linux-kernel - Re: [BUG] arm64: an infinite loop in generic_perform

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Message-ID: <YNRpYli/5/GWvaTT@casper.infradead.org>
Date:   Thu, 24 Jun 2021 12:15:46 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Chen Huang <chenhuang5@...wei.com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Al Viro <viro@...iv.linux.org.uk>,
        Randy Dunlap <rdunlap@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-mm <linux-mm@...ck.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] arm64: an infinite loop in generic_perform_write()

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.