On Tue, Oct 27, 2020 at 12:54 AM Sasha Levin sashal@kernel.org 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.