bpf: Fix use-after-free in arena_vm_close on fork

commit 4fddde2 upstream.

arena_vm_open() only bumps vml->mmap_count but never registers the
child VMA in arena->vma_list. The vml->vma always points at the
parent VMA, so after parent munmap the pointer dangles. If the child
then calls bpf_arena_free_pages(), zap_pages() reads the stale
vml->vma triggering use-after-free.

Fix this by preventing the arena VMA from being inherited across
fork with VM_DONTCOPY, and preventing VMA splits via the may_split
callback.

Also reject mremap with a .mremap callback returning -EINVAL. A
same-size mremap(MREMAP_FIXED) on the full arena VMA reaches
copy_vma() through the following path:

  check_prep_vma()       - returns 0 early: new_len == old_len
                           skips VM_DONTEXPAND check
  prep_move_vma()        - vm_start == old_addr and
                           vm_end == old_addr + old_len
                           so may_split is never called
  move_vma()
    copy_vma_and_data()
      copy_vma()
        vm_area_dup()    - copies vm_private_data (vml pointer)
        vm_ops->open()   - bumps vml->mmap_count
      vm_ops->mremap()   - returns -EINVAL, rollback unmaps new VMA

The refcount ensures the rollback's arena_vm_close does not free
the vml shared with the original VMA.

Reported-by: Weiming Shi <bestswngs@gmail.com>
Reported-by: Xiang Mei <xmei5@asu.edu>
Fixes: 3174603 ("bpf: Introduce bpf_arena.")
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260413194245.21449-1-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Alexei Starovoitov

kernel/bpf/arena.c

@@ -341,6 +341,16 @@ static void arena_vm_open(struct vm_area_struct *vma)
 	refcount_inc(&vml->mmap_count);
 }
 
+static int arena_vm_may_split(struct vm_area_struct *vma, unsigned long addr)
+{
+	return -EINVAL;
+}
+
+static int arena_vm_mremap(struct vm_area_struct *vma)
+{
+	return -EINVAL;
+}
+
 static void arena_vm_close(struct vm_area_struct *vma)
 {
 	struct bpf_map *map = vma->vm_file->private_data;
@@ -417,6 +427,8 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf)
 
 static const struct vm_operations_struct arena_vm_ops = {
 	.open		= arena_vm_open,
+	.may_split	= arena_vm_may_split,
+	.mremap		= arena_vm_mremap,
 	.close		= arena_vm_close,
 	.fault          = arena_vm_fault,
 };
@@ -486,10 +498,11 @@ static int arena_map_mmap(struct bpf_map *map, struct vm_area_struct *vma)
 	arena->user_vm_end = vma->vm_end;
 	/*
 	 * bpf_map_mmap() checks that it's being mmaped as VM_SHARED and
-	 * clears VM_MAYEXEC. Set VM_DONTEXPAND as well to avoid
-	 * potential change of user_vm_start.
+	 * clears VM_MAYEXEC. Set VM_DONTEXPAND to avoid potential change
+	 * of user_vm_start. Set VM_DONTCOPY to prevent arena VMA from
+	 * being copied into the child process on fork.
 	 */
-	vm_flags_set(vma, VM_DONTEXPAND);
+	vm_flags_set(vma, VM_DONTEXPAND | VM_DONTCOPY);
 	vma->vm_ops = &arena_vm_ops;
 	return 0;
 }