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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 11:39 PM, Cyrill Gorcunov <> wrote:
> 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.

And changing it once does not harm security?
I'm sure that rootkit writers will like this feature...

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