Subject: Re: [PATCH] Restrict initial stack space expansion to rlimit

From: Andrew Morton <[email-masked]>
Date: Tue, 9 Feb 2010 13:25:29 -0800

On Tue, 09 Feb 2010 19:59:27 +1100
Michael Neuling <[email-masked]> wrote:

> > > +	/* Initial stack must not cause stack overflow. */
> > > +	if (stack_expand > stack_expand_lim)
> > > +		stack_expand = stack_expand_lim;
> > >  #ifdef CONFIG_STACK_GROWSUP
> > > -	stack_base = vma->vm_end + EXTRA_STACK_VM_PAGES * PAGE_SIZE;
> > > +	stack_base = vma->vm_end + stack_expand;
> > >  #else
> > > -	stack_base = vma->vm_start - EXTRA_STACK_VM_PAGES * PAGE_SIZE;
> > > +	stack_base = vma->vm_start - stack_expand;
> > >  #endif
> > >  	ret = expand_stack(vma, stack_base);
> > >  	if (ret)
> > 
> > Umm.. It looks correct. but the nested complex if statement seems a bit ugly.
> > Instead, How about following?
> 
> I don't like the duplicated code in the #ifdef/else but I can live with it.

cleanup the cleanup:

--- a/fs/exec.c~fs-execc-restrict-initial-stack-space-expansion-to-rlimit-cleanup-cleanup
+++ a/fs/exec.c
@@ -637,20 +637,17 @@ int setup_arg_pages(struct linux_binprm 
 	 * will align it up.
 	 */
 	rlim_stack = rlimit(RLIMIT_STACK) & PAGE_MASK;
-	if (rlim_stack < stack_size)
-		rlim_stack = stack_size;
+	rlim_stack = min(rlim_stack, stack_size);
 #ifdef CONFIG_STACK_GROWSUP
-	if (stack_size + stack_expand > rlim_stack) {
+	if (stack_size + stack_expand > rlim_stack)
 		stack_base = vma->vm_start + rlim_stack;
-	} else {
+	else
 		stack_base = vma->vm_end + stack_expand;
-	}
 #else
-	if (stack_size + stack_expand > rlim_stack) {
+	if (stack_size + stack_expand > rlim_stack)
 		stack_base = vma->vm_end - rlim_stack;
-	} else {
+	else
 		stack_base = vma->vm_start - stack_expand;
-	}
 #endif
 	ret = expand_stack(vma, stack_base);
 	if (ret)
_

> > note: it's untested.
> 
> Works for me on ppc64 with 4k and 64k pages.  Thanks!
> 
> I'd still like someone with a CONFIG_STACK_GROWSUP arch to test/ACK it
> as well.

There's only one CONFIG_GROWSUP arch - parisc.

Guys, here's the rolled-up patch.  Could someone please test it on
parisc?

err, I'm not sure what one needs to do to test it, actually. 
Presumably it involves setting an unusual `ulimit -s'.  Can someone
please suggest a test plan?

From: Michael Neuling <[email-masked]>

When reserving stack space for a new process, make sure we're not
attempting to expand the stack by more than rlimit allows.

This fixes a bug caused by b6a2fea39318e43fee84fa7b0b90d68bed92d2ba ("mm:
variable length argument support") and unmasked by
fc63cf237078c86214abcb2ee9926d8ad289da9b ("exec: setup_arg_pages() fails
to return errors").

This bug means that when limiting the stack to less the 20*PAGE_SIZE (eg. 
80K on 4K pages or 'ulimit -s 79') all processes will be killed before
they start.  This is particularly bad with 64K pages, where a ulimit below
1280K will kill every process.

Signed-off-by: Michael Neuling <[email-masked]>
Cc: KOSAKI Motohiro <[email-masked]>
Cc: Americo Wang <[email-masked]>
Cc: Anton Blanchard <[email-masked]>
Cc: Oleg Nesterov <[email-masked]>
Cc: James Morris <[email-masked]>
Cc: Ingo Molnar <[email-masked]>
Cc: Serge Hallyn <[email-masked]>
Cc: Benjamin Herrenschmidt <[email-masked]>
Cc: <[email-masked]>

 fs/exec.c |   21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff -puN fs/exec.c~fs-execc-restrict-initial-stack-space-expansion-to-rlimit fs/exec.c
--- a/fs/exec.c~fs-execc-restrict-initial-stack-space-expansion-to-rlimit
+++ a/fs/exec.c
@@ -571,6 +571,9 @@ int setup_arg_pages(struct linux_binprm 
 	struct vm_area_struct *prev = NULL;
 	unsigned long vm_flags;
 	unsigned long stack_base;
+	unsigned long stack_size;
+	unsigned long stack_expand;
+	unsigned long rlim_stack;
 
 #ifdef CONFIG_STACK_GROWSUP
 	/* Limit stack size to 1GB */
@@ -627,10 +630,24 @@ int setup_arg_pages(struct linux_binprm 
 			goto out_unlock;
 	}
 
+	stack_expand = EXTRA_STACK_VM_PAGES * PAGE_SIZE;
+	stack_size = vma->vm_end - vma->vm_start;
+	/*
+	 * Align this down to a page boundary as expand_stack
+	 * will align it up.
+	 */
+	rlim_stack = rlimit(RLIMIT_STACK) & PAGE_MASK;
+	rlim_stack = min(rlim_stack, stack_size);
 #ifdef CONFIG_STACK_GROWSUP
-	stack_base = vma->vm_end + EXTRA_STACK_VM_PAGES * PAGE_SIZE;
+	if (stack_size + stack_expand > rlim_stack)
+		stack_base = vma->vm_start + rlim_stack;
+	else
+		stack_base = vma->vm_end + stack_expand;
 #else
-	stack_base = vma->vm_start - EXTRA_STACK_VM_PAGES * PAGE_SIZE;
+	if (stack_size + stack_expand > rlim_stack)
+		stack_base = vma->vm_end - rlim_stack;
+	else
+		stack_base = vma->vm_start - stack_expand;
 #endif
 	ret = expand_stack(vma, stack_base);
 	if (ret)
_