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

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Date:   Sun, 8 Dec 2019 19:43:54 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Will Deacon <will@...nel.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        security@...nel.org, ben.dooks@...ethink.co.uk
Subject: Re: [PATCH] exec: warn if process starts with executable stack

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@...nel.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@...il.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.