[PATCH 02/46] fork: take mmap_lock before copying mm_struct

[Pedro Falcato]

On Tue, Oct 27, 2026 at 12:17:22AM +0100, David Hildenbrand (Arm) wrote:
> dup_mm() copies the parent's mm_struct before dup_mmap() takes the
> mmap_lock. A concurrent brk() can leave the child with a stale mm->brk
> and newer heap mappings, causing later brk() requests to fail.
> 
> Other operations (e.g., PR_SET_MM) on the parent's mm_struct can similarly
> cause inconsistencies.
> 
> As a simple and robust fix, grab the parent's mmap_lock earlier. This
> way we don't need to update individual mm fields in dup_mmap() and
> keep missing ones. Stop updating the 4 existing members (total_vm, data_vm,
> exec_vm, stack_vm), as that is now no longer required.
> 
> Keep releasing the parent's mmap_lock in dup_mmap(), before
> userfaultfd completion, to avoid moving the userfaultfd handling into the
> caller. This should get cleaned up later.
> 
> Move uprobe_start_dup_mmap() to preserve the lock ordering.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable at vger.kernel.org
> Reported-by: Josef Bacik <josef at toxicpanda.com>
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
> Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>

Reviewed-by: Pedro Falcato <pfalcato at suse.de>

FWIW, strictly speaking malloc() isn't usable in a forked
(multi-threaded) child, but glibc/musl provide this in a best-effort way.

-- 
Pedro