On Wed Aug 21, 2024 at 1:02 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 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 enclave page X

*/ 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 on same page X */ 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 * * *BUG*: a PTE is installed for a * page in process of being removed */ 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); }

Here, 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, and any subsequent access to this page raises a fault: with the kernel believing there to be a valid VMA, the unlikely error code X86_PF_SGX encountered by code path do_user_addr_fault() -> access_error() causes the SGX driver's sgx_vma_fault() to be skipped and user space receives a SIGSEGV instead. The userspace SIGSEGV handler cannot perform EACCEPT because the page was not EAUGed. Thus, the user space is stuck with the inaccessible page.

Fix this race by forcing the fault handler on CPU2 to back off if the page is currently being removed (on CPU1). This is achieved by setting SGX_ENCL_PAGE_BUSY flag right-before the first mutex_unlock() in sgx_encl_remove_pages(). Upon loading the page, CPU2 checks whether this page is busy, 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.

Additionally fix a similar race: user space converts a normal enclave page to a TCS page (via SGX_IOC_ENCLAVE_MODIFY_TYPES) on CPU1, and at the same time, user space performs a memory access on the same page on CPU2. This fix is not strictly necessary (this particular race would indicate a bug in a user space application), but it gives a consistent rule: if an enclave page is under certain operation by the kernel with the mapping removed, then other threads trying to access that page are temporarily blocked and should retry.

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.h | 3 ++- arch/x86/kernel/cpu/sgx/ioctl.c | 17 +++++++++++++++++ 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/sgx/encl.h b/arch/x86/kernel/cpu/sgx/encl.h index b566b8ad5f33..96b11e8fb770 100644 --- a/arch/x86/kernel/cpu/sgx/encl.h +++ b/arch/x86/kernel/cpu/sgx/encl.h @@ -22,7 +22,8 @@ /* 'desc' bits holding the offset in the VA (version array) page. */ #define SGX_ENCL_PAGE_VA_OFFSET_MASK GENMASK_ULL(11, 3) -/* 'desc' bit indicating that the page is busy (being reclaimed). */ +/* 'desc' bit indicating that the page is busy (being reclaimed, removed or

• • converted to a TCS page). */

#define SGX_ENCL_PAGE_BUSY BIT(2) /* diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c index 5d390df21440..ee619f2b3414 100644 --- a/arch/x86/kernel/cpu/sgx/ioctl.c +++ b/arch/x86/kernel/cpu/sgx/ioctl.c @@ -969,12 +969,22 @@ static long sgx_enclave_modify_types(struct sgx_encl *encl, /* * Do not keep encl->lock because of dependency on * mmap_lock acquired in sgx_zap_enclave_ptes().

• 	 *
  

• 	 * Releasing encl->lock leads to a data race: while CPU1
  

• 	 * performs sgx_zap_enclave_ptes() and removes the PTE
  

• 	 * entry for the enclave page, CPU2 may attempt to load
  

• 	 * this page (because the page is still in enclave's
  

• 	 * xarray). To prevent CPU2 from loading the page, mark
  

• 	 * the page as busy before unlock and unmark after lock
  

• 	 * again.
   */
  

• 	entry->desc |= SGX_ENCL_PAGE_BUSY;
  mutex_unlock(&encl->lock);
  

sgx_zap_enclave_ptes(encl, addr); mutex_lock(&encl->lock);

• 	entry->desc &= ~SGX_ENCL_PAGE_BUSY;
  

sgx_mark_page_reclaimable(entry->epc_page); } @@ -1141,7 +1151,14 @@ 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().

•  *
  

•  * Releasing encl->lock leads to a data race: while CPU1
  

•  * performs sgx_zap_enclave_ptes() and removes the PTE entry
  

•  * for the enclave page, CPU2 may attempt to load this page
  

•  * (because the page is still in enclave's xarray). To prevent
  

•  * CPU2 from loading the page, mark the page as busy.
  

  */

• entry->desc |= SGX_ENCL_PAGE_BUSY;
  

  mutex_unlock(&encl->lock);

sgx_zap_enclave_ptes(encl, addr);

Reviewed-by: Jarkko Sakkinen jarkko@kernel.org

BR, Jarkko