NAK: [SRU T: CVE-2017-1000365] fs/exec.c: account for argv/envp pointers

[Stefan Bader]

On 12.07.2017 15:30, Stefan Bader wrote:
> From: Kees Cook <keescook at chromium.org>
> 
> When limiting the argv/envp strings during exec to 1/4 of the stack limit,
> the storage of the pointers to the strings was not included.  This means
> that an exec with huge numbers of tiny strings could eat 1/4 of the stack
> limit in strings and then additional space would be later used by the
> pointers to the strings.
> 
> For example, on 32-bit with a 8MB stack rlimit, an exec with 1677721
> single-byte strings would consume less than 2MB of stack, the max (8MB /
> 4) amount allowed, but the pointers to the strings would consume the
> remaining additional stack space (1677721 * 4 == 6710884).
> 
> The result (1677721 + 6710884 == 8388605) would exhaust stack space
> entirely.  Controlling this stack exhaustion could result in
> pathological behavior in setuid binaries (CVE-2017-1000365).
> 
> [akpm at linux-foundation.org: additional commenting from Kees]
> Fixes: b6a2fea39318 ("mm: variable length argument support")
> Link: http://lkml.kernel.org/r/20170622001720.GA32173@beast
> Signed-off-by: Kees Cook <keescook at chromium.org>
> Acked-by: Rik van Riel <riel at redhat.com>
> Acked-by: Michal Hocko <mhocko at suse.com>
> Cc: Alexander Viro <viro at zeniv.linux.org.uk>
> Cc: Qualys Security Advisory <qsa at qualys.com>
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> 
> CVE-2017-1000365
> 
> (cherry-picked from commit 98da7d08850fb8bdeb395d6368ed15753304aa0c)
> Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
> ---
> 
> Fix already applied to Xenial via upstream stable updates. Can be
> cherry-picked into Z/Y/T.
> 
> -Stefan
> 
>  fs/exec.c | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 72934df..9041990 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -220,8 +220,26 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
>  
>  	if (write) {
>  		unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start;
> +		unsigned long ptr_size;
>  		struct rlimit *rlim;
>  
> +		/*
> +		 * Since the stack will hold pointers to the strings, we
> +		 * must account for them as well.
> +		 *
> +		 * The size calculation is the entire vma while each arg page is
> +		 * built, so each time we get here it's calculating how far it
> +		 * is currently (rather than each call being just the newly
> +		 * added size from the arg page).  As a result, we need to
> +		 * always add the entire size of the pointers, so that on the
> +		 * last call to get_arg_page() we'll actually have the entire
> +		 * correct size.
> +		 */
> +		ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
> +		if (ptr_size > ULONG_MAX - size)
> +			goto fail;
> +		size += ptr_size;
> +
>  		acct_arg_size(bprm, size / PAGE_SIZE);
>  
>  		/*
> @@ -239,13 +257,15 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
>  		 *    to work from.
>  		 */
>  		rlim = current->signal->rlim;
> -		if (size > ACCESS_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4) {
> -			put_page(page);
> -			return NULL;
> -		}
> +		if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
                           ^
Should have compiled first. We need to keep this as ACCESS_ONCE in Trusty. Will
send a v2 for Trusty.

-Stefan

> +			goto fail;
>  	}
>  
>  	return page;
> +
> +fail:
> +	put_page(page);
> +	return NULL;
>  }
>  
>  static void put_arg_page(struct page *page)
> 


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