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SubjectRe: [patch 1/2] c/r: prctl: Add ability to set new mm_struct::exe_file
On Mon, Mar 19, 2012 at 03:15:07PM -0700, Andrew Morton wrote:
...
> >
> > Also this action is one-shot only. For security reason
> > we don't allow to change the symlink several times.
>
> What is this mysterious "security reason"?
>

Oh, sorry I should have included Matt's comment here

| Before this patch that state was rather ephemeral and almost entirely
| under the control of the kernel. The only way userspace could change it
| was by unmapping the region(s) mapped during exec*(). At that point it
| could not "lie" and insert some other symlink there and the admin would
| be better able to determine what had happened.
|
| With this patch -- especially the multi-shot form -- the symlink will
| be entirely under the control of (potentially untrusted) userspace code
| and the admin is totally at the mercy of the userspace code. In
| single-shot form programs could use the prctl() to ensure the symlink
| could not be changed later -- the restart tool would be the only program
| that would need to ensure that prctl() had not been used since the last
| exec*().
...
>
> It should be explained here also. The comment is pretty useless - if
> we don't tell people what this "security reason" is, how can future
> developers be sure that they aren't violating it?
>

Actually I liked multi-shot version more but Matt arguments convinced
me that one-short fashion is more "secure" in terms of overall kernel
state and potential transitions/changes of this /proc/pid/exe symlink.

At least with one-shot version the admin may be sure that the symlink
is never changed more than once, ever.

Cyrill


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