The SGX reclaimer code lacks page poison handling in its main
free path. This can lead to avoidable machine checks if a
poisoned page is freed and reallocated instead of being
isolated.
A troublesome scenario is:
1. Machine check (#MC) occurs (asynchronous, !MF_ACTION_REQUIRED)
2. arch_memory_failure() is eventually called
3. (SGX) page->poison set to 1
4. Page is reclaimed
5. Page added to normal free lists by sgx_reclaim_pages()
^ This is the bug (poison pages should be isolated on the
sgx_poison_page_list instead)
6. Page is reallocated by some innocent enclave, a second (synchronous)
in-kernel #MC is induced, probably during EADD instruction.
^ This is the fallout from the bug
(6) is unfortunate and can be avoided by replacing the open coded
enclave page freeing code in the reclaimer with sgx_free_epc_page()
to obtain support for poison page handling that includes placing the
poisoned page on the correct list.
Fixes: d6d261bded8a ("x86/sgx: Add new sgx_epc_page flag bit to mark free pages")
Fixes: 992801ae9243 ("x86/sgx: Initial poison handling for dirty and free pages")
Reviewed-by: Jarkko Sakkinen <[email protected]>
Signed-off-by: Reinette Chatre <[email protected]>
---
Changes since V2:
- V2: https://lore.kernel.org/linux-sgx/be5af586f667c7bcb8ef01286ce75675de5d100f.1642630582.git.reinette.chatre@intel.com/
- Add Reviewed-by from Jarkko.
- Remove unused variables.
Changes since V1:
- V1: https://lore.kernel.org/lkml/ef74bd9548df61f77e802e7505affcfb5159c48c.1642545829.git.reinette.chatre@intel.com/
- Complete rewrite of commit message with significant guidance from Dave
who provided the summary as well as troublesome scenario.
arch/x86/kernel/cpu/sgx/main.c | 10 +---------
1 file changed, 1 insertion(+), 9 deletions(-)
diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
index 4b41efc9e367..8e4bc6453d26 100644
--- a/arch/x86/kernel/cpu/sgx/main.c
+++ b/arch/x86/kernel/cpu/sgx/main.c
@@ -344,10 +344,8 @@ static void sgx_reclaim_pages(void)
{
struct sgx_epc_page *chunk[SGX_NR_TO_SCAN];
struct sgx_backing backing[SGX_NR_TO_SCAN];
- struct sgx_epc_section *section;
struct sgx_encl_page *encl_page;
struct sgx_epc_page *epc_page;
- struct sgx_numa_node *node;
pgoff_t page_index;
int cnt = 0;
int ret;
@@ -418,13 +416,7 @@ static void sgx_reclaim_pages(void)
kref_put(&encl_page->encl->refcount, sgx_encl_release);
epc_page->flags &= ~SGX_EPC_PAGE_RECLAIMER_TRACKED;
- section = &sgx_epc_sections[epc_page->section];
- node = section->node;
-
- spin_lock(&node->lock);
- list_add_tail(&epc_page->list, &node->free_page_list);
- spin_unlock(&node->lock);
- atomic_long_inc(&sgx_nr_free_pages);
+ sgx_free_epc_page(epc_page);
}
}
--
2.25.1
Dear Maintainers,
On 2/2/2022 11:41 AM, Reinette Chatre wrote:
> The SGX reclaimer code lacks page poison handling in its main
> free path. This can lead to avoidable machine checks if a
> poisoned page is freed and reallocated instead of being
> isolated.
>
> A troublesome scenario is:
> 1. Machine check (#MC) occurs (asynchronous, !MF_ACTION_REQUIRED)
> 2. arch_memory_failure() is eventually called
> 3. (SGX) page->poison set to 1
> 4. Page is reclaimed
> 5. Page added to normal free lists by sgx_reclaim_pages()
> ^ This is the bug (poison pages should be isolated on the
> sgx_poison_page_list instead)
> 6. Page is reallocated by some innocent enclave, a second (synchronous)
> in-kernel #MC is induced, probably during EADD instruction.
> ^ This is the fallout from the bug
>
> (6) is unfortunate and can be avoided by replacing the open coded
> enclave page freeing code in the reclaimer with sgx_free_epc_page()
> to obtain support for poison page handling that includes placing the
> poisoned page on the correct list.
>
> Fixes: d6d261bded8a ("x86/sgx: Add new sgx_epc_page flag bit to mark free pages")
> Fixes: 992801ae9243 ("x86/sgx: Initial poison handling for dirty and free pages")
> Reviewed-by: Jarkko Sakkinen <[email protected]>
> Signed-off-by: Reinette Chatre <[email protected]>
Could you please consider this fix for inclusion?
Thank you very much
Reinette
On 2/17/2022 10:26 AM, Dave Hansen wrote:
> On 2/16/22 16:25, Reinette Chatre wrote:
>>> Fixes: d6d261bded8a ("x86/sgx: Add new sgx_epc_page flag bit to mark free pages")
>>> Fixes: 992801ae9243 ("x86/sgx: Initial poison handling for dirty and free pages")
>>> Reviewed-by: Jarkko Sakkinen <[email protected]>
>>> Signed-off-by: Reinette Chatre <[email protected]>
>> Could you please consider this fix for inclusion?
>
> Thanks for the reminder. It's merged here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=x86/urgent&id=e5733d8c89c3b5
Thank you very much. Also thank you for improving the patch subject.
Reinette
On 2/16/22 16:25, Reinette Chatre wrote:
>> Fixes: d6d261bded8a ("x86/sgx: Add new sgx_epc_page flag bit to mark free pages")
>> Fixes: 992801ae9243 ("x86/sgx: Initial poison handling for dirty and free pages")
>> Reviewed-by: Jarkko Sakkinen <[email protected]>
>> Signed-off-by: Reinette Chatre <[email protected]>
> Could you please consider this fix for inclusion?
Thanks for the reminder. It's merged here:
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=x86/urgent&id=e5733d8c89c3b5