Re: [RFC 2/2] prctl: PR_SET_MM -- Introduce PR_SET_MM_MAP operation

[Cyrill Gorcunov]

On Thu, Jul 03, 2014 at 06:33:20PM +0400, Cyrill Gorcunov wrote:
> During development of c/r we've noticed that in case if we need to
> support user namespaces we face a problem with capabilities in
> prctl(PR_SET_MM, ...) call.
> 
> Current PR_SET_MM code forbids to modify fields if no CAP_SYS_RESOURCE
> granted, but rather relies on one who use this interface is passing
> more-less sane values (though the values must pass the basic validation
> procedure).
> 
> It seems a better approach is to eliminate CAP_SYS_RESOURCE check but
> provide all new values in one bundle, which would allow the kernel to make
> more intensive test for sanity of values and same time allow us to
> support checkpoint/restore of user namespaces.
> 
> Thus a new command (PR_SET_MM_MAP) introduced. It takes a pointer of
> prctl_mm_map structure which carries all members to be updated.
> 
> Most intensive work is done in validate_prctl_map_locked helper,
> because we need to make sure the values are valid. Thus we do
> 
>  - check the values are laying inside available user address space
>  - stack, brk, command line arguments and evnironment variables
>    must point to already existing VMA
>  - values must be ordered (start < end)
>  - if RLIMITs are defined don't allow to exceed it with new values
> 
> Since it uses prctl_set_mm_exe_file_locked helper, updating the
> exe-file link is one-shot action for security reason.
> 
> I believe the old interface should be deprecated and ripped off
> in a couple of kernel releases if noone against.
> 
> To test if new interface is implemented in the kernel one
> can pass PR_SET_MM_MAP_SIZE opcode and the kernel returns
> the size of currently supported struct prctl_mm_map.
> 
> Signed-off-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andrew Vagin <avagin@xxxxxxxxxx>
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>
> Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> Cc: Vasiliy Kulikov <segoon@xxxxxxxxxxxx>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>

Ping. Guys, any commens please?
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