Re: [kernel-hardening] [RFC V2 4/6] lib: vsprintf: default kptr_restrict to the maximum value

From: Jann Horn
Date: Wed Oct 04 2017 - 15:14:05 EST

On Wed, Oct 4, 2017 at 7:28 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Oct 4, 2017 at 10:08 AM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> So I honestly doubt the value of kptr_restrict. Any *sane* policy
>> pretty much has to be in the caller, and by thinking about what you
>> print out. IOW, things like proc_pid_wchan().
> Looking at /proc/kallsyms is actually a prime example of this.
> IOW, the old "open /proc/kallsyms as a normal user, then make it stdin
> for some suid-root program that can be fooled to output it probably
> works on it.

Actually, /proc/kallsyms uses %pK, which hacks around this issue
by checking for `euid != uid` in addition to the capability check - so this
isn't exploitable through a typical setuid program.