Re: modules: add ro_after_init support

Jessica Yu <jeyu@xxxxxxxxxx> · Wed, 29 Jun 2016 17:27:14 -0400

+++ Rusty Russell [29/06/16 10:38 +0930]:
Jessica Yu <jeyu@xxxxxxxxxx> writes:
Add ro_after_init support for modules by adding a new page-aligned section
in the module layout (after rodata) for ro_after_init data and enabling RO
protection for that section after module init runs.

Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>

I would prefer a "bool after_init" flag to module_enable_ro().  It's
more explicit.

Sure thing, I was just initially worried about the
module_{enable,disable}_ro() asymmetry. :)

Exposing the flags via uapi looks like a wart, but it's kind of a
feature, since we don't *unset* it in any section; userspace may want to
know about it.

Hm, I'm still unsure about this. I'm starting to think it might be a
bit overkill to expose SHF_RO_AFTER_INIT through uapi (although that
is where all the other SHF_* flags are defined) SHF_RO_AFTER_INIT
would technically be used only internally in the kernel (i.e. module
loader), and it'd also be considered a non-standard flag, using a bit
from SHF_MASKOS (OS-specific range). What do you think?

If you could re-spin, that would be great.

Once I figure out where to put SHF_RO_AFTER_INIT, I'll send a v2.

Thanks for the review!
Jessica


---
 include/linux/module.h   |  2 ++
 include/uapi/linux/elf.h |  1 +
 kernel/module.c          | 73 +++++++++++++++++++++++++++++++++++++++++-------
 3 files changed, 66 insertions(+), 10 deletions(-)

diff --git a/include/linux/module.h b/include/linux/module.h
index f777164..a698d23 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -311,6 +311,8 @@ struct module_layout {
 	unsigned int text_size;
 	/* Size of RO section of the module (text+rodata) */
 	unsigned int ro_size;
+	/* Size of RO after init section */
+	unsigned int ro_after_init_size;

 #ifdef CONFIG_MODULES_TREE_LOOKUP
 	struct mod_tree_node mtn;
diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
index cb4a72f..70b172ba 100644
--- a/include/uapi/linux/elf.h
+++ b/include/uapi/linux/elf.h
@@ -286,6 +286,7 @@ typedef struct elf64_phdr {
 #define SHF_ALLOC		0x2
 #define SHF_EXECINSTR		0x4
 #define SHF_RELA_LIVEPATCH	0x00100000
+#define SHF_RO_AFTER_INIT	0x00200000
 #define SHF_MASKPROC		0xf0000000

 /* special section indexes */
diff --git a/kernel/module.c b/kernel/module.c
index 7f21ab2..055bf6f 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1857,10 +1857,11 @@ static void mod_sysfs_teardown(struct module *mod)
  * from modification and any data from execution.
  *
  * General layout of module is:
- *          [text] [read-only-data] [writable data]
- * text_size -----^                ^               ^
- * ro_size ------------------------|               |
- * size -------------------------------------------|
+ *          [text] [read-only-data] [ro-after-init] [writable data]
+ * text_size -----^                ^               ^               ^
+ * ro_size ------------------------|               |               |
+ * ro_after_init_size -----------------------------|               |
+ * size -----------------------------------------------------------|
  *
  * These values are always page-aligned (as is base)
  */
@@ -1883,14 +1884,24 @@ static void frob_rodata(const struct module_layout *layout,
 		   (layout->ro_size - layout->text_size) >> PAGE_SHIFT);
 }

+static void frob_ro_after_init(const struct module_layout *layout,
+				int (*set_memory)(unsigned long start, int num_pages))
+{
+	BUG_ON((unsigned long)layout->base & (PAGE_SIZE-1));
+	BUG_ON((unsigned long)layout->ro_size & (PAGE_SIZE-1));
+	BUG_ON((unsigned long)layout->ro_after_init_size & (PAGE_SIZE-1));
+	set_memory((unsigned long)layout->base + layout->ro_size,
+		   (layout->ro_after_init_size - layout->ro_size) >> PAGE_SHIFT);
+}
+
 static void frob_writable_data(const struct module_layout *layout,
 			       int (*set_memory)(unsigned long start, int num_pages))
 {
 	BUG_ON((unsigned long)layout->base & (PAGE_SIZE-1));
-	BUG_ON((unsigned long)layout->ro_size & (PAGE_SIZE-1));
+	BUG_ON((unsigned long)layout->ro_after_init_size & (PAGE_SIZE-1));
 	BUG_ON((unsigned long)layout->size & (PAGE_SIZE-1));
-	set_memory((unsigned long)layout->base + layout->ro_size,
-		   (layout->size - layout->ro_size) >> PAGE_SHIFT);
+	set_memory((unsigned long)layout->base + layout->ro_after_init_size,
+		   (layout->size - layout->ro_after_init_size) >> PAGE_SHIFT);
 }

 /* livepatching wants to disable read-only so it can frob module. */
@@ -1898,6 +1909,7 @@ void module_disable_ro(const struct module *mod)
 {
 	frob_text(&mod->core_layout, set_memory_rw);
 	frob_rodata(&mod->core_layout, set_memory_rw);
+	frob_ro_after_init(&mod->core_layout, set_memory_rw);
 	frob_text(&mod->init_layout, set_memory_rw);
 	frob_rodata(&mod->init_layout, set_memory_rw);
 }
@@ -1908,11 +1920,16 @@ void module_enable_ro(const struct module *mod)
 	frob_rodata(&mod->core_layout, set_memory_ro);
 	frob_text(&mod->init_layout, set_memory_ro);
 	frob_rodata(&mod->init_layout, set_memory_ro);
+
+	/* after init */
+	if (mod->state == MODULE_STATE_LIVE)
+		frob_ro_after_init(&mod->core_layout, set_memory_ro);
 }

 static void module_enable_nx(const struct module *mod)
 {
 	frob_rodata(&mod->core_layout, set_memory_nx);
+	frob_ro_after_init(&mod->core_layout, set_memory_nx);
 	frob_writable_data(&mod->core_layout, set_memory_nx);
 	frob_rodata(&mod->init_layout, set_memory_nx);
 	frob_writable_data(&mod->init_layout, set_memory_nx);
@@ -1921,6 +1938,7 @@ static void module_enable_nx(const struct module *mod)
 static void module_disable_nx(const struct module *mod)
 {
 	frob_rodata(&mod->core_layout, set_memory_x);
+	frob_ro_after_init(&mod->core_layout, set_memory_x);
 	frob_writable_data(&mod->core_layout, set_memory_x);
 	frob_rodata(&mod->init_layout, set_memory_x);
 	frob_writable_data(&mod->init_layout, set_memory_x);
@@ -1963,6 +1981,8 @@ static void disable_ro_nx(const struct module_layout *layout)
 	frob_text(layout, set_memory_rw);
 	frob_rodata(layout, set_memory_rw);
 	frob_rodata(layout, set_memory_x);
+	frob_ro_after_init(layout, set_memory_rw);
+	frob_ro_after_init(layout, set_memory_x);
 	frob_writable_data(layout, set_memory_x);
 }

@@ -2305,6 +2325,7 @@ static void layout_sections(struct module *mod, struct load_info *info)
 		 * finder in the two loops below */
 		{ SHF_EXECINSTR | SHF_ALLOC, ARCH_SHF_SMALL },
 		{ SHF_ALLOC, SHF_WRITE | ARCH_SHF_SMALL },
+		{ SHF_RO_AFTER_INIT | SHF_ALLOC, ARCH_SHF_SMALL },
 		{ SHF_WRITE | SHF_ALLOC, ARCH_SHF_SMALL },
 		{ ARCH_SHF_SMALL | SHF_ALLOC, 0 }
 	};
@@ -2336,7 +2357,11 @@ static void layout_sections(struct module *mod, struct load_info *info)
 			mod->core_layout.size = debug_align(mod->core_layout.size);
 			mod->core_layout.ro_size = mod->core_layout.size;
 			break;
-		case 3: /* whole core */
+		case 2: /* RO after init */
+			mod->core_layout.size = debug_align(mod->core_layout.size);
+			mod->core_layout.ro_after_init_size = mod->core_layout.size;
+			break;
+		case 4: /* whole core */
 			mod->core_layout.size = debug_align(mod->core_layout.size);
 			break;
 		}
@@ -2366,7 +2391,14 @@ static void layout_sections(struct module *mod, struct load_info *info)
 			mod->init_layout.size = debug_align(mod->init_layout.size);
 			mod->init_layout.ro_size = mod->init_layout.size;
 			break;
-		case 3: /* whole init */
+		case 2:
+			/*
+			 * RO after init doesn't apply to init_layout (only
+			 * core_layout), so it just takes the value of ro_size.
+			 */
+			mod->init_layout.ro_after_init_size = mod->init_layout.ro_size;
+			break;
+		case 4: /* whole init */
 			mod->init_layout.size = debug_align(mod->init_layout.size);
 			break;
 		}
@@ -3172,6 +3204,7 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
 {
 	/* Module within temporary copy. */
 	struct module *mod;
+	unsigned int ndx;
 	int err;

 	mod = setup_load_info(info, flags);
@@ -3191,6 +3224,15 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
 	/* We will do a special allocation for per-cpu sections later. */
 	info->sechdrs[info->index.pcpu].sh_flags &= ~(unsigned long)SHF_ALLOC;

+	/*
+	 * Mark ro_after_init section with SHF_RO_AFTER_INIT so that
+	 * layout_sections() can put it in the right place.
+	 * Note: ro_after_init sections also have SHF_{WRITE,ALLOC} set.
+	 */
+	ndx = find_sec(info, ".data..ro_after_init");
+	if (ndx)
+		info->sechdrs[ndx].sh_flags |= SHF_RO_AFTER_INIT;
+
 	/* Determine total sizes, and put offsets in sh_entsize.  For now
 	   this is done generically; there doesn't appear to be any
 	   special cases for the architectures. */
@@ -3357,12 +3399,19 @@ static noinline int do_init_module(struct module *mod)
 	/* Switch to core kallsyms now init is done: kallsyms may be walking! */
 	rcu_assign_pointer(mod->kallsyms, &mod->core_kallsyms);
 #endif
+	/*
+	 * RO and NX regions should already be protected at this point,
+	 * so the below module_enable_ro() call enables additional RO
+	 * protection for the ro_after_init section only.
+	 */
+	module_enable_ro(mod);
 	mod_tree_remove_init(mod);
 	disable_ro_nx(&mod->init_layout);
 	module_arch_freeing_init(mod);
 	mod->init_layout.base = NULL;
 	mod->init_layout.size = 0;
 	mod->init_layout.ro_size = 0;
+	mod->init_layout.ro_after_init_size = 0;
 	mod->init_layout.text_size = 0;
 	/*
 	 * We want to free module_init, but be aware that kallsyms may be
@@ -3454,7 +3503,11 @@ static int complete_formation(struct module *mod, struct load_info *info)
 	/* This relies on module_mutex for list integrity. */
 	module_bug_finalize(info->hdr, info->sechdrs, mod);

-	/* Set RO and NX regions */
+	/*
+	 * Set RO and NX regions. Since module is not LIVE yet,
+	 * the ro_after_init section will remain RW until after
+	 * do_one_initcall().
+	 */
 	module_enable_ro(mod);
 	module_enable_nx(mod);

--
2.4.3
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