Re: [PATCH 2/2] mmap.2: MAP_FIXED updated documentation

[John Hubbard <jhubbard@nvidia.com>]

On 12/13/2017 06:52 PM, Jann Horn wrote:
> On Wed, Dec 13, 2017 at 10:31 AM, Michal Hocko <mhocko@kernel.org> wrote:
>> From: John Hubbard <jhubbard@nvidia.com>
[...]
>> +.IP
>> +Furthermore, this option is extremely hazardous (when used on its own), because
>> +it forcibly removes pre-existing mappings, making it easy for a multi-threaded
>> +process to corrupt its own address space.
> 
> I think this is worded unfortunately. It is dangerous if used
> incorrectly, and it's a good tool when used correctly.
> 
> [...]
>> +Thread B need not create a mapping directly; simply making a library call
>> +that, internally, uses
>> +.I dlopen(3)
>> +to load some other shared library, will
>> +suffice. The dlopen(3) call will map the library into the process's address
>> +space. Furthermore, almost any library call may be implemented using this
>> +technique.
>> +Examples include brk(2), malloc(3), pthread_create(3), and the PAM libraries
>> +(http://www.linux-pam.org).
> 
> This is arkward. This first mentions dlopen(), which is a very niche
> case, and then just very casually mentions the much bigger
> problem that tons of library functions can allocate memory through
> malloc(), causing mmap() calls, sometimes without that even being
> a documented property of the function.
> 

Hi Jann,

Here is some proposed new wording, to address your two comments above. What do 
you think of this:

NOTE:  this  option  can  be hazardous (when used on its own), because it
forcibly removes pre-existing mappings,  making  it  easy  for  a  multi-
threaded  process to corrupt its own address space. For example, thread A
looks through /proc/<pid>/maps and locates an  available  address  range,
while  thread  B simultaneously acquires part or all of that same address
range. Thread A then calls mmap(MAP_FIXED), effectively  overwriting  the
mapping that thread B created.

Thread B need not create a mapping directly; simply making a library call
whose implementation calls malloc(3), mmap(), or dlopen(3) will  suffice,
because those calls all create new mappings.

>> +.IP
>> +Newer kernels
>> +(Linux 4.16 and later) have a
>> +.B MAP_FIXED_SAFE
>> +option that avoids the corruption problem; if available, MAP_FIXED_SAFE
>> +should be preferred over MAP_FIXED.
> 
> This is bad advice. MAP_FIXED is completely safe if you use it on an address
> range you've allocated, and it is used in this way by core system libraries to
> place multiple VMAs in virtually contiguous memory, for example:
[...] 
> MAP_FIXED is a better solution for these usecases than MAP_FIXED_SAFE,
> or whatever it ends up being called. Please remove this advice or, better,
> clarify what MAP_FIXED should be used for (creation of virtually contiguous
> VMAs) and what MAP_FIXED_SAFE should be used for (attempting to
> allocate memory at a fixed address for some reason, with a failure instead of
> the normal fallback to using a different address).
> 

Rather than risk another back-and-forth with Michal (who doesn't want any advice
on how to use this safely, in the man page), I've simply removed this advice
entirely.

thanks,
-- 
John Hubbard
NVIDIA

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