Re: [RFC PATCH 4/9] mm: Introduce vm_ops->mprotect()

[Jarkko Sakkinen]

On Fri, May 31, 2019 at 04:31:54PM -0700, Sean Christopherson wrote:
> SGX will use the mprotect() hook to prevent userspace from circumventing
> various security checks, i.e. Linux Security Modules.
> 
> Enclaves are built by copying data from normal memory into the Enclave
> Page Cache (EPC).  Due to the nature of SGX, the EPC is represented by a
> single file that must be MAP_SHARED, i.e. mprotect() only ever sees a
> single MAP_SHARED vm_file.  Furthermore, all enclaves will need read,
> write and execute pages in the EPC.

What does the last sentence is pointing out? Enclaves read, write and
execute pages, so?

> As a result, LSM policies cannot be meaningfully applied, e.g. an LSM
> can deny access to the EPC as a whole, but can't deny PROT_EXEC on page
> that originated in a non-EXECUTE file (which is long gone by the time
> mprotect() is called).

I'm not sure what kind of scenario this is describing where some LSM
can't dent PROT_EXEC. Kind of cryptic paragraph, have to say.

> By hooking mprotect(), SGX can make explicit LSM upcalls while an
> enclave is being built, i.e. when the kernel has a handle to origin of
> each enclave page, and enforce the result of the LSM policy whenever
> userspace maps the enclave page in the future.

How does mprotect() enabled adding new LSM hooks?

> Alternatively, SGX could play games with MAY_{READ,WRITE,EXEC}, but
> that approach is quite ugly, e.g. would require userspace to call an
> SGX ioctl() prior to using mprotect() to extend a page's protections.

Not really sure I got this. SGX gets page permissions in SECINFO.
Also recurring comment about MAY_* constants.

> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>  include/linux/mm.h |  2 ++
>  mm/mprotect.c      | 15 +++++++++++----
>  2 files changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0e8834ac32b7..50a42364a885 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -458,6 +458,8 @@ struct vm_operations_struct {
>  	void (*close)(struct vm_area_struct * area);
>  	int (*split)(struct vm_area_struct * area, unsigned long addr);
>  	int (*mremap)(struct vm_area_struct * area);
> +	int (*mprotect)(struct vm_area_struct * area, unsigned long start,
> +			unsigned long end, unsigned long prot);

Right, the hook must be here obviously because mprotect() can be called
when /dev/sgx/enclave is closed. Can you describe start and end i.e.
what range they are in?

>  	vm_fault_t (*fault)(struct vm_fault *vmf);
>  	vm_fault_t (*huge_fault)(struct vm_fault *vmf,
>  			enum page_entry_size pe_size);
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index bf38dfbbb4b4..e466ca5e4fe0 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -547,13 +547,20 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
>  			goto out;
>  		}
>  
> -		error = security_file_mprotect(vma, reqprot, prot);
> -		if (error)
> -			goto out;
> -
>  		tmp = vma->vm_end;
>  		if (tmp > end)
>  			tmp = end;
> +
> +		if (vma->vm_ops && vma->vm_ops->mprotect) {
> +			error = vma->vm_ops->mprotect(vma, nstart, tmp, prot);
> +			if (error)
> +				goto out;
> +		}
> +
> +		error = security_file_mprotect(vma, reqprot, prot);

Why is mprotect callback called post the LSM hook?

> +		if (error)
> +			goto out;

/Jarkko