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x86/sgx: Wipe out EREMOVE from sgx_free_epc_page() 

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 pages. Rather than add the page back to the pool
of available EPC pages, 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 an EPC page assigned to a 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 the error message when EREMOVE fails, and
add documentation to explain to the user what that failure means and
to suggest to the user what to do when this bug happens in the case it
happens.

 [ bp: Massage commit message, fix typos and sanitize text, simplify. ]

Signed-off-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lkml.kernel.org/r/20210325093057.122834-1-kai.huang@intel.com
This commit is contained in:
Kai Huang 2021-03-25 22:30:57 +13:00 committed by Borislav Petkov
parent b8921dccf3
commit b0c7459be0
6 changed files with 64 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
Documentation/x86/sgx.rst
View File

@ -209,3 +209,28 @@ 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
---------
When EPC page leaks happen, a WARNING like this is shown in dmesg:
"EREMOVE returned ... and an EPC page was leaked. SGX may become unusable..."
This is effectively a kernel use-after-free of an EPC page, and due
to the way SGX works, the bug is detected at freeing. Rather than
adding the page back to the pool of available EPC pages, the kernel
intentionally leaks the page to avoid additional errors in the future.
When this happens, the kernel will likely soon leak more EPC pages, and
SGX will likely become unusable because the memory available to SGX is
limited. However, while this may be fatal to SGX, the rest of the kernel
is 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 memory, the bug
should be reported to Linux developers.

31
arch/x86/kernel/cpu/sgx/encl.c
View File

@ -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 +735,24 @@ bool sgx_va_page_full(struct sgx_va_page *va_page)
return slot == SGX_VA_SLOT_COUNT;
}
/**
* sgx_encl_free_epc_page - free an EPC page assigned to an enclave
* @page: EPC page to be freed
*
* Free an 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.
*/
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);
}

1
arch/x86/kernel/cpu/sgx/encl.h
View File

@ -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 */

6
arch/x86/kernel/cpu/sgx/ioctl.c
View File

@ -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 @@ err_out_unlock:
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;

14
arch/x86/kernel/cpu/sgx/main.c
View File

@ -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);

4
arch/x86/kernel/cpu/sgx/sgx.h
View File

@ -13,6 +13,10 @@
#undef pr_fmt
#define pr_fmt(fmt) "sgx: " fmt
#define EREMOVE_ERROR_MESSAGE \
"EREMOVE returned %d (0x%x) and an EPC page was leaked. SGX may become unusable. " \
"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