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Message-ID: <20170119212718.GC20931@redhat.com>
Date: Thu, 19 Jan 2017 16:27:18 -0500
From: "Frank Ch. Eigler" <fche@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: David Smith <dsmith@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andy Lutomirski <luto@...capital.net>,
Ingo Molnar <mingo@...nel.org>,
"H. Peter Anvin" <hpa@...or.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] x86: Verify access_ok() context
Hi, Thomas -
> Well, if you are not in thread context then the check is pointless:
> __range_not_ok(addr, size, user_addr_max())
> and:
> #define user_addr_max() (current->thread.addr_limit.seg)
>
> So what guarantees when you are not in context of current, i.e. in thread
> context, that the addr/size which is checked against the limits of current
> actually belongs to current?
We're probably in task context in that there is a valid current(), but
running with preemption and/or interrupts and/or pagefaults disabled
at that point, so in_task() objects. Think of it like from a kprobes
handler callback, except maybe more temporary preemption blocking.
> I assume this is about systemtap modules. Can you please explain
> what you are trying to achieve? I guess you know that you actually
> access current, but then we need a seperate special function and not
> relaxing of the checks.
This part is used in a part of the runtime that is a userspace
analogue of probe_kernel_address(), where we're given a potential
userspace address. We would like to quickly test whether it's even
plausible as a userspace address, before doing a (pagefault-disabled)
trial fetch/store to it.
- FChE
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