serial = serial;
padata_do_parallel();
wait_for_completion(&done);
padata_free_shell();
}
```

In the corresponding padata.c file, there's the following code:

```c
static void padata_serial_worker(struct work_struct *serial_work) {
...
cnt = 0;

while (!list_empty(&local_list)) {
...
padata->serial(padata);
cnt++;
}

local_bh_enable();

if (refcount_sub_and_test(cnt, &pd->refcnt))
padata_free_pd(pd);
}
```

Because of the high system load and the accumulation of unexecuted
softirq at this moment, `local_bh_enable()` in padata takes longer
to execute than usual. Subsequently, when accessing `pd->refcnt`,
`pd` has already been released by `padata_free_shell()`, resulting
in a UAF issue with `pd->refcnt`.

The fix is straightforward: add `refcount_dec_and_test` before calling
`padata_free_pd` in `padata_free_shell`.

Fixes: 07928d9bfc81 ("padata: Remove broken queue flushing")

Signed-off-by: WangJinchao <wangjinchao@xxxxxxxxxxx>
Acked-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
---
V4:
Included Daniel's ack
Included Herbert's ack
V3: https://lore.kernel.org/all/ZSDWAcUxXcwD4YUZ@fedora/
Included Daniel's ack
introduced wrong patch
V2: https://lore.kernel.org/all/ZRTLHY5A+VqIKhA2@fedora/
To satisfy Sparse, use rcu_dereference_protected.
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202309270829.xHgTOMKw-lkp@xxxxxxxxx/

V1: https://lore.kernel.org/all/ZRE4XvOOhz4HSOgR@fedora/
kernel/padata.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/padata.c b/kernel/padata.c
index 222d60195de6..73108ac75f03 100644
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -1102,12 +1102,15 @@ EXPORT_SYMBOL(padata_alloc_shell);
*/
void padata_free_shell(struct padata_shell *ps)
{
+ struct parallel_data *pd;
if (!ps)
return;

mutex_lock(&ps->pinst->lock);
list_del(&ps->list);
- padata_free_pd(rcu_dereference_protected(ps->pd, 1));
+ pd = rcu_dereference_protected(ps->pd, 1);
+ if (refcount_dec_and_test(&pd->refcnt))
+ padata_free_pd(pd);
mutex_unlock(&ps->pinst->lock);

kfree(ps);
--
2.40.0