On Wed Aug 21, 2024 at 1:02 PM EEST, Dmitrii Kuvaiskii wrote:
> Imagine an mmap()'d file. Two threads touch the same address at the same
> time and fault. Both allocate a physical page and race to install a PTE
> for that page. Only one will win the race. The loser frees its page, but
> still continues handling the fault as a success and returns
> VM_FAULT_NOPAGE from the fault handler.
>
> The same race can happen with SGX. But there's a bug: the loser in the
> SGX steers into a failure path. The loser EREMOVE's the winner's EPC
> page, then returns SIGBUS, likely killing the app.
>
> Fix the SGX loser's behavior. Check whether another thread already
> allocated the page and if yes, return with VM_FAULT_NOPAGE.
>
> The race can be illustrated as follows:
>
> /* /*
> * Fault on CPU1 * Fault on CPU2
> * on enclave page X * on enclave page X
> */ */
> sgx_vma_fault() { sgx_vma_fault() {
>
> xa_load(&encl->page_array) xa_load(&encl->page_array)
> == NULL --> == NULL -->
>
> sgx_encl_eaug_page() { sgx_encl_eaug_page() {
>
> ... ...
>
> /* /*
> * alloc encl_page * alloc encl_page
> */ */
> mutex_lock(&encl->lock);
> /*
> * alloc EPC page
> */
> epc_page = sgx_alloc_epc_page(...);
> /*
> * add page to enclave's xarray
> */
> xa_insert(&encl->page_array, ...);
> /*
> * add page to enclave via EAUG
> * (page is in pending state)
> */
> /*
> * add PTE entry
> */
> vmf_insert_pfn(...);
>
> mutex_unlock(&encl->lock);
> return VM_FAULT_NOPAGE;
> }
> }
> /*
> * All good up to here: enclave page
> * successfully added to enclave,
> * ready for EACCEPT from user space
> */
> mutex_lock(&encl->lock);
> /*
> * alloc EPC page
> */
> epc_page = sgx_alloc_epc_page(...);
> /*
> * add page to enclave's xarray,
> * this fails with -EBUSY as this
> * page was already added by CPU2
> */
> xa_insert(&encl->page_array, ...);
>
> err_out_shrink:
> sgx_encl_free_epc_page(epc_page) {
> /*
> * remove page via EREMOVE
> *
> * *BUG*: page added by CPU2 is
> * yanked from enclave while it
> * remains accessible from OS
> * perspective (PTE installed)
> */
> /*
> * free EPC page
> */
> sgx_free_epc_page(epc_page);
> }
>
> mutex_unlock(&encl->lock);
> /*
> * *BUG*: SIGBUS is returned
> * for a valid enclave page
> */
> return VM_FAULT_SIGBUS;
> }
> }
>
> Fixes: 5a90d2c3f5ef ("x86/sgx: Support adding of pages to an initialized enclave")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Marcelina Kościelnicka <mwk@xxxxxxxxxxxxxxxxxxxxxx>
> Suggested-by: Kai Huang <kai.huang@xxxxxxxxx>
> Signed-off-by: Dmitrii Kuvaiskii <dmitrii.kuvaiskii@xxxxxxxxx>
> ---
> arch/x86/kernel/cpu/sgx/encl.c | 36 ++++++++++++++++++++--------------
> 1 file changed, 21 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index c0a3c00284c8..2aa7ced0e4a0 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -337,6 +337,16 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
> if (!test_bit(SGX_ENCL_INITIALIZED, &encl->flags))
> return VM_FAULT_SIGBUS;
>
> + mutex_lock(&encl->lock);
> +
> + /*
> + * Multiple threads may try to fault on the same page concurrently.
> + * Re-check if another thread has already done that.
> + */
> + encl_page = xa_load(&encl->page_array, PFN_DOWN(addr));
> + if (encl_page)
> + goto done;
> +
> /*
> * Ignore internal permission checking for dynamically added pages.
> * They matter only for data added during the pre-initialization
> @@ -345,23 +355,23 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
> */
> secinfo_flags = SGX_SECINFO_R | SGX_SECINFO_W | SGX_SECINFO_X;
> encl_page = sgx_encl_page_alloc(encl, addr - encl->base, secinfo_flags);
> - if (IS_ERR(encl_page))
> - return VM_FAULT_OOM;
> -
> - mutex_lock(&encl->lock);
> + if (IS_ERR(encl_page)) {
> + vmret = VM_FAULT_OOM;
> + goto err_out_unlock;
> + }
>
> epc_page = sgx_encl_load_secs(encl);
> if (IS_ERR(epc_page)) {
> if (PTR_ERR(epc_page) == -EBUSY)
> vmret = VM_FAULT_NOPAGE;
> - goto err_out_unlock;
> + goto err_out_encl;
> }
>
> epc_page = sgx_alloc_epc_page(encl_page, false);
> if (IS_ERR(epc_page)) {
> if (PTR_ERR(epc_page) == -EBUSY)
> vmret = VM_FAULT_NOPAGE;
> - goto err_out_unlock;
> + goto err_out_encl;
> }
>
> va_page = sgx_encl_grow(encl, false);
> @@ -376,10 +386,6 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
>
> ret = xa_insert(&encl->page_array, PFN_DOWN(encl_page->desc),
> encl_page, GFP_KERNEL);
> - /*
> - * If ret == -EBUSY then page was created in another flow while
> - * running without encl->lock
> - */
> if (ret)
> goto err_out_shrink;
>
> @@ -389,7 +395,7 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
>
> ret = __eaug(&pginfo, sgx_get_epc_virt_addr(epc_page));
> if (ret)
> - goto err_out;
> + goto err_out_eaug;
>
> encl_page->encl = encl;
> encl_page->epc_page = epc_page;
> @@ -408,20 +414,20 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
> mutex_unlock(&encl->lock);
> return VM_FAULT_SIGBUS;
> }
> +done:
> mutex_unlock(&encl->lock);
> return VM_FAULT_NOPAGE;
>
> -err_out:
> +err_out_eaug:
> xa_erase(&encl->page_array, PFN_DOWN(encl_page->desc));
> -
> err_out_shrink:
> sgx_encl_shrink(encl, va_page);
> err_out_epc:
> sgx_encl_free_epc_page(epc_page);
> +err_out_encl:
> + kfree(encl_page);
> err_out_unlock:
> mutex_unlock(&encl->lock);
> - kfree(encl_page);
> -
> return vmret;
> }
>
Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
BR, Jarkko
