Date:   Fri, 26 Nov 2021 17:42:52 +0100
From:   David Sterba <dsterba@suse.cz>
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will@kernel.org>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware
 with sub-page faults
Message-ID: <20211126164252.GB28560@suse.cz>
Reply-To: dsterba@suse.cz
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        Catalin Marinas <catalin.marinas@arm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Josef Bacik <josef@toxicpanda.com>, David Sterba <dsterba@suse.com>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will@kernel.org>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-btrfs@vger.kernel.org
References: <20211124192024.2408218-1-catalin.marinas@arm.com>
 <20211124192024.2408218-4-catalin.marinas@arm.com>
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On Wed, Nov 24, 2021 at 07:20:24PM +0000, Catalin Marinas wrote:
> Commit a48b73eca4ce ("btrfs: fix potential deadlock in the search
> ioctl") addressed a lockdep warning by pre-faulting the user pages and
> attempting the copy_to_user_nofault() in an infinite loop. On
> architectures like arm64 with MTE, an access may fault within a page at
> a location different from what fault_in_writeable() probed. Since the
> sk_offset is rewound to the previous struct btrfs_ioctl_search_header
> boundary, there is no guaranteed forward progress and search_ioctl() may
> live-lock.
> 
> Use fault_in_exact_writeable() instead which probes the entire user
> buffer for faults at sub-page granularity.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Reported-by: Al Viro <viro@zeniv.linux.org.uk>

Acked-by: David Sterba <dsterba@suse.com>