[PATCH 2/4] binfmt_elf: Take the mmap lock around find_extend_vma()

[Jann Horn <jannh@google.com>]

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.

(Yes, this diff looks suspicious because we drop the lock before doing
anything with `vma`, but that's because we actually don't do anything with
it apart from the NULL check.)

Signed-off-by: Jann Horn <jannh@google.com>
---
 fs/binfmt_elf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 40ec0b9b4b4f..cd7c574a91a4 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -309,7 +309,10 @@ create_elf_tables(struct linux_binprm *bprm,
const struct elfhdr *exec,
 	 * Grow the stack manually; some architectures have a limit on how
 	 * far ahead a user-space access may be in order to grow the stack.
 	 */
+	if (mmap_read_lock_killable(mm))
+		return -EINTR;
 	vma = find_extend_vma(mm, bprm->p);
+	mmap_read_unlock(mm);
 	if (!vma)
 		return -EFAULT;

-- 
2.28.0.709.gb0816b6eb0-goog