linux-kernel - Re: [RFC][arm64] possible infinite loop in btrfs search

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Message-ID: <YS5KudP4DBwlbPEp@zeniv-ca.linux.org.uk>
Date:   Tue, 31 Aug 2021 15:28:57 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>,
        Josef Bacik <josef@...icpanda.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [RFC][arm64] possible infinite loop in btrfs search_ioctl()

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).

Can't say I'm fond of their ABI, but then I guess it could've been worse -
iterating over btree, running a user-supplied chunk of INTERCAL over it,
with all details of internal representation cast in stone by that exposure...