mm: abstract initial stack setup to mm subsystem

There are peculiarities within the kernel where what is very clearly mm
code is performed elsewhere arbitrarily.

This violates separation of concerns and makes it harder to refactor code
to make changes to how fundamental initialisation and operation of mm
logic is performed.

One such case is the creation of the VMA containing the initial stack upon
execve()'ing a new process.  This is currently performed in
__bprm_mm_init() in fs/exec.c.

Abstract this operation to create_init_stack_vma().  This allows us to
limit use of vma allocation and free code to fork and mm only.

We previously did the same for the step at which we relocate the initial
stack VMA downwards via relocate_vma_down(), now we move the initial VMA
establishment too.

Take the opportunity to also move insert_vm_struct() to mm/vma.c as it's
no longer needed anywhere outside of mm.

Link: https://lkml.kernel.org/r/118c950ef7a8dd19ab20a23a68c3603751acd30e.1745853549.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 5 insertions, 61 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 477bc3f2e966..f9bbcf0016a4 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -245,60 +245,6 @@ static void flush_arg_page(struct linux_binprm *bprm, unsigned long pos,
 	flush_cache_page(bprm->vma, pos, page_to_pfn(page));
 }
 
-static int __bprm_mm_init(struct linux_binprm *bprm)
-{
-	int err;
-	struct vm_area_struct *vma = NULL;
-	struct mm_struct *mm = bprm->mm;
-
-	bprm->vma = vma = vm_area_alloc(mm);
-	if (!vma)
-		return -ENOMEM;
-	vma_set_anonymous(vma);
-
-	if (mmap_write_lock_killable(mm)) {
-		err = -EINTR;
-		goto err_free;
-	}
-
-	/*
-	 * Need to be called with mmap write lock
-	 * held, to avoid race with ksmd.
-	 */
-	err = ksm_execve(mm);
-	if (err)
-		goto err_ksm;
-
-	/*
-	 * Place the stack at the largest stack address the architecture
-	 * supports. Later, we'll move this to an appropriate place. We don't
-	 * use STACK_TOP because that can depend on attributes which aren't
-	 * configured yet.
-	 */
-	BUILD_BUG_ON(VM_STACK_FLAGS & VM_STACK_INCOMPLETE_SETUP);
-	vma->vm_end = STACK_TOP_MAX;
-	vma->vm_start = vma->vm_end - PAGE_SIZE;
-	vm_flags_init(vma, VM_SOFTDIRTY | VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP);
-	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
-
-	err = insert_vm_struct(mm, vma);
-	if (err)
-		goto err;
-
-	mm->stack_vm = mm->total_vm = 1;
-	mmap_write_unlock(mm);
-	bprm->p = vma->vm_end - sizeof(void *);
-	return 0;
-err:
-	ksm_exit(mm);
-err_ksm:
-	mmap_write_unlock(mm);
-err_free:
-	bprm->vma = NULL;
-	vm_area_free(vma);
-	return err;
-}
-
 static bool valid_arg_len(struct linux_binprm *bprm, long len)
 {
 	return len <= MAX_ARG_STRLEN;
@@ -351,12 +297,6 @@ static void flush_arg_page(struct linux_binprm *bprm, unsigned long pos,
 {
 }
 
-static int __bprm_mm_init(struct linux_binprm *bprm)
-{
-	bprm->p = PAGE_SIZE * MAX_ARG_PAGES - sizeof(void *);
-	return 0;
-}
-
 static bool valid_arg_len(struct linux_binprm *bprm, long len)
 {
 	return len <= bprm->p;
@@ -385,9 +325,13 @@ static int bprm_mm_init(struct linux_binprm *bprm)
 	bprm->rlim_stack = current->signal->rlim[RLIMIT_STACK];
 	task_unlock(current->group_leader);
 
-	err = __bprm_mm_init(bprm);
+#ifndef CONFIG_MMU
+	bprm->p = PAGE_SIZE * MAX_ARG_PAGES - sizeof(void *);
+#else
+	err = create_init_stack_vma(bprm->mm, &bprm->vma, &bprm->p);
 	if (err)
 		goto err;
+#endif
 
 	return 0;