Hi Dmitrii,

Thank you very much for uncovering and fixing this issue.

On 4/30/2024 7:38 AM, Dmitrii Kuvaiskii wrote:

On Mon, Apr 29, 2024 at 04:11:03PM +0300, Jarkko Sakkinen wrote:

...

On Mon Apr 29, 2024 at 1:43 PM EEST, Dmitrii Kuvaiskii wrote:

...

Two enclave threads may try to add and remove the same enclave page simultaneously (e.g., if the SGX runtime supports both lazy allocation and `MADV_DONTNEED` semantics). Consider this race:

1. T1 performs page removal in sgx_encl_remove_pages() and stops right after removing the page table entry and right before re-acquiring the enclave lock to EREMOVE and xa_erase(&encl->page_array) the page.
2. T2 tries to access the page, and #PF[not_present] is raised. The condition to EAUG in sgx_vma_fault() is not satisfied because the page is still present in encl->page_array, thus the SGX driver assumes that the fault happened because the page was swapped out. The driver continues on a code path that installs a page table entry *without* performing EAUG.
3. The enclave page metadata is in inconsistent state: the PTE is installed but there was no EAUG. Thus, T2 in userspace infinitely receives SIGSEGV on this page (and EACCEPT always fails).

Fix this by making sure that T1 (the page-removing thread) always wins this data race. In particular, the page-being-removed is marked as such, and T2 retries until the page is fully removed.

Fixes: 9849bb27152c ("x86/sgx: Support complete page removal") Cc: stable@vger.kernel.org Signed-off-by: Dmitrii Kuvaiskii dmitrii.kuvaiskii@intel.com

---

arch/x86/kernel/cpu/sgx/encl.c | 3 ++- arch/x86/kernel/cpu/sgx/encl.h | 3 +++ arch/x86/kernel/cpu/sgx/ioctl.c | 1 + 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c index 41f14b1a3025..7ccd8b2fce5f 100644 --- a/arch/x86/kernel/cpu/sgx/encl.c +++ b/arch/x86/kernel/cpu/sgx/encl.c @@ -257,7 +257,8 @@ static struct sgx_encl_page *__sgx_encl_load_page(struct sgx_encl *encl, /* Entry successfully located. */ if (entry->epc_page) {

• if (entry->desc & SGX_ENCL_PAGE_BEING_RECLAIMED)
  

• if (entry->desc & (SGX_ENCL_PAGE_BEING_RECLAIMED |
  

• 		   SGX_ENCL_PAGE_BEING_REMOVED))
  return ERR_PTR(-EBUSY);
  

return entry; diff --git a/arch/x86/kernel/cpu/sgx/encl.h b/arch/x86/kernel/cpu/sgx/encl.h index f94ff14c9486..fff5f2293ae7 100644 --- a/arch/x86/kernel/cpu/sgx/encl.h +++ b/arch/x86/kernel/cpu/sgx/encl.h @@ -25,6 +25,9 @@ /* 'desc' bit marking that the page is being reclaimed. */ #define SGX_ENCL_PAGE_BEING_RECLAIMED BIT(3) +/* 'desc' bit marking that the page is being removed. */ +#define SGX_ENCL_PAGE_BEING_REMOVED BIT(2)

struct sgx_encl_page { unsigned long desc; unsigned long vm_max_prot_bits:8; diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c index b65ab214bdf5..c542d4dd3e64 100644 --- a/arch/x86/kernel/cpu/sgx/ioctl.c +++ b/arch/x86/kernel/cpu/sgx/ioctl.c @@ -1142,6 +1142,7 @@ static long sgx_encl_remove_pages(struct sgx_encl *encl, * Do not keep encl->lock because of dependency on * mmap_lock acquired in sgx_zap_enclave_ptes(). */

• entry->desc |= SGX_ENCL_PAGE_BEING_REMOVED;
  

  mutex_unlock(&encl->lock);

sgx_zap_enclave_ptes(encl, addr);

It is somewhat trivial to NAK this as the commit message does not do any effort describing the new flag. By default at least I have strong opposition against any new flags related to reclaiming even if it needs a bit of extra synchronization work in the user space.

One way to describe concurrency scenarios would be to take example from https://www.kernel.org/doc/Documentation/memory-barriers.txt

I.e. see the examples with CPU 1 and CPU 2.

Thank you for the suggestion. Here is my new attempt at describing the racy scenario:

Consider some enclave page added to the enclave. User space decides to temporarily remove this page (e.g., emulating the MADV_DONTNEED semantics) on CPU1. At the same time, user space performs a memory access on the same page on CPU2, which results in a #PF and ultimately in sgx_vma_fault(). Scenario proceeds as follows:

/*

• CPU1: User space performs
• ioctl(SGX_IOC_ENCLAVE_REMOVE_PAGES)
• on a single enclave page

*/ sgx_encl_remove_pages() {

mutex_lock(&encl->lock);

entry = sgx_encl_load_page(encl); /*

• verify that page is
• trimmed and accepted

*/

mutex_unlock(&encl->lock);

/*

• remove PTE entry; cannot
• be performed under lock

*/ sgx_zap_enclave_ptes(encl); /* * Fault on CPU2 */

Please highlight that this fault is related to the page that is in process of being removed on CPU1.

                               sgx_vma_fault() {
                                 /*
                                  * PTE entry was removed, but the
                                  * page is still in enclave's xarray
                                  */
                                 xa_load(&encl->page_array) != NULL ->
                                 /*
                                  * SGX driver thinks that this page
                                  * was swapped out and loads it
                                  */
                                 mutex_lock(&encl->lock);
                                 /*
                                  * this is effectively a no-op
                                  */
                                 entry = sgx_encl_load_page_in_vma();
                                 /*
                                  * add PTE entry
                                  */

It may be helpful to highlight that this is a problem: "BUG: A PTE is installed for a page in process of being removed." (please feel free to expand)

                                 vmf_insert_pfn(...);

                                 mutex_unlock(&encl->lock);
                                 return VM_FAULT_NOPAGE;
                               }

/*

• continue with page removal

*/ mutex_lock(&encl->lock);

sgx_encl_free_epc_page(epc_page) { /* * remove page via EREMOVE */ /* * free EPC page */ sgx_free_epc_page(epc_page); }

xa_erase(&encl->page_array);

mutex_unlock(&encl->lock); }

CPU1 removed the page. However CPU2 installed the PTE entry on the same page. This enclave page becomes perpetually inaccessible (until another SGX_IOC_ENCLAVE_REMOVE_PAGES ioctl). This is because the page is marked accessible in the PTE entry but is not EAUGed. Because of this combination, any subsequent access to this page raises a fault, and the #PF handler sees the SGX bit set in the #PF error code and does not call

Which #PF handler?

sgx_vma_fault() but instead raises a SIGSEGV. The userspace SIGSEGV handler cannot perform EACCEPT because the page was not EAUGed. Thus, the user space is stuck with the inaccessible page.

This race can be fixed by forcing the fault handler on CPU2 to back off if the page is currently being removed (on CPU1). Thus a simple change is to introduce a new flag SGX_ENCL_PAGE_BEING_REMOVED, which is unset by default and set only right-before the first mutex_unlock() in sgx_encl_remove_pages(). Upon loading the page, CPU2 checks whether this page is being removed, and if yes then CPU2 backs off and waits until the page is completely removed. After that, any memory access to this page results in a normal "allocate and EAUG a page on #PF" flow.

I have been tripped by these page flags before so would appreciate another opinion. From my side this looks like an appropriate fix.

Reinette