[PATCH v4 03/25] x86/sgx: Wipe out EREMOVE from sgx_free_epc_page()

From: Kai Huang
Date: Thu Mar 25 2021 - 05:32:18 EST

EREMOVE takes a page and removes any association between that page and
an enclave. It must be run on a page before it can be added into
another enclave. Currently, EREMOVE is run as part of pages being freed
into the SGX page allocator. It is not expected to fail, as it would
indicate a use-after-free of EPC. Rather than add the page back to the
pool of available EPC, the kernel intentionally leaks the page to avoid
additional errors in the future.

However, KVM does not track how guest pages are used, which means that
SGX virtualization use of EREMOVE might fail. Specifically, it is
legitimate that EREMOVE returns SGX_CHILD_PRESENT for EPC assigned to
KVM guest, because KVM/kernel doesn't track SECS pages.

To allow SGX/KVM to introduce a more permissive EREMOVE helper and to
let the SGX virtualization code use the allocator directly, break out
the EREMOVE call from the SGX page allocator. Rename the original
sgx_free_epc_page() to sgx_encl_free_epc_page(), indicating that it is
used to free EPC page assigned host enclave. Replace sgx_free_epc_page()
with sgx_encl_free_epc_page() in all call sites so there's no functional
change.

At the same time improve error message when EREMOVE fails, and add
documentation to explain to user what is the bug and suggest user what
to do when this bug happens, although extremely unlikely.

Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
---
Documentation/x86/sgx.rst | 27 +++++++++++++++++++++++++++
arch/x86/kernel/cpu/sgx/encl.c | 32 +++++++++++++++++++++++++++-----
arch/x86/kernel/cpu/sgx/encl.h | 1 +
arch/x86/kernel/cpu/sgx/ioctl.c | 6 +++---
arch/x86/kernel/cpu/sgx/main.c | 14 +++++---------
arch/x86/kernel/cpu/sgx/sgx.h | 5 +++++
6 files changed, 68 insertions(+), 17 deletions(-)

diff --git a/Documentation/x86/sgx.rst b/Documentation/x86/sgx.rst
index eaee1368b4fd..5ec7d17e65e0 100644
--- a/Documentation/x86/sgx.rst
+++ b/Documentation/x86/sgx.rst
@@ -209,3 +209,30 @@ An application may be loaded into a container enclave which is specially
configured with a library OS and run-time which permits the application to run.
The enclave run-time and library OS work together to execute the application
when a thread enters the enclave.
+
+Impact of Potential Kernel SGX Bugs
+===================================
+
+EPC leaks
+---------
+
+EPC leaks can happen if kernel SGX bug happens, when a WARNING with below
+message is shown in dmesg:
+
+"...EREMOVE returned ... and an EPC page was leaked. SGX may become unusuable.
+This is likely a kernel bug. Refer to Documentation/x86/sgx.rst for more
+information."
+
+This is effectively a kernel use-after-free of EPC, and due to the way SGX
+works, the bug is detected at freeing. Rather than add the page back to the pool
+of available EPC, the kernel intentionally leaks the page to avoid additional
+errors in the future.
+
+When this happens, kernel will likely soon leak majority of EPC pages, and SGX
+will likely become unusable. However while this may be fatal to SGX, other
+kernel functionalities are unlikely to be impacted, and should continue to work.
+
+As a result, when this happpens, user should stop running any new SGX workloads,
+(or just any new workloads), and migrate all valuable workloads. Although a
+machine reboot can recover all EPC, the bug should be reported to Linux
+developers.
diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index 7449ef33f081..26c0987153de 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -78,7 +78,7 @@ static struct sgx_epc_page *sgx_encl_eldu(struct sgx_encl_page *encl_page,

ret = __sgx_encl_eldu(encl_page, epc_page, secs_page);
if (ret) {
- sgx_free_epc_page(epc_page);
+ sgx_encl_free_epc_page(epc_page);
return ERR_PTR(ret);
}

@@ -404,7 +404,7 @@ void sgx_encl_release(struct kref *ref)
if (sgx_unmark_page_reclaimable(entry->epc_page))
continue;

- sgx_free_epc_page(entry->epc_page);
+ sgx_encl_free_epc_page(entry->epc_page);
encl->secs_child_cnt--;
entry->epc_page = NULL;
}
@@ -415,7 +415,7 @@ void sgx_encl_release(struct kref *ref)
xa_destroy(&encl->page_array);

if (!encl->secs_child_cnt && encl->secs.epc_page) {
- sgx_free_epc_page(encl->secs.epc_page);
+ sgx_encl_free_epc_page(encl->secs.epc_page);
encl->secs.epc_page = NULL;
}

@@ -423,7 +423,7 @@ void sgx_encl_release(struct kref *ref)
va_page = list_first_entry(&encl->va_pages, struct sgx_va_page,
list);
list_del(&va_page->list);
- sgx_free_epc_page(va_page->epc_page);
+ sgx_encl_free_epc_page(va_page->epc_page);
kfree(va_page);
}

@@ -686,7 +686,7 @@ struct sgx_epc_page *sgx_alloc_va_page(void)
ret = __epa(sgx_get_epc_virt_addr(epc_page));
if (ret) {
WARN_ONCE(1, "EPA returned %d (0x%x)", ret, ret);
- sgx_free_epc_page(epc_page);
+ sgx_encl_free_epc_page(epc_page);
return ERR_PTR(-EFAULT);
}

@@ -735,3 +735,25 @@ bool sgx_va_page_full(struct sgx_va_page *va_page)

return slot == SGX_VA_SLOT_COUNT;
}
+
+/**
+ * sgx_encl_free_epc_page - free EPC page assigned to an enclave
+ * @page: EPC page to be freed
+ *
+ * Free EPC page assigned to an enclave. It does EREMOVE for the page, and
+ * only upon success, it puts the page back to free page list. Otherwise, it
+ * gives a WARNING to indicate page is leaked, and require reboot to retrieve
+ * leaked pages.
+ */
+void sgx_encl_free_epc_page(struct sgx_epc_page *page)
+{
+ int ret;
+
+ WARN_ON_ONCE(page->flags & SGX_EPC_PAGE_RECLAIMER_TRACKED);
+
+ ret = __eremove(sgx_get_epc_virt_addr(page));
+ if (WARN_ONCE(ret, EREMOVE_ERROR_MESSAGE, ret, ret))
+ return;
+
+ sgx_free_epc_page(page);
+}
diff --git a/arch/x86/kernel/cpu/sgx/encl.h b/arch/x86/kernel/cpu/sgx/encl.h
index d8d30ccbef4c..6e74f85b6264 100644
--- a/arch/x86/kernel/cpu/sgx/encl.h
+++ b/arch/x86/kernel/cpu/sgx/encl.h
@@ -115,5 +115,6 @@ struct sgx_epc_page *sgx_alloc_va_page(void);
unsigned int sgx_alloc_va_slot(struct sgx_va_page *va_page);
void sgx_free_va_slot(struct sgx_va_page *va_page, unsigned int offset);
bool sgx_va_page_full(struct sgx_va_page *va_page);
+void sgx_encl_free_epc_page(struct sgx_epc_page *page);

#endif /* _X86_ENCL_H */
diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
index 90a5caf76939..772b9c648cf1 100644
--- a/arch/x86/kernel/cpu/sgx/ioctl.c
+++ b/arch/x86/kernel/cpu/sgx/ioctl.c
@@ -47,7 +47,7 @@ static void sgx_encl_shrink(struct sgx_encl *encl, struct sgx_va_page *va_page)
encl->page_cnt--;

if (va_page) {
- sgx_free_epc_page(va_page->epc_page);
+ sgx_encl_free_epc_page(va_page->epc_page);
list_del(&va_page->list);
kfree(va_page);
}
@@ -117,7 +117,7 @@ static int sgx_encl_create(struct sgx_encl *encl, struct sgx_secs *secs)
return 0;

err_out:
- sgx_free_epc_page(encl->secs.epc_page);
+ sgx_encl_free_epc_page(encl->secs.epc_page);
encl->secs.epc_page = NULL;

err_out_backing:
@@ -365,7 +365,7 @@ static int sgx_encl_add_page(struct sgx_encl *encl, unsigned long src,
mmap_read_unlock(current->mm);

err_out_free:
- sgx_free_epc_page(epc_page);
+ sgx_encl_free_epc_page(epc_page);
kfree(encl_page);

return ret;
diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
index 13a7599ce7d4..b227629b1e9c 100644
--- a/arch/x86/kernel/cpu/sgx/main.c
+++ b/arch/x86/kernel/cpu/sgx/main.c
@@ -294,7 +294,7 @@ static void sgx_reclaimer_write(struct sgx_epc_page *epc_page,

sgx_encl_ewb(encl->secs.epc_page, &secs_backing);

- sgx_free_epc_page(encl->secs.epc_page);
+ sgx_encl_free_epc_page(encl->secs.epc_page);
encl->secs.epc_page = NULL;

sgx_encl_put_backing(&secs_backing, true);
@@ -609,19 +609,15 @@ struct sgx_epc_page *sgx_alloc_epc_page(void *owner, bool reclaim)
* sgx_free_epc_page() - Free an EPC page
* @page: an EPC page
*
- * Call EREMOVE for an EPC page and insert it back to the list of free pages.
+ * Put the EPC page back to the list of free pages. It's the caller's
+ * responsibility to make sure that the page is in uninitialized state. In other
+ * words, do EREMOVE, EWB or whatever operation is necessary before calling
+ * this function.
*/
void sgx_free_epc_page(struct sgx_epc_page *page)
{
struct sgx_epc_section *section = &sgx_epc_sections[page->section];
struct sgx_numa_node *node = section->node;
- int ret;
-
- WARN_ON_ONCE(page->flags & SGX_EPC_PAGE_RECLAIMER_TRACKED);
-
- ret = __eremove(sgx_get_epc_virt_addr(page));
- if (WARN_ONCE(ret, "EREMOVE returned %d (0x%x)", ret, ret))
- return;

spin_lock(&node->lock);

diff --git a/arch/x86/kernel/cpu/sgx/sgx.h b/arch/x86/kernel/cpu/sgx/sgx.h
index 653af8ca1a25..6b21a165500e 100644
--- a/arch/x86/kernel/cpu/sgx/sgx.h
+++ b/arch/x86/kernel/cpu/sgx/sgx.h
@@ -13,6 +13,11 @@
#undef pr_fmt
#define pr_fmt(fmt) "sgx: " fmt

+/* Error message for EREMOVE failure, when kernel is about to leak EPC page */
+#define EREMOVE_ERROR_MESSAGE \
+ "EREMOVE returned %d (0x%x) and an EPC page was leaked. SGX may become unusuable. " \
+ "This is likely a kernel bug. Refer to Documentation/x86/sgx.rst for more information."
+
#define SGX_MAX_EPC_SECTIONS 8
#define SGX_EEXTEND_BLOCK_SIZE 256
#define SGX_NR_TO_SCAN 16
--
2.30.2