[PATCH] ELF: warn if process starts with executable stack

[PATCH] ELF: warn if process starts with executable stack

[Alexey Dobriyan]

PT_GNU_STACK is fail open design, at least warn people that something
isn't right.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 fs/exec.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -762,6 +762,13 @@ int setup_arg_pages(struct linux_binprm *bprm,
 		goto out_unlock;
 	BUG_ON(prev != vma);
 
+#ifdef CONFIG_MMU
+	if (vm_flags & VM_EXEC) {
+		pr_warn_once("process '%s'/%u started with executable stack\n",
+			     current->comm, current->pid);
+	}
+#endif
+
 	/* Move stack pages down in memory. */
 	if (stack_shift) {
 		ret = shift_arg_pages(vma, stack_shift);

Re: [PATCH] ELF: warn if process starts with executable stack

[Ben Dooks]

On 18/11/2019 14:51, Alexey Dobriyan wrote:
> PT_GNU_STACK is fail open design, at least warn people that something
> isn't right.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> ---
> 
>   fs/exec.c |    7 +++++++
>   1 file changed, 7 insertions(+)
> 
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -762,6 +762,13 @@ int setup_arg_pages(struct linux_binprm *bprm,
>   		goto out_unlock;
>   	BUG_ON(prev != vma);
>   

it might be worth to use:
  if (IS_ENABLED(CONFIG_MMU) && vm_flags & VM_EXEC) {

instead of the #ifdef


> +#ifdef CONFIG_MMU
> +	if (vm_flags & VM_EXEC) {
> +		pr_warn_once("process '%s'/%u started with executable stack\n",
> +			     current->comm, current->pid);
> +	}
> +#endif
> +
>   	/* Move stack pages down in memory. */
>   	if (stack_shift) {
>   		ret = shift_arg_pages(vma, stack_shift);
> 


-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

https://www.codethink.co.uk/privacy.html

Re: [PATCH] ELF: warn if process starts with executable stack

[Andrew Morton]

On Mon, 18 Nov 2019 17:51:15 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote:

> PT_GNU_STACK is fail open design,

Not sure what this means.  Please expand on the motivation for this
change.

> at least warn people that something
> isn't right.

People who use an executable stack get a kernel splat.  How is that
useful?

> ...
>
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -762,6 +762,13 @@ int setup_arg_pages(struct linux_binprm *bprm,
>  		goto out_unlock;
>  	BUG_ON(prev != vma);
>  
> +#ifdef CONFIG_MMU
> +	if (vm_flags & VM_EXEC) {
> +		pr_warn_once("process '%s'/%u started with executable stack\n",
> +			     current->comm, current->pid);
> +	}
> +#endif
>
>  	/* Move stack pages down in memory. */
>  	if (stack_shift) {
>  		ret = shift_arg_pages(vma, stack_shift);

Re: [PATCH] ELF: warn if process starts with executable stack

[Alexey Dobriyan]

On Mon, Nov 18, 2019 at 12:54:57PM -0800, Andrew Morton wrote:
> On Mon, 18 Nov 2019 17:51:15 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote:
> 
> > PT_GNU_STACK is fail open design,
> 
> Not sure what this means.  Please expand on the motivation for this
> change.
> 
> > at least warn people that something
> > isn't right.
> 
> People who use an executable stack get a kernel splat.  How is that
> useful?

There were two stories about silent downgrade to an executable stack:

1)
compiling .S file and linking it to normal code:

	$ cat f.S
	.intel_syntax noprefix
	.text
	.globl f
	f:
	        ret

will silently add PT_GNU_STACK segment with RWE permissions

2)
closures with nested functions will require executable stack
https://nullprogram.com/blog/2019/11/15/

> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -762,6 +762,13 @@ int setup_arg_pages(struct linux_binprm *bprm,
> >  		goto out_unlock;
> >  	BUG_ON(prev != vma);
> >  
> > +#ifdef CONFIG_MMU

This code is already under CONFIG_MMU. I'll resend.

[PATCH] exec: warn if process starts with executable stack

[Alexey Dobriyan]

There were few episodes of silent downgrade to an executable stack:

1) linking innocent looking assembly file

	$ cat f.S
	.intel_syntax noprefix
	.text
	.globl f
	f:
	        ret

	$ cat main.c
	void f(void);
	int main(void)
	{
	        f();
	        return 0;
	}

	$ gcc main.c f.S
	$ readelf -l ./a.out
	  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                         0x0000000000000000 0x0000000000000000  RWE    0x10

2) converting C99 nested function into a closure
https://nullprogram.com/blog/2019/11/15/

	void intsort2(int *base, size_t nmemb, _Bool invert)
	{
	    int cmp(const void *a, const void *b)
	    {
	        int r = *(int *)a - *(int *)b;
	        return invert ? -r : r;
	    }
	    qsort(base, nmemb, sizeof(*base), cmp);
	}

will silently require stack trampolines while non-closure version will not.

While without a double this behaviour is documented somewhere, add a warning
so that developers and users can at least notice. After so many years of x86_64
having proper executable stack support it should not cause too much problems.

If the system is old or CPU is old, then there will be an early warning
against init and/or support personnel will write that "uh-oh, our Enterprise
Software absolutely requires executable stack" and close tickets and customers
will nod heads and life moves on.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 fs/exec.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -762,6 +762,11 @@ int setup_arg_pages(struct linux_binprm *bprm,
 		goto out_unlock;
 	BUG_ON(prev != vma);
 
+	if (vm_flags & VM_EXEC) {
+		pr_warn_once("process '%s'/%u started with executable stack\n",
+			     current->comm, current->pid);
+	}
+
 	/* Move stack pages down in memory. */
 	if (stack_shift) {
 		ret = shift_arg_pages(vma, stack_shift);

Re: [PATCH] exec: warn if process starts with executable stack

[Dan Carpenter]

On Tue, Nov 19, 2019 at 12:52:27AM +0300, Alexey Dobriyan wrote:
> There were few episodes of silent downgrade to an executable stack:
> 
> 1) linking innocent looking assembly file
> 
> 	$ cat f.S
> 	.intel_syntax noprefix
> 	.text
> 	.globl f
> 	f:
> 	        ret
> 
> 	$ cat main.c
> 	void f(void);
> 	int main(void)
> 	{
> 	        f();
> 	        return 0;
> 	}
> 
> 	$ gcc main.c f.S
> 	$ readelf -l ./a.out
> 	  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
>                          0x0000000000000000 0x0000000000000000  RWE    0x10
> 
> 2) converting C99 nested function into a closure
> https://nullprogram.com/blog/2019/11/15/
> 
> 	void intsort2(int *base, size_t nmemb, _Bool invert)
> 	{
> 	    int cmp(const void *a, const void *b)
> 	    {
> 	        int r = *(int *)a - *(int *)b;
> 	        return invert ? -r : r;
> 	    }
> 	    qsort(base, nmemb, sizeof(*base), cmp);
> 	}
> 
> will silently require stack trampolines while non-closure version will not.
> 
> While without a double this behaviour is documented somewhere, add a warning
                  ^^^^^^
doubt

> so that developers and users can at least notice. After so many years of x86_64
> having proper executable stack support it should not cause too much problems.
> 
> If the system is old or CPU is old, then there will be an early warning
> against init and/or support personnel will write that "uh-oh, our Enterprise
> Software absolutely requires executable stack" and close tickets and customers
> will nod heads and life moves on.
> 

regards,
dan carpenter

Re: [PATCH] exec: warn if process starts with executable stack

[Will Deacon]

On Tue, Nov 19, 2019 at 12:52:27AM +0300, Alexey Dobriyan wrote:
> There were few episodes of silent downgrade to an executable stack:
> 
> 1) linking innocent looking assembly file
> 
> 	$ cat f.S
> 	.intel_syntax noprefix
> 	.text
> 	.globl f
> 	f:
> 	        ret
> 
> 	$ cat main.c
> 	void f(void);
> 	int main(void)
> 	{
> 	        f();
> 	        return 0;
> 	}
> 
> 	$ gcc main.c f.S
> 	$ readelf -l ./a.out
> 	  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
>                          0x0000000000000000 0x0000000000000000  RWE    0x10
> 
> 2) converting C99 nested function into a closure
> https://nullprogram.com/blog/2019/11/15/
> 
> 	void intsort2(int *base, size_t nmemb, _Bool invert)
> 	{
> 	    int cmp(const void *a, const void *b)
> 	    {
> 	        int r = *(int *)a - *(int *)b;
> 	        return invert ? -r : r;
> 	    }
> 	    qsort(base, nmemb, sizeof(*base), cmp);
> 	}
> 
> will silently require stack trampolines while non-closure version will not.
> 
> While without a double this behaviour is documented somewhere, add a warning
> so that developers and users can at least notice. After so many years of x86_64
> having proper executable stack support it should not cause too much problems.
> 
> If the system is old or CPU is old, then there will be an early warning
> against init and/or support personnel will write that "uh-oh, our Enterprise
> Software absolutely requires executable stack" and close tickets and customers
> will nod heads and life moves on.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> ---
> 
>  fs/exec.c |    5 +++++
>  1 file changed, 5 insertions(+)
> 
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -762,6 +762,11 @@ int setup_arg_pages(struct linux_binprm *bprm,
>  		goto out_unlock;
>  	BUG_ON(prev != vma);
>  
> +	if (vm_flags & VM_EXEC) {
> +		pr_warn_once("process '%s'/%u started with executable stack\n",
> +			     current->comm, current->pid);
> +	}

Given that this is triggerable by userspace, is there a concern about PID
namespaces here?

Will

Re: [PATCH] exec: warn if process starts with executable stack

[Eric W. Biederman]

Will Deacon <will@kernel.org> writes:

> On Tue, Nov 19, 2019 at 12:52:27AM +0300, Alexey Dobriyan wrote:
>> There were few episodes of silent downgrade to an executable stack:
>> 
>> 1) linking innocent looking assembly file
>> 
>> 	$ cat f.S
>> 	.intel_syntax noprefix
>> 	.text
>> 	.globl f
>> 	f:
>> 	        ret
>> 
>> 	$ cat main.c
>> 	void f(void);
>> 	int main(void)
>> 	{
>> 	        f();
>> 	        return 0;
>> 	}
>> 
>> 	$ gcc main.c f.S
>> 	$ readelf -l ./a.out
>> 	  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
>>                          0x0000000000000000 0x0000000000000000  RWE    0x10
>> 
>> 2) converting C99 nested function into a closure
>> https://nullprogram.com/blog/2019/11/15/
>> 
>> 	void intsort2(int *base, size_t nmemb, _Bool invert)
>> 	{
>> 	    int cmp(const void *a, const void *b)
>> 	    {
>> 	        int r = *(int *)a - *(int *)b;
>> 	        return invert ? -r : r;
>> 	    }
>> 	    qsort(base, nmemb, sizeof(*base), cmp);
>> 	}
>> 
>> will silently require stack trampolines while non-closure version will not.
>> 
>> While without a double this behaviour is documented somewhere, add a warning
>> so that developers and users can at least notice. After so many years of x86_64
>> having proper executable stack support it should not cause too much problems.
>> 
>> If the system is old or CPU is old, then there will be an early warning
>> against init and/or support personnel will write that "uh-oh, our Enterprise
>> Software absolutely requires executable stack" and close tickets and customers
>> will nod heads and life moves on.
>> 
>> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
>> ---
>> 
>>  fs/exec.c |    5 +++++
>>  1 file changed, 5 insertions(+)
>> 
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -762,6 +762,11 @@ int setup_arg_pages(struct linux_binprm *bprm,
>>  		goto out_unlock;
>>  	BUG_ON(prev != vma);
>>  
>> +	if (vm_flags & VM_EXEC) {
>> +		pr_warn_once("process '%s'/%u started with executable stack\n",
>> +			     current->comm, current->pid);
>> +	}
>
> Given that this is triggerable by userspace, is there a concern about PID
> namespaces here?

In what sense?  Are you thinking about the printing of the pid?

Pretty much by fiat and by definition the kernel log always print things
in the initial pid namespace.  Which this printk does.

Eric

Re: [PATCH] exec: warn if process starts with executable stack

[Will Deacon]

On Wed, Nov 20, 2019 at 02:28:37PM -0600, Eric W. Biederman wrote:
> Will Deacon <will@kernel.org> writes:
> 
> > On Tue, Nov 19, 2019 at 12:52:27AM +0300, Alexey Dobriyan wrote:
> >> There were few episodes of silent downgrade to an executable stack:
> >> 
> >> 1) linking innocent looking assembly file
> >> 
> >> 	$ cat f.S
> >> 	.intel_syntax noprefix
> >> 	.text
> >> 	.globl f
> >> 	f:
> >> 	        ret
> >> 
> >> 	$ cat main.c
> >> 	void f(void);
> >> 	int main(void)
> >> 	{
> >> 	        f();
> >> 	        return 0;
> >> 	}
> >> 
> >> 	$ gcc main.c f.S
> >> 	$ readelf -l ./a.out
> >> 	  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
> >>                          0x0000000000000000 0x0000000000000000  RWE    0x10
> >> 
> >> 2) converting C99 nested function into a closure
> >> https://nullprogram.com/blog/2019/11/15/
> >> 
> >> 	void intsort2(int *base, size_t nmemb, _Bool invert)
> >> 	{
> >> 	    int cmp(const void *a, const void *b)
> >> 	    {
> >> 	        int r = *(int *)a - *(int *)b;
> >> 	        return invert ? -r : r;
> >> 	    }
> >> 	    qsort(base, nmemb, sizeof(*base), cmp);
> >> 	}
> >> 
> >> will silently require stack trampolines while non-closure version will not.
> >> 
> >> While without a double this behaviour is documented somewhere, add a warning
> >> so that developers and users can at least notice. After so many years of x86_64
> >> having proper executable stack support it should not cause too much problems.
> >> 
> >> If the system is old or CPU is old, then there will be an early warning
> >> against init and/or support personnel will write that "uh-oh, our Enterprise
> >> Software absolutely requires executable stack" and close tickets and customers
> >> will nod heads and life moves on.
> >> 
> >> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> >> ---
> >> 
> >>  fs/exec.c |    5 +++++
> >>  1 file changed, 5 insertions(+)
> >> 
> >> --- a/fs/exec.c
> >> +++ b/fs/exec.c
> >> @@ -762,6 +762,11 @@ int setup_arg_pages(struct linux_binprm *bprm,
> >>  		goto out_unlock;
> >>  	BUG_ON(prev != vma);
> >>  
> >> +	if (vm_flags & VM_EXEC) {
> >> +		pr_warn_once("process '%s'/%u started with executable stack\n",
> >> +			     current->comm, current->pid);
> >> +	}
> >
> > Given that this is triggerable by userspace, is there a concern about PID
> > namespaces here?
> 
> In what sense?  Are you thinking about the printing of the pid?
> 
> Pretty much by fiat and by definition the kernel log always print things
> in the initial pid namespace.  Which this printk does.

Ok, fair enough. Just wanted to make sure it was ok, since we're not using
a task_pid_nr*() accessor and it might have been overlooked.

Will

Will

Re: [PATCH] exec: warn if process starts with executable stack

[Alexey Dobriyan]

On Thu, Nov 21, 2019 at 09:38:06AM +0000, Will Deacon wrote:
> On Wed, Nov 20, 2019 at 02:28:37PM -0600, Eric W. Biederman wrote:
> > Will Deacon <will@kernel.org> writes:
> > 
> > > On Tue, Nov 19, 2019 at 12:52:27AM +0300, Alexey Dobriyan wrote:
> > >> There were few episodes of silent downgrade to an executable stack:
> > >> 
> > >> 1) linking innocent looking assembly file
> > >> 
> > >> 	$ cat f.S
> > >> 	.intel_syntax noprefix
> > >> 	.text
> > >> 	.globl f
> > >> 	f:
> > >> 	        ret
> > >> 
> > >> 	$ cat main.c
> > >> 	void f(void);
> > >> 	int main(void)
> > >> 	{
> > >> 	        f();
> > >> 	        return 0;
> > >> 	}
> > >> 
> > >> 	$ gcc main.c f.S
> > >> 	$ readelf -l ./a.out
> > >> 	  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
> > >>                          0x0000000000000000 0x0000000000000000  RWE    0x10
> > >> 
> > >> 2) converting C99 nested function into a closure
> > >> https://nullprogram.com/blog/2019/11/15/
> > >> 
> > >> 	void intsort2(int *base, size_t nmemb, _Bool invert)
> > >> 	{
> > >> 	    int cmp(const void *a, const void *b)
> > >> 	    {
> > >> 	        int r = *(int *)a - *(int *)b;
> > >> 	        return invert ? -r : r;
> > >> 	    }
> > >> 	    qsort(base, nmemb, sizeof(*base), cmp);
> > >> 	}
> > >> 
> > >> will silently require stack trampolines while non-closure version will not.
> > >> 
> > >> While without a double this behaviour is documented somewhere, add a warning
> > >> so that developers and users can at least notice. After so many years of x86_64
> > >> having proper executable stack support it should not cause too much problems.
> > >> 
> > >> If the system is old or CPU is old, then there will be an early warning
> > >> against init and/or support personnel will write that "uh-oh, our Enterprise
> > >> Software absolutely requires executable stack" and close tickets and customers
> > >> will nod heads and life moves on.
> > >> 
> > >> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> > >> ---
> > >> 
> > >>  fs/exec.c |    5 +++++
> > >>  1 file changed, 5 insertions(+)
> > >> 
> > >> --- a/fs/exec.c
> > >> +++ b/fs/exec.c
> > >> @@ -762,6 +762,11 @@ int setup_arg_pages(struct linux_binprm *bprm,
> > >>  		goto out_unlock;
> > >>  	BUG_ON(prev != vma);
> > >>  
> > >> +	if (vm_flags & VM_EXEC) {
> > >> +		pr_warn_once("process '%s'/%u started with executable stack\n",
> > >> +			     current->comm, current->pid);
> > >> +	}
> > >
> > > Given that this is triggerable by userspace, is there a concern about PID
> > > namespaces here?
> > 
> > In what sense?  Are you thinking about the printing of the pid?
> > 
> > Pretty much by fiat and by definition the kernel log always print things
> > in the initial pid namespace.  Which this printk does.
> 
> Ok, fair enough. Just wanted to make sure it was ok, since we're not using
> a task_pid_nr*() accessor and it might have been overlooked.

PID is printed both as ->pid and a task_pid_vnr().
I'll just print filename, so that executable can be easily found.