[PATCH 1/2] x86/sgx: Do not fail on incomplete sanitization on premature stop of ksgxd

[Jarkko Sakkinen <jarkko@kernel.org>]

Unsanitized pages trigger WARN_ON() unconditionally, which can panic the
whole computer, if /proc/sys/kernel/panic_on_warn is set.

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 unsanitized pages, which will result a
false warning.

Refine __sgx_sanitize_pages() to return:

1. Zero when the sanitization process is complete or ksgxd has been
   requested to stop.
2. The number of unsanitized pages otherwise.

Use the return value as the criteria for triggering output, and tone down
the output to pr_err() to prevent the whole system to be taken down if for
some reason sanitization process does not complete.

Link: https://lore.kernel.org/linux-sgx/20220825051827.246698-1-jarkko@kernel.org/T/#u
Fixes: 51ab30eb2ad4 ("x86/sgx: Replace section->init_laundry_list with sgx_dirty_page_list")
Cc: stable@vger.kernel.org # v5.13+
Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
v7:
- Rewrote commit message.
- Do not return -ECANCELED on premature stop. Instead use zero both
  premature stop and complete sanitization.

v6:
- Address Reinette's feedback:
  https://lore.kernel.org/linux-sgx/Yw6%2FiTzSdSw%2FY%2FVO@kernel.org/

v5:
- Add the klog dump and sysctl option to the commit message.

v4:
- Explain expectations for dirty_page_list in the function header, instead
  of an inline comment.
- Improve commit message to explain the conditions better.
- Return the number of pages left dirty to ksgxd() and print warning after
  the 2nd call, if there are any.

v3:
- Remove WARN_ON().
- Tuned comments and the commit message a bit.

v2:
- Replaced WARN_ON() with optional pr_info() inside
  __sgx_sanitize_pages().
- Rewrote the commit message.
- Added the fixes tag.
---
 arch/x86/kernel/cpu/sgx/main.c | 33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
index 515e2a5f25bb..c0a5ce19c608 100644
--- a/arch/x86/kernel/cpu/sgx/main.c
+++ b/arch/x86/kernel/cpu/sgx/main.c
@@ -49,17 +49,23 @@ 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.
+ *
+ * Return 0 when sanitization was successful or kthread was stopped, and the
+ * number of unsanitized pages otherwise.
  */
-static void __sgx_sanitize_pages(struct list_head *dirty_page_list)
+static unsigned long __sgx_sanitize_pages(struct list_head *dirty_page_list)
 {
+	unsigned long left_dirty = 0;
 	struct sgx_epc_page *page;
 	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;
+			return 0;
 
 		page = list_first_entry(dirty_page_list, struct sgx_epc_page, list);
 
@@ -92,12 +98,14 @@ 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)
@@ -388,17 +396,28 @@ void sgx_reclaim_direct(void)
 
 static int ksgxd(void *p)
 {
+	unsigned long left_dirty;
+
 	set_freezable();
 
 	/*
 	 * Sanitize pages in order to recover from kexec(). The 2nd pass is
 	 * required for SECS pages, whose child pages blocked EREMOVE.
 	 */
-	__sgx_sanitize_pages(&sgx_dirty_page_list);
-	__sgx_sanitize_pages(&sgx_dirty_page_list);
+	left_dirty = __sgx_sanitize_pages(&sgx_dirty_page_list);
+	pr_debug("%ld unsanitized pages\n", left_dirty);
 
-	/* sanity check: */
-	WARN_ON(!list_empty(&sgx_dirty_page_list));
+	left_dirty = __sgx_sanitize_pages(&sgx_dirty_page_list);
+	/*
+	 * Never expected to happen in a working driver. If it happens the bug
+	 * is expected to be in the sanitization process, but successfully
+	 * sanitized pages are still valid and driver can be used and most
+	 * importantly debugged without issues. To put short, the global state
+	 * of kernel is not corrupted so no reason to do any more complicated
+	 * rollback.
+	 */
+	if (left_dirty)
+		pr_err("%ld unsanitized pages\n", left_dirty);
 
 	while (!kthread_should_stop()) {
 		if (try_to_freeze())
-- 
2.37.2