Re: [PATCH] prctl: remove one-shot limitation for changing exe link

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Tue, Jul 12, 2016 at 07:30:29PM +0400, Stanislav Kinsburskiy wrote:
> This limitation came with the reason to remove "another
> way for malicious code to obscure a compromised program and
> masquerade as a benign process" by allowing "security-concious program can use
> this prctl once during its early initialization to ensure the prctl cannot
> later be abused for this purpose":
> 
> http://marc.info/?l=linux-kernel&m=133160684517468&w=2
> 
> But the way how the feature can be used is the following:
> 
> 1) Attach to process via ptrace (protected by CAP_SYS_PTRACE)
> 2) Unmap all the process file mappings, related to "exe" file.
> 3) Change exe link (protected by CAP_SYS_RESOURCE).
> 
> IOW, some other process already has an access to process internals (and thus
> it's already compromised), and can inject fork and use the child of the
> compromised program to masquerade.
> Which means this limitation doesn't solve the problem it was aimed to.
> 
> While removing this limitation allow to replace files from underneath of a
> running process as many times as required. One of the use cases is network
> file systems migration (NFS, to be precise) by CRIU.
> 
> NFS mount can't be mounted on restore stage because network is locked.
> To overcome this limitation, another file system (FUSE-based) is used. Then
> opened files replaced by the proper ones NFS is remounted.
> Thus exe link replace has to be done twice: first on restore stage and second
> - when actual NFS was remounted.
> 
> Signed-off-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>

Persistent exe-link doesn't guarantee anything if you have rights to ptrace
task and inject own code into (from security POV). So lets rip it out.

Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>