[RFC][PATCHSET] mremap/mmap mess

[Al Viro]

[NOTE: the patch series below is not for merge until ACKed by arch maintainers]

	We have a bunch of interesting problems with mmap/mremap.

1) MREMAP_FIXED allows remap to any location, regardless of what
the architecture has to say about it.  The only check is TASK_SIZE.
That's not enough - e.g. there are architectures where some ranges
are simply absent (itanic, sparc), there are some that have cache
coherency requirements on alignments of shared mapping (a lot -
anything with VIPT cache, itanic where it's not a coherency but
a performance issue).  There are architectures where specific ranges
are reserved for hugetlb and they either simply do not allow normal
mappings in there or need to do something to make them possible (as
ppc64 does).  sparc tried to deal with that problem, but it hadn't
been complete (alignment issues) and it had been actually wrong for
non-MREMAP_FIXED calls of mremap().

2) without MREMAP_FIXED we happily allowed extension into a hole in
address space - the only check for being able to extend without
move had been for TASK_SIZE (and for non-overlap with other vmas).
Victims: sparc, itanic due to extending into holes, powerpc due to
extending into hugetlb range.

3) in case of relocation without MREMAP_FIXED we ended up doing
get_unmapped_area() with wrong pgoff if the starting address had
been in the middle of a mapping.  New vma gets the right pgoff,
the checks are done for the wrong one.  Cache coherency issues
on all VIPT architectures.

4) mmap() with MAP_HUGETLB leaks struct file if it bails out anywhere
past the allocation of struct file (by do_mmap_pgoff())

5) brk() into a hugetlb range failed without trying to do anything;
known thing, ppc folks had been unhappy about that.

Series below should deal with those, mostly by switching to consistent
use of get_unmapped_area() and sanitizing mmap/mremap code in general.

There is one case where we still have a serious PITA and I'm not sure
how to deal with that; it's expand_stack().  We can trigger that by
creating a VM_GROWS{UP,DOWN} mapping and either hitting a pagefault
on address {below,above} it or doing PTRACE_POKEDATA on such address.
As it is, we only check that range we are expanding into is not a
hugetlb-only one.  The thing is, we *can't* just do the normal checks
as-is there.

For cases when we do expand_stack() for our own mm that would work just
fine and do the right thing.  Unfortunately, we have places that hit
it from get_user_pages() for another process.  And checks (starting with
"what's the maximal address we allow") are process-dependent on biarch
architectures.  Worse yet, execve() does that when we have no other
process - it creates new mm, puts an anonymous mapping as high as
possible in it and copies argv/envp in there.  And that's done with
get_user_pages() on new mm.  If we have a 32bit task on e.g. amd64,
we'll have that mapping at addresses far above the TASK_SIZE of caller.
Later, when  ->load_binary() figures out what personality we'll get,
it turns that mapping into a valid vma for stack, possibly relocating
the entire thing to address suitable for resulting process.

Breaking execve() from 32bit processes on biarch architectures would
be a bad thing, so we can't just add the normal set of checks to
expand_stack() (acct_stack_growth(), actually).  The problem is quite
real, though, since e.g. on itanic PTRACE_POKEDATA can be used to get
a vma hanging down into a gap in address space quite easily.  Results
are not pretty...

One way to deal with that would be to put enough information into mm_struct
so that all these checks wouldn't have to look at the caller's personality.
I'm not sure how much PITA would that be, though; I've been digging through
the arch/* VM code for several weeks by now, but I certainly don't pretend
to be able to spot e.g. performance implications of such change.

Comments (both on that issue and on following patch series) would be very
welcome.
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