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Date:   Thu, 24 Jun 2021 12:15:46 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Chen Huang <chenhuang5@huawei.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Randy Dunlap <rdunlap@infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-mm <linux-mm@kvack.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] arm64: an infinite loop in generic_perform_write()
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References: <da9c2fa9-a545-0c48-4490-d6134cc31425@huawei.com>
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On Thu, Jun 24, 2021 at 08:04:07AM +0100, Christoph Hellwig wrote:
> On Thu, Jun 24, 2021 at 04:24:46AM +0100, Matthew Wilcox wrote:
> > On Thu, Jun 24, 2021 at 11:10:41AM +0800, Chen Huang wrote:
> > > In userspace, I perform such operation:
> > > 
> > >  	fd = open("/tmp/test", O_RDWR | O_SYNC);
> > >         access_address = (char *)mmap(NULL, uio_size, PROT_READ, MAP_SHARED, uio_fd, 0);
> > >         ret = write(fd, access_address + 2, sizeof(long));
> > 
> > ... you know that accessing this at unaligned offsets isn't going to
> > work.  It's completely meaningless.  Why are you trying to do it?
> 
> We still should not cause an infinite loop in kernel space due to a
> a userspace programmer error.

They're running as root and they've mapped some device memory.  We can't
save them from themself.  Imagine if they'd done this to the NVMe BAR.