Re: [PATCH AUTOSEL 5.8 100/132] binfmt_elf: take the mmap lock around find_extend_vma()

[Jann Horn]

On Tue, Oct 27, 2020 at 12:54 AM Sasha Levin <sashal@xxxxxxxxxx> wrote:
> [ Upstream commit b2767d97f5ff758250cf28684aaa48bbfd34145f ]
>
> create_elf_tables() runs after setup_new_exec(), so other tasks can
> already access our new mm and do things like process_madvise() on it.  (At
> the time I'm writing this commit, process_madvise() is not in mainline
> yet, but has been in akpm's tree for some time.)
>
> While I believe that there are currently no APIs that would actually allow
> another process to mess up our VMA tree (process_madvise() is limited to
> MADV_COLD and MADV_PAGEOUT, and uring and userfaultfd cannot reach an mm
> under which no syscalls have been executed yet), this seems like an
> accident waiting to happen.
>
> Let's make sure that we always take the mmap lock around GUP paths as long
> as another process might be able to see the mm.

While this commit makes the kernel less prone to future accidents, and
it is arguably fixing locking misbehavior, I don't think it belongs
into stable trees? As far as I know, it is not fixing any bugs that
can actually materialize in current or past kernels.