linux-kernel - Re: [PATCH] mm: fix use-after-free in sys_remap_file

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Date:	Fri, 13 Dec 2013 12:10:17 +0100
From:	"PaX Team" <pageexec@...email.hu>
To:	Kees Cook <keescook@...omium.org>, Rik van Riel <riel@...hat.com>
CC:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH] mm: fix use-after-free in sys_remap_file_pages

On 12 Dec 2013 at 22:41, Rik van Riel wrote:

> If the vma has been freed by the time the code jumps to the
> out label (because it was freed by a function called from
> mmap_region), surely it will also already have been freed
> by the time this patch dereferences it?

oops, yes, i meant to save the flags away before mmap_region,
no idea how i ended up with this ;).

> Also, setting vma = NULL to avoid the if (vma) branch at
> the out: label is unnecessarily obfuscated. Lets make things
> clear by documenting what is going on, and having a label
> after that dereference.

on that note, how about this as well:

> --- a/mm/fremap.c
> +++ b/mm/fremap.c
> @@ -203,6 +203,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
>  		if (mapping_cap_account_dirty(mapping)) {
>  			unsigned long addr;
>  			struct file *file = get_file(vma->vm_file);
> +			vm_flags = vma->vm_flags;
>  
>  			addr = mmap_region(file, start, size,
>  					vma->vm_flags, pgoff);
                                    ^^^^^^^^^^^^^
pass in vm_flags instead of vma->vm_flags just to prevent someone
from 'optimizing' away the read in the future?

> @@ -213,7 +214,8 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
>  				BUG_ON(addr != start);
>  				err = 0;
>  			}
> -			goto out;
> +			/* mmap_region may have freed vma */
> +			goto out_freed;

perhaps {copy,move} this comment above the previous hunk since that's
where the relevant action is?

cheers,
  PaX Team

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