Re: [PATCH c/r -mm] c/r: prctl: Simplify PR_SET_MM on mm::code/data assignment

[Serge Hallyn <serge.hallyn@canonical.com>]

Quoting Cyrill Gorcunov (gorcunov@openvz.org):
> On Tue, Apr 17, 2012 at 11:19:16PM +0400, Cyrill Gorcunov wrote:
> ...
> > > Since this is CAP_SYS_RESOURCE, and mmap_min_addr is CAP_SYS_RAWIO,
> > > how about a lower-bounds check against mmap_min_addr? (We're already
> > > doing the TASK_SIZE upper check, so this additional sanity checking
> > > seems reasonable to me.)
> > 
> > I think this is good idea, thanks Kees. I'll check it out.
> 
> Updated and tested version is below.
> 
> 	Cyrill
> ---
> From: Cyrill Gorcunov <gorcunov@openvz.org>
> Subject: [PATCH] c/r: prctl: Simplify PR_SET_MM on mm::code/data assignment v2
> 
> The mm::start_code, end_code, start_data, end_data members
> are set during startup of executable file and are not changed
> after.
> 
> But the program itself might map new executable or/and data areas in
> time so the original values written into mm fields mentioned above
> might not have correspond VMA area at all, thus if one try to
> use this prctl codes without underlied VMA, the error will be
> returned.
> 
> Drop this requirement. This shrinks the code and eliminates
> redundant calls to vma_flags_mismatch. The worst thing one can
> do (if say to write some bad values here) -- the weird results
> will be shown in /proc/$pid/statm or in /proc/pid/stat.
> 
> Still, assignement of data on stack (such as command line and
> environment variables) requires the underlied VMA to exist.
> 
> v2:
>  Also make sure the address being set is greater than mmap_min_addr.
>  Suggested by Kees Cook.

Thanks for that, Kees.

> 
> Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Serge Hallyn <serge.hallyn@canonical.com>

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>

> Cc: Pavel Emelyanov <xemul@parallels.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> ---
>  kernel/sys.c |   36 +++++++++---------------------------
>  1 file changed, 9 insertions(+), 27 deletions(-)
> 
> Index: linux-2.6.git/kernel/sys.c
> ===================================================================
> --- linux-2.6.git.orig/kernel/sys.c
> +++ linux-2.6.git/kernel/sys.c
> @@ -1771,44 +1771,24 @@ static int prctl_set_mm(int opt, unsigne
>  	if (opt == PR_SET_MM_EXE_FILE)
>  		return prctl_set_mm_exe_file(mm, (unsigned int)addr);
>  
> -	if (addr >= TASK_SIZE)
> +	if (addr >= TASK_SIZE || addr < mmap_min_addr)
>  		return -EINVAL;
>  
>  	down_read(&mm->mmap_sem);
>  	vma = find_vma(mm, addr);
>  
> -	if (opt != PR_SET_MM_START_BRK &&
> -	    opt != PR_SET_MM_BRK &&
> -	    opt != PR_SET_MM_AUXV) {
> -		/* It must be existing VMA */
> -		if (!vma || vma->vm_start > addr)
> -			goto out;
> -	}
> -
> -	error = -EINVAL;
>  	switch (opt) {
>  	case PR_SET_MM_START_CODE:
> +		mm->start_code = addr;
> +		break;
>  	case PR_SET_MM_END_CODE:
> -		if (vma_flags_mismatch(vma, VM_READ | VM_EXEC,
> -				       VM_WRITE | VM_MAYSHARE))
> -			goto out;
> -
> -		if (opt == PR_SET_MM_START_CODE)
> -			mm->start_code = addr;
> -		else
> -			mm->end_code = addr;
> +		mm->end_code = addr;
>  		break;
> -
>  	case PR_SET_MM_START_DATA:
> +		mm->start_data = addr;
> +		break;
>  	case PR_SET_MM_END_DATA:
> -		if (vma_flags_mismatch(vma, VM_READ | VM_WRITE,
> -				       VM_EXEC | VM_MAYSHARE))
> -			goto out;
> -
> -		if (opt == PR_SET_MM_START_DATA)
> -			mm->start_data = addr;
> -		else
> -			mm->end_data = addr;
> +		mm->end_data = addr;
>  		break;
>  
>  	case PR_SET_MM_START_BRK:
> @@ -1847,6 +1827,8 @@ static int prctl_set_mm(int opt, unsigne
>  	case PR_SET_MM_ARG_END:
>  	case PR_SET_MM_ENV_START:
>  	case PR_SET_MM_ENV_END:
> +		if (!vma)
> +			goto out;
>  #ifdef CONFIG_STACK_GROWSUP
>  		if (vma_flags_mismatch(vma, VM_READ | VM_WRITE | VM_GROWSUP, 0))
>  #else