| Message ID | 20201116222531.4834-1-dave.hansen@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | x86/sgx: clarify 'laundry_list' locking | expand |
On Mon, Nov 16, 2020 at 02:25:31PM -0800, Dave Hansen wrote: > From: Dave Hansen <dave.hansen@linux.intel.com> > > Short Version: > > The SGX section->laundry_list structure is effectively thread-local, > but declared next to some shared structures. Its semantics are clear > as mud. Fix that. No functional changes. Compile tested only. > > Long Version: > > The SGX hardware keeps per-page metadata. This can provide things like > permissions, integrity and replay protection. It also prevents things > like having an enclave page mapped multiple times or shared between > enclaves. > > But, that presents a problem for kexec()'d kernels (or any other kernel > that does not run immediately after a hardware reset). This is because > the last kernel may have been rude and forgotten to reset pages, which > would trigger the the "shared page" sanity check. > > To fix this, the SGX code "launders" the pages by running the EREMOVE > instruction on all pages at boot. This is slow and can take a long > time, so it is performed off in the SGX-specific ksgxd instead of being > synchronous at boot. The init code hands the list of pages to launder > in a per-SGX-section list: ->laundry_list. The only code to touch this > list is the init code and ksgxd. This means that no locking is > necessary for ->laundry_list. > > However, a lock is required for section->page_list, which is accessed > while creating enclaves and by ksgxd. This lock (section->lock is > acquired by ksgxd while also processing ->laundry_list. It is easy > to confuse the purpose of the locking as being for ->laundry_list > and ->page_list. > > Rename ->laundry_list to ->init_laundry_list to make it clear that > this is not normally used at runtime. Also add some comments > clarifying the locking, and reorganize 'sgx_epc_section' to put 'lock' > near the things it protects. > > Note: init_laundry_list is 128 bytes of wasted space at runtime. It > could theoretically be dynamically allocated and then freed after the > laundering process. But, I suspect it would take nearly 128 bytes > of extra instructions to do that. > > Cc: Jethro Beekman <jethro@fortanix.com> > Cc: Serge Ayoun <serge.ayoun@intel.com> > Cc: Jarkko Sakkinen <jarkko@kernel.org> > Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org> > Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Thansk, this does make sense to me. I'll squash it. /Jarkko > --- > > b/arch/x86/kernel/cpu/sgx/main.c | 14 ++++++++------ > b/arch/x86/kernel/cpu/sgx/sgx.h | 15 ++++++++++++--- > 2 files changed, 20 insertions(+), 9 deletions(-) > > diff -puN arch/x86/kernel/cpu/sgx/main.c~sgx-laundry-comments arch/x86/kernel/cpu/sgx/main.c > --- a/arch/x86/kernel/cpu/sgx/main.c~sgx-laundry-comments 2020-11-16 13:55:42.624972349 -0800 > +++ b/arch/x86/kernel/cpu/sgx/main.c 2020-11-16 13:58:10.652971980 -0800 > @@ -36,13 +36,15 @@ static void sgx_sanitize_section(struct > LIST_HEAD(dirty); > int ret; > > - while (!list_empty(§ion->laundry_list)) { > + /* init_laundry_list is thread-local, no need for a lock: */ > + while (!list_empty(§ion->init_laundry_list)) { > if (kthread_should_stop()) > return; > > + /* needed for access to ->page_list: */ > spin_lock(§ion->lock); > > - page = list_first_entry(§ion->laundry_list, > + page = list_first_entry(§ion->init_laundry_list, > struct sgx_epc_page, list); > > ret = __eremove(sgx_get_epc_virt_addr(page)); > @@ -56,7 +58,7 @@ static void sgx_sanitize_section(struct > cond_resched(); > } > > - list_splice(&dirty, §ion->laundry_list); > + list_splice(&dirty, §ion->init_laundry_list); > } > > static bool sgx_reclaimer_age(struct sgx_epc_page *epc_page) > @@ -418,7 +420,7 @@ static int ksgxswapd(void *p) > sgx_sanitize_section(&sgx_epc_sections[i]); > > /* Should never happen. */ > - if (!list_empty(&sgx_epc_sections[i].laundry_list)) > + if (!list_empty(&sgx_epc_sections[i].init_laundry_list)) > WARN(1, "EPC section %d has unsanitized pages.\n", i); > } > > @@ -632,13 +634,13 @@ static bool __init sgx_setup_epc_section > section->phys_addr = phys_addr; > spin_lock_init(§ion->lock); > INIT_LIST_HEAD(§ion->page_list); > - INIT_LIST_HEAD(§ion->laundry_list); > + INIT_LIST_HEAD(§ion->init_laundry_list); > > for (i = 0; i < nr_pages; i++) { > section->pages[i].section = index; > section->pages[i].flags = 0; > section->pages[i].owner = NULL; > - list_add_tail(§ion->pages[i].list, §ion->laundry_list); > + list_add_tail(§ion->pages[i].list, §ion->init_laundry_list); > } > > section->free_cnt = nr_pages; > diff -puN arch/x86/kernel/cpu/sgx/sgx.h~sgx-laundry-comments arch/x86/kernel/cpu/sgx/sgx.h > --- a/arch/x86/kernel/cpu/sgx/sgx.h~sgx-laundry-comments 2020-11-16 13:55:42.627972349 -0800 > +++ b/arch/x86/kernel/cpu/sgx/sgx.h 2020-11-16 13:55:42.631972349 -0800 > @@ -34,15 +34,24 @@ struct sgx_epc_page { > * physical memory e.g. for memory areas of the each node. This structure is > * used to store EPC pages for one EPC section and virtual memory area where > * the pages have been mapped. > + * > + * 'lock' must be held before accessing 'page_list' or 'free_cnt'. > */ > struct sgx_epc_section { > unsigned long phys_addr; > void *virt_addr; > - struct list_head page_list; > - struct list_head laundry_list; > struct sgx_epc_page *pages; > - unsigned long free_cnt; > + > spinlock_t lock; > + struct list_head page_list; > + unsigned long free_cnt; > + > + /* > + * Pages which need EREMOVE run on them before they can be > + * used. Only safe to be accessed in ksgxd and init code. > + * Not protected by locks. > + */ > + struct list_head init_laundry_list; > }; > > extern struct sgx_epc_section sgx_epc_sections[SGX_MAX_EPC_SECTIONS]; > _ >
diff -puN arch/x86/kernel/cpu/sgx/main.c~sgx-laundry-comments arch/x86/kernel/cpu/sgx/main.c --- a/arch/x86/kernel/cpu/sgx/main.c~sgx-laundry-comments 2020-11-16 13:55:42.624972349 -0800 +++ b/arch/x86/kernel/cpu/sgx/main.c 2020-11-16 13:58:10.652971980 -0800 @@ -36,13 +36,15 @@ static void sgx_sanitize_section(struct LIST_HEAD(dirty); int ret; - while (!list_empty(§ion->laundry_list)) { + /* init_laundry_list is thread-local, no need for a lock: */ + while (!list_empty(§ion->init_laundry_list)) { if (kthread_should_stop()) return; + /* needed for access to ->page_list: */ spin_lock(§ion->lock); - page = list_first_entry(§ion->laundry_list, + page = list_first_entry(§ion->init_laundry_list, struct sgx_epc_page, list); ret = __eremove(sgx_get_epc_virt_addr(page)); @@ -56,7 +58,7 @@ static void sgx_sanitize_section(struct cond_resched(); } - list_splice(&dirty, §ion->laundry_list); + list_splice(&dirty, §ion->init_laundry_list); } static bool sgx_reclaimer_age(struct sgx_epc_page *epc_page) @@ -418,7 +420,7 @@ static int ksgxswapd(void *p) sgx_sanitize_section(&sgx_epc_sections[i]); /* Should never happen. */ - if (!list_empty(&sgx_epc_sections[i].laundry_list)) + if (!list_empty(&sgx_epc_sections[i].init_laundry_list)) WARN(1, "EPC section %d has unsanitized pages.\n", i); } @@ -632,13 +634,13 @@ static bool __init sgx_setup_epc_section section->phys_addr = phys_addr; spin_lock_init(§ion->lock); INIT_LIST_HEAD(§ion->page_list); - INIT_LIST_HEAD(§ion->laundry_list); + INIT_LIST_HEAD(§ion->init_laundry_list); for (i = 0; i < nr_pages; i++) { section->pages[i].section = index; section->pages[i].flags = 0; section->pages[i].owner = NULL; - list_add_tail(§ion->pages[i].list, §ion->laundry_list); + list_add_tail(§ion->pages[i].list, §ion->init_laundry_list); } section->free_cnt = nr_pages; diff -puN arch/x86/kernel/cpu/sgx/sgx.h~sgx-laundry-comments arch/x86/kernel/cpu/sgx/sgx.h --- a/arch/x86/kernel/cpu/sgx/sgx.h~sgx-laundry-comments 2020-11-16 13:55:42.627972349 -0800 +++ b/arch/x86/kernel/cpu/sgx/sgx.h 2020-11-16 13:55:42.631972349 -0800 @@ -34,15 +34,24 @@ struct sgx_epc_page { * physical memory e.g. for memory areas of the each node. This structure is * used to store EPC pages for one EPC section and virtual memory area where * the pages have been mapped. + * + * 'lock' must be held before accessing 'page_list' or 'free_cnt'. */ struct sgx_epc_section { unsigned long phys_addr; void *virt_addr; - struct list_head page_list; - struct list_head laundry_list; struct sgx_epc_page *pages; - unsigned long free_cnt; + spinlock_t lock; + struct list_head page_list; + unsigned long free_cnt; + + /* + * Pages which need EREMOVE run on them before they can be + * used. Only safe to be accessed in ksgxd and init code. + * Not protected by locks. + */ + struct list_head init_laundry_list; }; extern struct sgx_epc_section sgx_epc_sections[SGX_MAX_EPC_SECTIONS];