Re: [PATCH] io_uring: avoid page allocation warnings

From: Jens Axboe
Date: Wed May 01 2019 - 08:42:01 EST

On 5/1/19 4:30 AM, Mark Rutland wrote:
> On Tue, Apr 30, 2019 at 12:11:59PM -0600, Jens Axboe wrote:
>> On 4/30/19 11:03 AM, Mark Rutland wrote:
>>> On Tue, Apr 30, 2019 at 10:21:03AM -0600, Jens Axboe wrote:
>>>> On 4/30/19 8:59 AM, Mark Rutland wrote:
>>>>> On Tue, Apr 30, 2019 at 07:18:10AM -0700, Matthew Wilcox wrote:
>>>>>> On Tue, Apr 30, 2019 at 02:24:05PM +0100, Mark Rutland wrote:
>>>>>>> In io_sqe_buffer_register() we allocate a number of arrays based on the
>>>>>>> iov_len from the user-provided iov. While we limit iov_len to SZ_1G,
>>>>>>> we can still attempt to allocate arrays exceeding MAX_ORDER.
>>>>>>> On a 64-bit system with 4KiB pages, for an iov where iov_base = 0x10 and
>>>>>>> iov_len = SZ_1G, we'll calculate that nr_pages = 262145. When we try to
>>>>>>> allocate a corresponding array of (16-byte) bio_vecs, requiring 4194320
>>>>>>> bytes, which is greater than 4MiB. This results in SLUB warning that
>>>>>>> we're trying to allocate greater than MAX_ORDER, and failing the
>>>>>>> allocation.
>>>>>>> Avoid this by passing __GFP_NOWARN when allocating arrays for the
>>>>>>> user-provided iov_len. We'll gracefully handle the failed allocation,
>>>>>>> returning -ENOMEM to userspace.
>>>>>>> We should probably consider lowering the limit below SZ_1G, or reworking
>>>>>>> the array allocations.
>>>>>> I'd suggest that kvmalloc is probably our friend here ... we don't really
>>>>>> want to return -ENOMEM to userspace for this case, I don't think.
>>>>> Sure. I'll go verify that the uring code doesn't assume this memory is
>>>>> physically contiguous.
>>>>> I also guess we should be passing GFP_KERNEL_ACCOUNT rateh than a plain
>>>> kvmalloc() is fine, the io_uring code doesn't care about the layout of
>>>> the memory, it just uses it as an index.
>>> I've just had a go at that, but when using kvmalloc() with or without
>>> GFP_KERNEL_ACCOUNT I hit OOM and my system hangs within a few seconds with the
>>> syzkaller prog below:
>>> ----
>>> Syzkaller reproducer:
>>> # {Threaded:false Collide:false Repeat:false RepeatTimes:0 Procs:1 Sandbox: Fault:false FaultCall:-1 FaultNth:0 EnableTun:false EnableNetDev:false EnableNetReset:false EnableCgroups:false EnableBinfmtMisc:false EnableCloseFds:false UseTmpDir:false HandleSegv:false Repro:false Trace:false}
>>> r0 = io_uring_setup(0x378, &(0x7f00000000c0))
>>> sendmsg$SEG6_CMD_SET_TUNSRC(0xffffffffffffffff, &(0x7f0000000240)={&(0x7f0000000000)={0x10, 0x0, 0x0, 0x40000000}, 0xc, 0x0, 0x1, 0x0, 0x0, 0x10}, 0x800)
>>> io_uring_register$IORING_REGISTER_BUFFERS(r0, 0x0, &(0x7f0000000000), 0x1)
>>> ----
>>> ... I'm a bit worried that opens up a trivial DoS.
>>> Thoughts?
>> Can you post the patch you used?
> Diff below.

And the reproducer, that was never posted. Patch looks fine to me. Note
that buffer registration is under the protection of RLIMIT_MEMLOCK.
That's usually very limited for non-root, as root you can of course
consume as much as you want and OOM the system.

Jens Axboe