Re: [PATCH v2] devtmpfs: mount with noexec and nosuid

[Kees Cook]

On Tue, Nov 20, 2012 at 1:13 PM, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 20 Nov 2012 12:42:38 -0800
> Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
>> Since devtmpfs is writable, make the default noexec,nosuid as well. This
>> protects from the case of a privileged process having an arbitrary file
>> write flaw and an argumentless arbitrary execution (i.e. it would lack
>> the ability to run "mount -o remount,exec,suid /dev").
>
> Ok this looks crap on two levels.
>
> 1. Why not just have your userspace mount -o remount the file system this
> way already in early boot. (and if you trojanned boot that early then any
> supposed security gain is already lost)

That's certainly possible, but I am hoping to avoid adding any extra
boot time. The kernel is responsible for this mount, so its flags
should be configurable, resulting in no time penalty anywhere.

> 2. If you want to do this right then you need to work out what you are
> trying to prevent. Your devtmpfs can force file permissions on the
> underlying device nodes by having its own operation handling for chmod.
>
> At that point you can force permissions on anything that you want to
> avoid floating around that filesystem with other rights, while not
> touching it on device or directory nodes where the meaning is different.
>
> In its current form however it appears to be a kernel implementation of
> "mount is too hard".

This change also stops mmap() with PROT_EXEC which a chmod handler
wouldn't be able to do. The noexec and nosuid mount options were
designed for this sort of thing, so I think that's where it should be
handled.

-Kees

-- 
Kees Cook
Chrome OS Security
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