Re: [patch 1/4] Add routine for generating an ID for kernel pointer

From: Andrew Morton
Date: Tue Dec 27 2011 - 18:23:46 EST

On Fri, 23 Dec 2011 16:47:42 +0400
Cyrill Gorcunov <gorcunov@xxxxxxxxxx> wrote:

> The routine XORs the given pointer with a random value
> producing an ID (32 or 64 bit, depending on the arch).
> Since it's a valuable information -- only CAP_SYS_ADMIN
> is allowed to obtain it.
> - 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.
> - Pekka proposed to initialized poison values in the late_initcall callback
> - ... and move the code to mm/util.c

I'm trying to remember what this is all about, and I don't want to have
to remember all the discussion from last time this came up!

Please, do cover all this in the changelogs: tell us what the code is
all for and try to capture the design decisions thus far. It's a
useful reminder for current reviewers and is very valuable for new

The root-only restriction sounds like a pretty bad one. I suspect it
really isn't that bad but again, the changelog should discuss the pros
and cons here.

A thought: if all we're trying to do here is to check for the sameness
of objects, can we push the comparison into the kernel so we don't have
this exporting-sensitive-info problem at all? Just return a boolean to

Something like

int sys_pid_fields_equal(pid_t pid1, pid_t pid2, enum pid_field field_id);


For /proc/pid/fdinfo/* userspace can open /proc/pid1/fdinfo/0 and
/proc/pid2/fdinfo/0 and call sys_are_these_files_the_same(fd1, fd2, ...).

Perhaps sys_pid_fields_equal() can use sys_are_these_files_the_same()
as well, if we can think up a way of passing it two fds to represent
the two pids.

Have a think about it ;)
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