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Date:   Tue, 30 Apr 2019 18:03:03 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH] io_uring: avoid page allocation warnings

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
> > GFP_KERNEL.
> 
> 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?

Thanks,
Mark.

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