Ack: [PATCH Lucid/Natty CVE-2012-3511] mm: Hold a file reference in madvise_remove

[Brad Figg]

On 08/30/2012 08:22 AM, Tim Gardner wrote:
> From: Andy Lutomirski <luto at amacapital.net>
> 
> CVE-2012-3511
> 
> BugLink: http://bugs.launchpad.net/bugs/1042447
> 
> Otherwise the code races with munmap (causing a use-after-free
> of the vma) or with close (causing a use-after-free of the struct
> file).
> 
> The bug was introduced by commit 90ed52ebe481 ("[PATCH] holepunch: fix
> mmap_sem i_mutex deadlock")
> 
> Cc: Hugh Dickins <hugh at veritas.com>
> Cc: Miklos Szeredi <mszeredi at suse.cz>
> Cc: Badari Pulavarty <pbadari at us.ibm.com>
> Cc: Nick Piggin <npiggin at suse.de>
> Cc: stable at vger.kernel.org
> Signed-off-by: Andy Lutomirski <luto at amacapital.net>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> (back ported from commit 9ab4233dd08036fe34a89c7dc6f47a8bf2eb29eb)
> 
> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
> ---
>  mm/madvise.c |   16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 35b1479..e51291d 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -5,6 +5,7 @@
>   * Copyright (C) 2002  Christoph Hellwig
>   */
>  
> +#include <linux/file.h>
>  #include <linux/mman.h>
>  #include <linux/pagemap.h>
>  #include <linux/syscalls.h>
> @@ -190,14 +191,16 @@ static long madvise_remove(struct vm_area_struct *vma,
>  	struct address_space *mapping;
>  	loff_t offset, endoff;
>  	int error;
> +	struct file *f;
>  
>  	*prev = NULL;	/* tell sys_madvise we drop mmap_sem */
>  
>  	if (vma->vm_flags & (VM_LOCKED|VM_NONLINEAR|VM_HUGETLB))
>  		return -EINVAL;
>  
> -	if (!vma->vm_file || !vma->vm_file->f_mapping
> -		|| !vma->vm_file->f_mapping->host) {
> +	f = vma->vm_file;
> +
> +	if (!f || !f->f_mapping || !f->f_mapping->host) {
>  			return -EINVAL;
>  	}
>  
> @@ -211,9 +214,16 @@ static long madvise_remove(struct vm_area_struct *vma,
>  	endoff = (loff_t)(end - vma->vm_start - 1)
>  			+ ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
>  
> -	/* vmtruncate_range needs to take i_mutex and i_alloc_sem */
> +	/*
> +	 * Filesystem's fallocate may need to take i_mutex.  We need to
> +	 * explicitly grab a reference because the vma (and hence the
> +	 * vma's reference to the file) can go away as soon as we drop
> +	 * mmap_sem.
> +	 */
> +	get_file(f);
>  	up_read(&current->mm->mmap_sem);
>  	error = vmtruncate_range(mapping->host, offset, endoff);
> +	fput(f);
>  	down_read(&current->mm->mmap_sem);
>  	return error;
>  }
> 


-- 
Brad Figg brad.figg at canonical.com http://www.canonical.com