Re: [Bug 11063][PATCH] exec: fix stack excutability without PT_GNU_STACK

[pageexec@freemail.hu]

On 10 Jul 2008 at 21:19, Hugh Dickins wrote:

> Kernel Bugzilla #11063 points out that on some architectures (e.g. x86_32)
> exec'ing an ELF without a PT_GNU_STACK program header should default to an
> executable stack; but this got broken by the unlimited argv feature because
> stack vma is now created before the right personality has been established:
> so breaking old binaries using nested function trampolines.
> 
> Therefore re-evaluate VM_STACK_FLAGS in setup_arg_pages, where stack
> vm_flags used to be set, before the mprotect_fixup.  Checking through
> our existing VM_flags, none would have changed since insert_vm_struct:
> so this seems safer than finding a way through the personality labyrinth.

alternatively, if there's a concern of stack_vma->vm_flags manipulation
during execve (maybe not now, but in the future or in non-ELF formats
that also want to rely on personality bits), you could opt for a safer

	vm_flags = vma->vm_flags | (VM_STACK_FLAGS & (VM_EXEC | VM_MAYEXEC));

to just recompute the exec rights related bits.

> Reported-by: pageexec@freemail.hu
> Signed-off-by: Hugh Dickins <hugh@veritas.com>
> Cc: stable@kernel.org
> ---
> 
>  fs/exec.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- 2.6.26-rc9/fs/exec.c	2008-06-21 08:41:19.000000000 +0100
> +++ linux/fs/exec.c	2008-07-10 20:02:25.000000000 +0100
> @@ -610,7 +610,7 @@ int setup_arg_pages(struct linux_binprm 
>  	bprm->exec -= stack_shift;
>  
>  	down_write(&mm->mmap_sem);
> -	vm_flags = vma->vm_flags;
> +	vm_flags = VM_STACK_FLAGS;
>  
>  	/*
>  	 * Adjust stack execute permissions; explicitly enable for
>