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SubjectRe: [RFC PATCH 4/9] mm: Introduce vm_ops->mprotect()
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@intel.com>
> ---
> 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

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