Re: [PATCH] io_uring/rsrc: fix RLIMIT_MEMLOCK bypass via compound page accounting

[Pavel Begunkov <asml.silence@gmail.com>]

On 1/9/26 03:02, Yuhao Jiang wrote:
> Hi Jens, Pavel, and all,
> 
> Just a gentle follow-up on this patch below.
> Please let me know if there are any concerns or if changes are needed.

I'm pretty this will break with buffer sharing / cloning. I'd
be tempted to remove all this cross buffer accounting logic
and overestimate it, the current accounting is not sane.
Otherwise, it'll likely need some proxy object shared b/w
buffers or some other overly overcomplicated solution.


> Thanks for your time.
> 
> Best regards,
> Yuhao Jiang
> 
> On Wed, Dec 17, 2025 at 9:00 PM Yuhao Jiang <danisjiang@gmail.com> wrote:
>>
>> When multiple registered buffers share the same compound page, only the
>> first buffer accounts for the memory via io_buffer_account_pin(). The
>> subsequent buffers skip accounting since headpage_already_acct() returns
>> true.
>>
>> When the first buffer is unregistered, the accounting is decremented,
>> but the compound page remains pinned by the remaining buffers. This
>> creates a state where pinned memory is not properly accounted against
>> RLIMIT_MEMLOCK.
>>
>> On systems with HugeTLB pages pre-allocated, an unprivileged user can
>> exploit this to pin memory beyond RLIMIT_MEMLOCK by cycling buffer
>> registrations. The bypass amount is proportional to the number of
>> available huge pages, potentially allowing gigabytes of memory to be
>> pinned while the kernel accounting shows near-zero.
>>
>> Fix this by recalculating the actual pages to unaccount when unmapping
>> a buffer. For regular pages, always unaccount. For compound pages, only
>> unaccount if no other registered buffer references the same compound
>> page. This ensures the accounting persists until the last buffer
>> referencing the compound page is released.
>>
>> Reported-by: Yuhao Jiang <danisjiang@gmail.com>
>> Fixes: 57bebf807e2a ("io_uring/rsrc: optimise registered huge pages")

That's not the right commit, the accounting is ancient, should
get blamed somewhere around first commits that added registered
buffers.

-- 
Pavel Begunkov