Re: brk() in v6.1-rc1 can expand file mappings, seemingly without taking file locks

[Liam Howlett]

* Jann Horn <jannh@xxxxxxxxxx> [221202 13:54]:
> As of commit ca57f02295f, brk() can expand ordinary file mappings (but
> not file mappings with weird flags), and I think it does it with
> insufficient locks. I think brk() probably needs some extra checks to
> make sure it's operating on a brk-like VMA (which means it should at
> least be anonymous, and perhaps pass the full can_vma_merge_after()
> check so that we're not creating unnecessary special cases?).


Thanks.  This is probably caused by commit 2e7ce7d354f2: "mm/mmap:
change do_brk_flags() to expand existing VMA and add do_brk_munmap()"

Specifically the checks around expanding the VMA. 

> user@vm:~/brk_stretch$ cat brk_file.c

Thanks for the testcase.  I have a fix that I'm testing, but it's worth
noting that the brk call will succeed - except a new VMA will be
created.  Is this what you expect?

...
> 
> The codepaths that are intended to expand file VMAs do stuff like
> i_mmap_lock_write() and vma_interval_tree_remove(), which
> do_brk_flags() doesn't seem to do (because it was never intended to
> operate on file VMAs?).

I don't think the locks were there before and I think you are correct
that it wasn't intended to operate on file VMAs.  I made sure all the
ltp tests around brk pass with my changes but this seems insufficient.