Re: [RFC PATCH 3/5] mm: introduce VM_EXEC_KEEP

Eric W. Biederman
2020-07-28T13:42:13Z

Anthony Yznaga <anthony.yznaga@oracle.com> writes:

> A vma with the VM_EXEC_KEEP flag is preserved across exec. For anonymous
> vmas only. For safety, overlap with fixed address VMAs created in the new
> mm during exec (e.g. the stack and elf load segments) is not permitted and
> will cause the exec to fail.
> (We are studying how to guarantee there are no conflicts. Comments welcome.)
>

> diff --git a/fs/exec.c b/fs/exec.c
> index 262112e5f9f8..1de09c4eef00 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1069,6 +1069,20 @@ ssize_t read_code(struct file *file, unsigned long addr, loff_t pos, size_t len)
> EXPORT_SYMBOL(read_code);
> #endif
>
> +static int vma_dup_some(struct mm_struct *old_mm, struct mm_struct *new_mm)
> +{
> + struct vm_area_struct *vma;
> + int ret;
> +
> + for (vma = old_mm->mmap; vma; vma = vma->vm_next)
> + if (vma->vm_flags & VM_EXEC_KEEP) {
> + ret = vma_dup(vma, new_mm);
> + if (ret)
> + return ret;
> + }
> + return 0;
> +}
> +
> /*
> * Maps the mm_struct mm into the current task struct.
> * On success, this function returns with the mutex
> @@ -1104,6 +1118,12 @@ static int exec_mmap(struct mm_struct *mm)
> mutex_unlock(&tsk->signal->exec_update_mutex);
> return -EINTR;
> }
> + ret = vma_dup_some(old_mm, mm);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Ouch! An unconditional loop through all of the vmas of the execing
process, just in case there is a VM_EXEC_KEEP vma.

I know we already walk the list in exit_mmap, but I get the feeling this
will slow exec down when this feature is not enabled, especially when
a process with a lot of vmas is calling exec.

> + if (ret) {
> + mmap_read_unlock(old_mm);
> + mutex_unlock(&tsk->signal->exec_update_mutex);
> + return ret;
> + }
> }
>
> task_lock(tsk);