Messages in this thread | | | Date | Thu, 17 Nov 2011 12:48:31 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH v2 0/4] Checkpoint/Restore: Show in proc IDs of objects that can be shared between tasks |
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On Thu, 17 Nov 2011 13:55:33 +0400 Pavel Emelyanov <xemul@parallels.com> wrote:
> While doing the checkpoint-restore in the userspace one need to determine > whether various kernel objects (like mm_struct-s of file_struct-s) are shared > between tasks and restore this state. > > The 2nd step can for now be solved by using respective CLONE_XXX flags and > the unshare syscall, while there's currently no ways for solving the 1st one. > > One of the ways for checking whether two tasks share e.g. an mm_struct is to > provide some mm_struct ID of a task to its proc file. The best from the > performance point of view ID is the object address in the kernel, but showing > them to the userspace is not good for security reasons. > > Thus the object address is XOR-ed with a "random" value of the same size and > then shown in proc. Providing this poison is not leaked into the userspace then > ID seem to be safe. The objects for which the IDs are shown are: > > * all namespaces living in /proc/pid/ns/ > * open files (shown in /proc/pid/fdinfo/) > * objects, that can be shared with CLONE_XXX flags (except for namespaces) > > Changes since > v1: * Tejun worried about the single poison value was a weak side - leaking one > makes all the IDs vulnerable. To address this several poison values - one > per object type - are introduced. They are stored in a plain array. Tejun, > is this enough from your POV, or you'd like to see them widely scattered > over the memory? > * Pekka proposed to initialized poison values in the late_initcall callback > * ... and move the code to mm/util.c > > Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
It doesn't *sound* terribly secure. There might be clever ways in which userspace can determine the secret mask, dunno. We should ask evil-minded security people to review this proposal.
Why not simply use a sequence number, increment it each time we create an mm_struct? On could use an idr tree to prevent duplicates but it would be simpler and sufficient to make it 64-bit and we never have to worry about wraparound causing duplicates.
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