linux-kernel - Re: [git pull] kgdb light, v5

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Message-ID: <20080210204124.GA26701@elte.hu>
Date:	Sun, 10 Feb 2008 21:41:24 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jan Kiszka <jan.kiszka@....de>, Ray Lee <ray-lk@...rabbit.org>,
	Sam Ravnborg <sam@...nborg.org>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@....com.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Wessel <jason.wessel@...driver.com>
Subject: Re: [git pull] kgdb light, v5

* Ingo Molnar <mingo@...e.hu> wrote:

> all other places already use probe_kernel_{read|write}. (Now, there 
> are a few stray TASK_SIZE checks still, i'll double check them and 
> convert them to access_ok() checks.)

all the TASK_SIZE checks relate to the soft breakpoint write accesses.

and access_ok() does not cut it: it's also a bit dangerous from debug 
context: uses current->address_space, which is task dependent and can 
accidentally allow an int3 write to userspace if executed in a kernel 
thread that has lazy-inherited the TLB from a user task, etc., and it 
also does not give enough protection on some other architectures.

is_kernel_text() is not good, because it does not cover modules. 
is_module_address() is not good either, because it also covers module 
data areas, and is a bit thick (hence crash-risky) as well. So there's 
no existing facility to cover this.

so i'd say the safest would be to remove the TASK_SIZE check altogether. 
If someone typoes a raw breakpoint - it is still enumerated by gdb and 
can still be cleared. It's not like kgdb cannot be used to shoot in 
one's own foot ...

	Ingo
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