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Date:   Tue, 31 Aug 2021 17:01:58 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        cluster-devel <cluster-devel@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "ocfs2-devel@oss.oracle.com" <ocfs2-devel@oss.oracle.com>,
        Josef Bacik <josef@toxicpanda.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [RFC][arm64] possible infinite loop in btrfs search_ioctl()
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On Tue, Aug 31, 2021 at 03:28:57PM +0000, Al Viro wrote:
> On Tue, Aug 31, 2021 at 02:54:50PM +0100, Catalin Marinas wrote:
> 
> > An arm64-specific workaround would be for pagefault_disable() to disable
> > tag checking. It's a pretty big hammer, weakening the out of bounds
> > access detection of MTE. My preference would be a fix in the btrfs code.
> > 
> > A btrfs option would be for copy_to_sk() to return an indication of
> > where the fault occurred and get fault_in_pages_writeable() to check
> > that location, even if the copying would restart from an earlier offset
> > (this requires open-coding copy_to_user_nofault()). An attempt below,
> > untested and does not cover read_extent_buffer_to_user_nofault():
> 
> Umm...  There's another copy_to_user_nofault() call in the same function
> (same story, AFAICS).

Yeah, I was too lazy to do it all and I don't have a setup to test the
patch quickly either. BTW, my hack is missing an access_ok() check.

I wonder whether copy_{to,from}_user_nofault() could actually return the
number of bytes left to copy, just like their non-nofault counterparts.
These are only used in a few places, so fairly easy to change. If we go
for a btrfs fix along the lines of my diff, it saves us from duplicating
the copy_to_user_nofault() code.

-- 
Catalin