Re: [PATCH 1/6] x86/sgx: Do not consider unsanitized pages an error

Jarkko Sakkinen
2022-08-31T01:59:03Z

On Wed, Aug 31, 2022 at 04:55:24AM +0300, Jarkko Sakkinen wrote:
> On Tue, Aug 30, 2022 at 03:54:27PM -0700, Reinette Chatre wrote:
> > Hi Jarkko,
> >
> > On 8/29/2022 8:12 PM, Jarkko Sakkinen wrote:
> > > In sgx_init(), if misc_register() for the provision device fails, and
> > > neither sgx_drv_init() nor sgx_vepc_init() succeeds, then ksgxd will be
> > > prematurely stopped.
> >
> > I do not think misc_register() is required to fail for the scenario to
> > be triggered (rather use "or" than "and"?). Perhaps just
> > "In sgx_init(), if a failure is encountered after ksgxd is started
> > (via sgx_page_reclaimer_init()) ...".
>
> This would be the fixed version of the sentence:
>
> "
> In sgx_init(), if misc_register() fails or misc_register() succeeds but
> neither sgx_drv_init() nor sgx_vepc_init() succeeds, then ksgxd will be
> prematurely stopped. This may leave some unsanitized pages, which does
> not matter, because SGX will be disabled for the whole power cycle.
> "
>
> I want to keep the end states listed and not make it more abstract.
>
> The second sentence addresses the remark below.
>
> > To help the reader understand the subject of this patch it may help
> > to explain that prematurely stopping ksgxd may leave some
> > unsanitized pages, but that is not a problem since SGX cannot
> > be used on the platform anyway.
> >
> > > This triggers WARN_ON() because sgx_dirty_page_list ends up being
> > > non-empty, and dumps the call stack:
> > >
> >
> > Traces like below can be frowned upon. I recommend that you follow the
> > guidance in "Backtraces in commit mesages"(sic) in
> > Documentation/process/submitting-patches.rst.
> >
> > > [ 0.268592] WARNING: CPU: 6 PID: 83 at
> > > arch/x86/kernel/cpu/sgx/main.c:401 ksgxd+0x1b7/0x1d0
>
> Is this good enough? I had not actually spotted this section before but
> nice that it exists. Apparently has been added in 5.12.
>
> >> >
> > > Ultimately this can crash the kernel, if the following is set:
> > >
> > > /proc/sys/kernel/panic_on_warn
> > >
> > > Print a simple warning instead, and improve the output by printing the
> > > number of unsanitized pages, in order to provide debug informnation for
> > > future needs.
> >
> > informnation -> information
>
> +1
>
> >
> >
> > ...
> >
> > > Link: https://lore.kernel.org/linux-sgx/20220825051827.246698-1-jarkko@kernel.org/T/#u
> > > Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
> > > Tested-by: Paul Menzel <pmenzel@molgen.mpg.de>
> > > Fixes: 51ab30eb2ad4 ("x86/sgx: Replace section->init_laundry_list with sgx_dirty_page_list")
> > > Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> >
> > Should this go to stable?
>
> I guess it should. The hard reason for this that it can panic
> the kernel.
>
> >
> > >
> > > diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> > > index 515e2a5f25bb..903100fcfce3 100644
> > > --- a/arch/x86/kernel/cpu/sgx/main.c
> > > +++ b/arch/x86/kernel/cpu/sgx/main.c
> > > @@ -49,17 +49,20 @@ static LIST_HEAD(sgx_dirty_page_list);
> > > * Reset post-kexec EPC pages to the uninitialized state. The pages are removed
> > > * from the input list, and made available for the page allocator. SECS pages
> > > * prepending their children in the input list are left intact.
> > > + *
> > > + * Contents of the @dirty_page_list must be thread-local, i.e.
> > > + * not shared by multiple threads.
> >
> > Did you intend to mention something about the needed locking here? It looks
> > like some information is lost during the move to the function description.
>
> Nothing about the locking that concerns the parameter, as the
> sentence defines clear constraints for the caller.
>
> >
> > > */
> > > -static void __sgx_sanitize_pages(struct list_head *dirty_page_list)
> > > +static int __sgx_sanitize_pages(struct list_head *dirty_page_list)
> > > {
> > > struct sgx_epc_page *page;
> > > + int left_dirty = 0;
> >
> > I do not know how many pages this code should be ready for but at least
> > this could handle more by being an unsigned int considering that it is
> > always positive ... maybe even unsigned long?
>
> I would go for 'long'. More information below.
>
> >
> > > LIST_HEAD(dirty);
> > > int ret;
> > >
> > > - /* dirty_page_list is thread-local, no need for a lock: */
> > > while (!list_empty(dirty_page_list)) {
> > > if (kthread_should_stop())
> > > - return;
> > > + break;
> > >
> > > page = list_first_entry(dirty_page_list, struct sgx_epc_page, list);
> > >
> > > @@ -92,12 +95,14 @@ static void __sgx_sanitize_pages(struct list_head *dirty_page_list)
> > > } else {
> > > /* The page is not yet clean - move to the dirty list. */
> > > list_move_tail(&page->list, &dirty);
> > > + left_dirty++;
> > > }
> > >
> > > cond_resched();
> > > }
> > >
> > > list_splice(&dirty, dirty_page_list);
> > > + return left_dirty;
> > > }
> > >
> > > static bool sgx_reclaimer_age(struct sgx_epc_page *epc_page)
> > > @@ -388,6 +393,8 @@ void sgx_reclaim_direct(void)
> > >
> > > static int ksgxd(void *p)
> > > {
> > > + int left_dirty;
> > > +
> > > set_freezable();
> > >
> > > /*
> > > @@ -395,10 +402,10 @@ static int ksgxd(void *p)
> > > * required for SECS pages, whose child pages blocked EREMOVE.
> > > */
> > > __sgx_sanitize_pages(&sgx_dirty_page_list);
> > > - __sgx_sanitize_pages(&sgx_dirty_page_list);
> > >
> > > - /* sanity check: */
> > > - WARN_ON(!list_empty(&sgx_dirty_page_list));
> > > + left_dirty = __sgx_sanitize_pages(&sgx_dirty_page_list);
> > > + if (left_dirty)
> > > + pr_warn("%d unsanitized pages\n", left_dirty);
> > >
> > > while (!kthread_should_stop()) {
> > > if (try_to_freeze())
> >
> >
> > Reinette
>
> We need to return -ECANCELED on premature stop, and number of
> pages otherwise.
>
> In premature stop, nothing should be printed, as the number
> is by practical means a random number. Otherwise, it is an
> indicator of a bug in the driver, and therefore a non-zero
> number should be printed pr_err(), if that happens after the
> second call.

I.e. even though we print less we get more *information* what
is going inside the kernel. Warning is not correct for either
path IMHO.

BR, Jarkko