linux-kernel - Re: [RFC PATCH 3/5] mm: introduce VM_EXEC

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Date:   Tue, 28 Jul 2020 08:38:58 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Anthony Yznaga <anthony.yznaga@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org, mhocko@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        arnd@...db.de, keescook@...omium.org, gerg@...ux-m68k.org,
        ktkhai@...tuozzo.com, christian.brauner@...ntu.com,
        peterz@...radead.org, esyr@...hat.com, jgg@...pe.ca,
        christian@...lner.me, areber@...hat.com, cyphar@...har.com,
        steven.sistare@...cle.com
Subject: Re: [RFC PATCH 3/5] mm: introduce VM_EXEC_KEEP

Anthony Yznaga <anthony.yznaga@...cle.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);