[PATCH] Restrict initial stack space expansion to rlimit

[KOSAKI Motohiro]

> 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 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 <mikey at neuling.org>
> Cc: stable at kernel.org
> ---
> Attempts to answer comments from Kosaki Motohiro.
> 
> Tested on PPC only, hence !CONFIG_STACK_GROWSUP.  Someone should
> probably ACK for an arch with CONFIG_STACK_GROWSUP.
> 
> As noted, stable needs the same patch, but 2.6.32 doesn't have the
> rlimit() helper.
> 
>  fs/exec.c |   21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6-ozlabs/fs/exec.c
> ===================================================================
> --- linux-2.6-ozlabs.orig/fs/exec.c
> +++ linux-2.6-ozlabs/fs/exec.c
> @@ -555,6 +555,7 @@ static int shift_arg_pages(struct vm_are
>  }
>  
>  #define EXTRA_STACK_VM_PAGES	20	/* random */
> +#define ALIGN_DOWN(addr,size)	((addr)&(~((size)-1)))
>  
>  /*
>   * Finalizes the stack vm_area_struct. The flags and permissions are updated,
> @@ -570,7 +571,7 @@ int setup_arg_pages(struct linux_binprm 
>  	struct vm_area_struct *vma = bprm->vma;
>  	struct vm_area_struct *prev = NULL;
>  	unsigned long vm_flags;
> -	unsigned long stack_base;
> +	unsigned long stack_base, stack_expand, stack_expand_lim, stack_size;
>  
>  #ifdef CONFIG_STACK_GROWSUP
>  	/* Limit stack size to 1GB */
> @@ -627,10 +628,24 @@ 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;
> +	if (rlimit(RLIMIT_STACK) < stack_size)
> +		stack_expand_lim = 0; /* don't shrick the stack */
> +	else
> +		/*
> +		 * Align this down to a page boundary as expand_stack
> +		 * will align it up.
> +		 */
> +		stack_expand_lim = ALIGN_DOWN(rlimit(RLIMIT_STACK) - stack_size,
> +					      PAGE_SIZE);
> +	/* 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?

note: it's untested.



===============
From: Michael Neuling <mikey at neuling.org>
Subject: Restrict initial stack space expansion to rlimit

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 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.

[kosaki.motohiro at jp.fujitsu.com: cleanups]
Signed-off-by: Michael Neuling <mikey at neuling.org>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro at jp.fujitsu.com>
Cc: stable at kernel.org
---
Attempts to answer comments from Kosaki Motohiro.

Tested on PPC only, hence !CONFIG_STACK_GROWSUP.  Someone should
probably ACK for an arch with CONFIG_STACK_GROWSUP.

As noted, stable needs the same patch, but 2.6.32 doesn't have the
rlimit() helper.

diff --git a/fs/exec.c b/fs/exec.c
index 6f7fb0c..325bad4 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -573,6 +573,9 @@ int setup_arg_pages(struct linux_binprm *bprm,
 	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 */
@@ -629,10 +632,27 @@ int setup_arg_pages(struct linux_binprm *bprm,
 			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;
+	if (rlim_stack < stack_size)
+		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)