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Date:   Fri, 24 Nov 2023 18:43:56 +0100
From:   David Sterba <dsterba@suse.cz>
To:     Jann Horn <jannh@google.com>
Cc:     Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Alex Lyakas <alex.bolshoy.btrfs@gmail.com>,
        Alexander Block <ablock84@googlemail.com>,
        linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com
Subject: Re: [PATCH] btrfs: send: Ensure send_fd is writable
Message-ID: <20231124174356.GH18929@suse.cz>
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On Fri, Nov 24, 2023 at 05:48:31PM +0100, Jann Horn wrote:
> kernel_write() requires the caller to ensure that the file is writable.
> Let's do that directly after looking up the ->send_fd.
> 
> (We don't need a separate bailout path because the "out" path already
> does fput() if ->send_filp is non-NULL.)
> 
> This has no security impact for two reasons:
> 
>  - the ioctl requires CAP_SYS_ADMIN
>  - __kernel_write() bails out on read-only files - but only since 5.8,
>    see commit a01ac27be472 ("fs: check FMODE_WRITE in __kernel_write")
> 
> Reported-and-tested-by: syzbot+12e098239d20385264d3@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=12e098239d20385264d3
> Fixes: 31db9f7c23fb ("Btrfs: introduce BTRFS_IOC_SEND for btrfs send/receive")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jann Horn <jannh@google.com>

Added to misc-next, thanks.