Re: [PATCH 04/25] x86/sgx: Add pfn_mkwrite() handler for present PTEs

[Jarkko Sakkinen]

On Mon, 2021-12-06 at 13:18 -0800, Reinette Chatre wrote:
> Hi Jarkko,
> 
> On 12/4/2021 2:43 PM, Jarkko Sakkinen wrote:
> > On Wed, Dec 01, 2021 at 11:23:02AM -0800, Reinette Chatre wrote:
> > > By default a write page fault on a present PTE inherits the permissions
> > > of the VMA. Enclave page permissions maintained in the hardware's
> > > Enclave Page Cache Map (EPCM) may change after a VMA accessing the page
> > > is created. A VMA's permissions may thus exceed the enclave page
> > > permissions even though the VMA was originally created not to exceed
> > > the enclave page permissions. Following the default behavior during
> > > a page fault on a present PTE while the VMA permissions exceed the
> > > enclave page permissions would result in the PTE for an enclave page
> > > to be writable even though the page is not writable according to the
> > > enclave's permissions.
> > > 
> > > Consider the following scenario:
> > > * An enclave page exists with RW EPCM permissions.
> > > * A RW VMA maps the range spanning the enclave page.
> > > * The enclave page's EPCM permissions are changed to read-only.
> > 
> > How could this happen in the existing mainline code?
> 
> This is a preparatory patch for SGX2 support. Restricting the 
> permissions of an enclave page would require OS support that is added in 
> a later patch.
> 
> > 
> > > * There is no page table entry for the enclave page.
> > > 
> > > Q.
> > >   What will user space observe when an attempt is made to write to the
> > >   enclave page from within the enclave?
> > > 
> > > A.
> > >   Initially the page table entry is not present so the following is
> > >   observed:
> > >   1) Instruction writing to enclave page is run from within the enclave.
> > >   2) A page fault with second and third bits set (0x6) is encountered
> > >      and handled by the SGX handler sgx_vma_fault() that installs a
> > >      read-only page table entry following previous patch that installs
> > >      page table entry with permissions that VMA and enclave agree on
> > >      (read-only in this case).
> > >   3) Instruction writing to enclave page is re-attempted.
> > >   4) A page fault with first three bits set (0x7) is encountered and
> > >      transparently (from SGX and user space perspective) handled by the
> > >      OS with the page table entry made writable because the VMA is
> > >      writable.
> > >   5) Instruction writing to enclave page is re-attempted.
> > >   6) Since the EPCM permissions prevents writing to the page a new page
> > >      fault is encountered, this time with the SGX flag set in the error
> > >      code (0x8007). No action is taken by OS for this page fault and
> > >      execution returns to user space.
> > >   7) Typically such a fault will be passed on to an application with a
> > >      signal but if the enclave is entered with the vDSO function provided
> > >      by the kernel then user space does not receive a signal but instead
> > >      the vDSO function returns successfully with exception information
> > >      (vector=14, error code=0x8007, and address) within the exception
> > >      fields within the vDSO function's struct sgx_enclave_run.
> > > 
> > > As can be observed it is not possible for user space to write to an
> > > enclave page if that page's enclave page permissions do not allow so,
> > > no matter what the VMA or PTE allows.
> > > 
> > > Even so, the OS should not allow writing to a page if that page is not
> > > writable. Thus the page table entry should accurately reflect the
> > > enclave page permissions.
> > > 
> > > Do not blindly accept VMA permissions on a page fault due to a write
> > > attempt to a present PTE. Install a pfn_mkwrite() handler that ensures
> > > that the VMA permissions agree with the enclave permissions in this
> > > regard.
> > > 
> > > Considering the same scenario as above after this change results in
> > > the following behavior change:
> > > 
> > > Q.
> > >   What will user space observe when an attempt is made to write to the
> > >   enclave page from within the enclave?
> > > 
> > > A.
> > >   Initially the page table entry is not present so the following is
> > >   observed:
> > >   1) Instruction writing to enclave page is run from within the enclave.
> > >   2) A page fault with second and third bits set (0x6) is encountered
> > >      and handled by the SGX handler sgx_vma_fault() that installs a
> > >      read-only page table entry following previous patch that installs
> > >      page table entry with permissions that VMA and enclave agree on
> > >      (read-only in this case).
> > >   3) Instruction writing to enclave page is re-attempted.
> > >   4) A page fault with first three bits set (0x7) is encountered and
> > >      passed to the pfn_mkwrite() handler for consideration. The handler
> > >      determines that the page should not be writable and returns SIGBUS.
> > >   5) Typically such a fault will be passed on to an application with a
> > >      signal but if the enclave is entered with the vDSO function provided
> > >      by the kernel then user space does not receive a signal but instead
> > >      the vDSO function returns successfully with exception information
> > >      (vector=14, error code=0x7, and address) within the exception fields
> > >      within the vDSO function's struct sgx_enclave_run.
> > > 
> > > The accurate exception information supports the SGX runtime, which is
> > > virtually always implemented inside a shared library, by providing
> > > accurate information in support of its management of the SGX enclave.
> > 
> > This QA-format is not a great idea, as it kind of tells what are the legit
> > questions to ask.
> 
> I will remove the QA-format and just describe the two (before/after) 
> scenarios.
> 
> > You should describe what the patch does and what are the
> > legit reasons for doing that. Unfortunately, in the current form it is very
> > hard to get grip of this patch.
> 
> That was the goal of the summary (the first paragraph) at the start of 
> the changelog. Could you please elaborate how you would like me to 
> improve it?

If I do a search "mktme" through the commit message, it gives
me zero results.

/Jarkko