linux-kernel - Re: access

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Date:	Tue, 14 Jul 2009 15:45:22 +0200
From:	Michal Simek <monstr@...str.eu>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Linux Kernel list <linux-kernel@...r.kernel.org>,
	LTP <ltp-list@...ts.sourceforge.net>
Subject: Re: access_ok macor

Arnd Bergmann wrote:
> On Tuesday 14 July 2009, Michal Simek wrote:
>> I found that I can setup text base in binutils/ld/emulparam/elf32mb_linux.sh
>>
>> The problem which I have is that if I run socketpair, getsockname, getpeername LTP
>> tests with invalid salen pointer there are addresses close to 0x0. Microblaze
>> has no text there and the sigsegv fault is generated.
> 
> This sounds like a classic NULL pointer dereference that is handled correctly
> by the kernel. The question is where the address came from.

It is not anly NULL pointer - is LTP tests are some fake addresses. From my tests I see
that I am not able to access place till 1000 0000 in dec.
Bad address come from tests to test it.

Look at
http://developer.petalogix.com/git/gitweb.cgi?p=ltp-microblaze.git;a=commitdiff;h=45f4cd783ce8b94f1267bb87c0c46e8536f62eca

There are three affected tests and my quick fixes which I am trying to solve now.


> 
>> This fault could be fixed by changed access_ok macro where I check bottom limit
>> at 0x1000 0000 too. After this change the LTP program not failed but I am not sure
>> if is the right solution because none arch do it. All archs just check upper limit
>> not lower.
>>
>> What is the correct solution for it? Moving .text base to 0x0 or is there any other
>> elegant solution?
> 
> Moving .text is not the right solution, because it only papers over real bugs.

I can confirm it - I moved it and rebuild toolchain.

> access_ok() is also not the right place to check this, the only purpose it has
> is to make sure that the argument is not a valid kernel address but either a
> valid user address or possibly invalid address. Also, access_ok() is only used
> together with the copy_from/to_user and get/put_user function families. These
> need to catch invalid addresses with a fixup table entry in the kernel.

ok - that mean that problem could be in bad fixup table?


> 
> I briefly looked at your implementation but could not find any problems in
> this area. Could you use gdb to find out whether the sigsegv happens in the
> kernel at all, or in user space?

We don't have gdb in place.

The problem should come from get_user macro. net/socket.c:212
I was looking for it in the morning. I am checking it again.

Thanks,
Michal


int move_addr_to_user(struct sockaddr *kaddr, int klen, void __user *uaddr,
		      int __user *ulen)
{
	int err;
	int len;

	err = get_user(len, ulen);
	if (err)
		return err;


> 
> 	Arnd <><


-- 
Michal Simek, Ing. (M.Eng)
w: www.monstr.eu p: +42-0-721842854
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