linux-kernel - Re: [PATCHv4 4/5] x86/mm: check in_compat_syscall() instead TIF_ADDR32 for mmap(MAP

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Message-ID: <a5ee6568-22fb-8829-524a-66d4d89b6325@virtuozzo.com>
Date:   Tue, 14 Feb 2017 19:11:40 +0300
From:   Dmitry Safonov <dsafonov@...tuozzo.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     <linux-kernel@...r.kernel.org>, <0x7f454c46@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>, <x86@...nel.org>,
        <linux-mm@...ck.org>
Subject: Re: [PATCHv4 4/5] x86/mm: check in_compat_syscall() instead
 TIF_ADDR32 for mmap(MAP_32BIT)

On 02/11/2017 11:13 PM, Thomas Gleixner wrote:
> On Mon, 30 Jan 2017, Dmitry Safonov wrote:
>
>> At this momet, logic in arch_get_unmapped_area{,_topdown} for mmaps with
>> MAP_32BIT flag checks TIF_ADDR32 which means:
>> o if 32-bit ELF changes mode to 64-bit on x86_64 and then tries to
>>   mmap() with MAP_32BIT it'll result in addr over 4Gb (as default is
>>   top-down allocation)
>> o if 64-bit ELF changes mode to 32-bit and tries mmap() with MAP_32BIT,
>>   it'll allocate only memory in 1GB space: [0x40000000, 0x80000000).
>>
>> Fix it by handeling MAP_32BIT in 64-bit syscalls only.
>
> I really have a hard time to understand what is fixed and how that is
> related to the $subject.
>
> Again. Please explain the problem first properly so one can understand the
> issue immediately.

Ok, rewrote the changes log.

>
>> As a little bonus it'll make thread flag a little less used.
>
> I really do not understand the bonus part here. You replace the thread flag
> check with a different one and AFAICT this looks like oart of the 'fix'.

It's a part of the fix, right.
What I meant here is that after those patches TIF_ADDR32 is no more
used after exec() time. That's bonus as if we manage to change exec()
code in some way (i.e., pass address restriction as a parameter), we'll
have additional free thread info flag.

> Thanks,
>
> 	tglx
>
>> @@ -101,7 +101,7 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
>>  static void find_start_end(unsigned long flags, unsigned long *begin,
>>  			   unsigned long *end)
>>  {
>> -	if (!test_thread_flag(TIF_ADDR32) && (flags & MAP_32BIT)) {
>> +	if (!in_compat_syscall() && (flags & MAP_32BIT)) {
>>  		/* This is usually used needed to map code in small
>>  		   model, so it needs to be in the first 31bit. Limit
>>  		   it to that.  This means we need to move the
>> @@ -195,7 +195,7 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
>>  		return addr;
>>
>>  	/* for MAP_32BIT mappings we force the legacy mmap base */
>> -	if (!test_thread_flag(TIF_ADDR32) && (flags & MAP_32BIT))
>> +	if (!in_compat_syscall() && (flags & MAP_32BIT))
>>  		goto bottomup;
>>
>>  	/* requesting a specific address */
>> --
>> 2.11.0
>>
>>


-- 
              Dmitry