x86/criu: shstk: restore SHSTK via premap loops

* call shstk_vma_restore() for VMA_AREA_SHSTK in vma_remap()
* delete map/copy/unmap from shstk_restore() and keep token setup + finalize
* before the loop naturally stopped at cet->ssp-8, so a -8 nudge is required here

Signed-off-by: Igor Svilenkov Bozic <svilenkov@gmail.com>
Co-Authored-By: Andrei Vagin <avagin@gmail.com>
[ alex: small code cleanups ]
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>

Author: 2 people

criu/arch/x86/include/asm/shstk.h

@@ -199,28 +199,11 @@ static always_inline int shstk_vma_restore(VmaEntry *vma_entry)
  */
 static always_inline int shstk_restore(struct rst_shstk_info *cet)
 {
-	unsigned long *shstk_data = (unsigned long *)cet->premmaped_addr;
-	unsigned long ssp = cet->vma_start + cet->vma_size - 8;
-	unsigned long shstk_top = cet->vma_size / 8 - 1;
-	unsigned long val;
-	long ret;
+	unsigned long ssp, val;
 
 	if (!(cet->cet & ARCH_SHSTK_SHSTK))
 		return 0;
 
-	if (shstk_map(cet->vma_start, cet->vma_size))
-		return -1;
-
-	/*
-	 * Switch shadow stack from temporary location to the actual task's
-	 * shadow stack VMA
-	 */
-	shstk_switch_ssp(ssp);
-
-	/* restore shadow stack contents */
-	for (; ssp >= cet->ssp; ssp -= 8, shstk_top--)
-		wrssq(ssp, shstk_data[shstk_top]);
-
 	/*
 	 * Add tokens for sigreturn frame and for switch of the shadow stack.
 	 * The sigreturn token will be checked by the kernel during
@@ -230,6 +213,7 @@ static always_inline int shstk_restore(struct rst_shstk_info *cet)
 	 */
 
 	/* token for sigreturn frame */
+	ssp = cet->ssp - 8;
 	val = ALIGN_DOWN(cet->ssp, 8) | SHSTK_DATA_BIT;
 	wrssq(ssp, val);
 
@@ -241,12 +225,6 @@ static always_inline int shstk_restore(struct rst_shstk_info *cet)
 	/* reset shadow stack pointer to the proper location */
 	shstk_switch_ssp(ssp);
 
-	ret = sys_munmap(shstk_data, cet->vma_size + PAGE_SIZE);
-	if (ret < 0) {
-		pr_err("Failed to unmap premmaped shadow stack\n");
-		return ret;
-	}
-
 	return shstk_finalize();
 }
 #define arch_shstk_restore shstk_restore

criu/pie/restorer.c

@@ -1112,6 +1112,23 @@ static int vma_remap(VmaEntry *vma_entry, int uffd)
 
 	pr_info("Remap %lx->%lx len %lx\n", src, dst, len);
 
+	/*
+	 * SHSTK VMAs are a bit special, in fact we create shstk vma right in the
+	 * shstk_vma_restore() and populate it with contents from a premapped VMA
+	 * (which in turns is just a normal anonymous VMA!). Then, we munmap() this
+	 * premapped VMA. After, we need to adjust vma_premmaped_start(vma_entry)
+	 * to point to a created shstk vma and treat it as a premmaped one in vma_remap().
+	 */
+	if (vma_entry_is(vma_entry, VMA_AREA_SHSTK)) {
+		if (shstk_vma_restore(vma_entry)) {
+			pr_err("Unable to prepare shadow stack vma for remap %lx -> %lx\n", src, dst);
+			return -1;
+		}
+
+		/* shstk_vma_restore() modifies vma premapped address */
+		src = vma_premmaped_start(vma_entry);
+	}
+
 	if (src - dst < len)
 		guard = dst;
 	else if (dst - src < len)
@@ -1811,13 +1828,6 @@ __visible long __export_restore_task(struct task_restore_args *args)
 		if (vma_entry->start > vma_entry->shmid)
 			break;
 
-		/*
-		 * shadow stack VMAs cannot be remapped, they must be
-		 * recreated with map_shadow_stack system call
-		 */
-		if (vma_entry_is(vma_entry, VMA_AREA_SHSTK))
-			continue;
-
 		if (vma_remap(vma_entry, args->uffd))
 			goto core_restore_end;
 	}
@@ -1835,13 +1845,6 @@ __visible long __export_restore_task(struct task_restore_args *args)
 		if (vma_entry->start < vma_entry->shmid)
 			break;
 
-		/*
-		 * shadow stack VMAs cannot be remapped, they must be
-		 * recreated with map_shadow_stack system call
-		 */
-		if (vma_entry_is(vma_entry, VMA_AREA_SHSTK))
-			continue;
-
 		if (vma_remap(vma_entry, args->uffd))
 			goto core_restore_end;
 	}