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

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Date:   Tue, 14 Feb 2017 19:14:27 +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 02/14/2017 07:11 PM, Dmitry Safonov wrote:
> 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.

Oh, I lied here: it'll be still used for TASK_SIZE macro and etc.
But, I don't see why in generic code we can't use TASK_SIZE_MAX,
at least at this moment.

>
>> 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