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Date: Wed, 15 May 2024 17:01:31 -0500
From: "Haitao Huang" <haitao.huang@...ux.intel.com>
To: dave.hansen@...ux.intel.com, jarkko@...nel.org, kai.huang@...el.com,
reinette.chatre@...el.com, linux-sgx@...r.kernel.org,
linux-kernel@...r.kernel.org, "Dmitrii Kuvaiskii"
<dmitrii.kuvaiskii@...el.com>
Cc: mona.vij@...el.com, kailun.qin@...el.com, stable@...r.kernel.org,
Marcelina Kościelnicka <mwk@...isiblethingslab.com>
Subject: Re: [PATCH v2 1/2] x86/sgx: Resolve EAUG race where losing thread
returns SIGBUS
On Wed, 15 May 2024 08:12:39 -0500, Dmitrii Kuvaiskii
<dmitrii.kuvaiskii@...el.com> wrote:
> Two enclave threads may try to access the same non-present enclave page
> simultaneously (e.g., if the SGX runtime supports lazy allocation). The
> threads will end up in sgx_encl_eaug_page(), racing to acquire the
> enclave lock. The winning thread will perform EAUG, set up the page
> table entry, and insert the page into encl->page_array. The losing
> thread will then get -EBUSY on xa_insert(&encl->page_array) and proceed
> to error handling path.
>
> This race condition 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;
> }
> }
>
> The err_out_shrink error handling path contains two bugs: (1) function
> sgx_encl_free_epc_page() is called that performs EREMOVE even though the
> enclave page was never intended to be removed, and (2) SIGBUS is sent to
> userspace even though the enclave page is correctly installed by another
> thread.
>
> The first bug renders the enclave page 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. The second bug is less severe: a spurious SIGBUS signal is
> unnecessarily sent to user space.
>
> Fix these two bugs (1) by returning VM_FAULT_NOPAGE to the generic Linux
> fault handler so that no signal is sent to userspace, and (2) by
> replacing sgx_encl_free_epc_page() with sgx_free_epc_page() so that no
> EREMOVE is performed.
>
> Note that sgx_encl_free_epc_page() performs an additional WARN_ON_ONCE
> check in comparison to sgx_free_epc_page(): whether the EPC page is
> being reclaimer tracked. However, the EPC page is allocated in
> sgx_encl_eaug_page() and has zeroed-out flags in all error handling
> paths. In other words, the page is marked as reclaimable only in the
> happy path of sgx_encl_eaug_page(). Therefore, in the particular code
> path affected in this commit, the "page reclaimer tracked" condition is
> always false and the warning is never printed. Thus, it is safe to
> replace sgx_encl_free_epc_page() with sgx_free_epc_page().
>
> Fixes: 5a90d2c3f5ef ("x86/sgx: Support adding of pages to an initialized
> enclave")
> Cc: stable@...r.kernel.org
> Reported-by: Marcelina Kościelnicka <mwk@...isiblethingslab.com>
> Suggested-by: Reinette Chatre <reinette.chatre@...el.com>
> Signed-off-by: Dmitrii Kuvaiskii <dmitrii.kuvaiskii@...el.com>
> ---
> arch/x86/kernel/cpu/sgx/encl.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c
> b/arch/x86/kernel/cpu/sgx/encl.c
> index 279148e72459..41f14b1a3025 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -382,8 +382,11 @@ static vm_fault_t sgx_encl_eaug_page(struct
> vm_area_struct *vma,
> * If ret == -EBUSY then page was created in another flow while
> * running without encl->lock
> */
> - if (ret)
> + if (ret) {
> + if (ret == -EBUSY)
> + vmret = VM_FAULT_NOPAGE;
> goto err_out_shrink;
> + }
> pginfo.secs = (unsigned long)sgx_get_epc_virt_addr(encl->secs.epc_page);
> pginfo.addr = encl_page->desc & PAGE_MASK;
> @@ -419,7 +422,7 @@ static vm_fault_t sgx_encl_eaug_page(struct
> vm_area_struct *vma,
> err_out_shrink:
> sgx_encl_shrink(encl, va_page);
> err_out_epc:
> - sgx_encl_free_epc_page(epc_page);
> + sgx_free_epc_page(epc_page);
> err_out_unlock:
> mutex_unlock(&encl->lock);
> kfree(encl_page);
Reviewed-by: Haitao Huang <haitao.huang@...ux.intel.com>
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