[PATCH v9 13/15] module: use a scripted approach for FG-KASLR

Alexander Lobakin
2021-12-23T00:24:10Z

Use the same methods and scripts to generate an LD script for every
module containing all the output text sections.
The only difference there is that we don't need to reserve any space
as the memory for every section is being allocated dynamically.

I picked ".ko.lds" extension since there's a fistful of ".lds" files
inside the tree, so I couldn't count all of them as generated /
build artifacts. OTOH, we're limited in heuristics when cleaning as
dotconfig doesn't get included and stuff like ".mod.c" is just being
wiped using `find`.

Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
---
.gitignore | 1 +
include/asm-generic/vmlinux.lds.h | 12 ++++++++++++
init/Kconfig | 15 ++++++++++++++-
scripts/Makefile.modfinal | 20 +++++++++++++++++---
scripts/generate_text_sections.pl | 9 ++++++++-
scripts/module.lds.S | 14 +++++++++++++-
6 files changed, 65 insertions(+), 6 deletions(-)

diff --git a/.gitignore b/.gitignore
index 7afd412dadd2..d1b48f01037a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -26,6 +26,7 @@
*.gz
*.i
*.ko
+*.ko.lds
*.lex.c
*.ll
*.lst
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index e63d5a69f1bc..9f67660ace18 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -130,6 +130,18 @@
#define TEXT_MAIN .text
#endif

+/*
+ * Same for modules. However, LD_DEAD_CODE_DATA_ELIMINATION doesn't touch
+ * them, so no need to check for it here.
+ */
+#if defined(CONFIG_LTO_CLANG) && !defined(CONFIG_MODULE_FG_KASLR)
+#define TEXT_MAIN_MODULE SECT_WILDCARD(.text)
+#elif defined(CONFIG_MODULE_FG_KASLR)
+#define TEXT_MAIN_MODULE .text.__unused__
+#else
+#define TEXT_MAIN_MODULE .text
+#endif
+
/*
* Used by scripts/generate_text_sections.pl to inject text sections,
* harmless if FG-KASLR is disabled.
diff --git a/init/Kconfig b/init/Kconfig
index 38c82e21efd7..d4000885b27d 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2381,7 +2381,6 @@ config MODULE_FG_KASLR
depends on $(cc-option,-ffunction-sections)
depends on LD_HAS_Z_UNIQUE_SYMBOL || !LIVEPATCH
default FG_KASLR
- depends on BROKEN
help
This option randomizes the module text section by reordering the text
section by function at module load time. In order to use this
@@ -2390,6 +2389,20 @@ config MODULE_FG_KASLR

If unsure, say N.

+config MODULE_FG_KASLR_SHIFT
+ int "Module FG-KASLR granularity (functions per section shift)"
+ depends on MODULE_FG_KASLR
+ range 0 16
+ default 0
+ help
+ This sets the number of functions that will be put in each section
+ as a power of two.
+ Decreasing the value increases the randomization, but also increases
+ the size of the final kernel module due to the amount of sections.
+ 0 means that a separate section will be created for each function.
+ 16 almost disables the randomization, leaving only the manual
+ separation.
+
endif # MODULES

config MODULES_TREE_LOOKUP
diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 7f39599e9fae..4ca9d8fc978d 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -28,13 +28,25 @@ quiet_cmd_cc_o_c = CC [M] $@
%.mod.o: %.mod.c FORCE
$(call if_changed_dep,cc_o_c)

+ifdef CONFIG_MODULE_FG_KASLR
+quiet_cmd_gen_modules_lds = GEN [M] $@
+ cmd_gen_modules_lds = \
+ $(PERL) $(srctree)/scripts/generate_text_sections.pl \
+ $(if $(CONFIG_HAVE_ASM_FUNCTION_SECTIONS),-a) \
+ -s $(CONFIG_MODULE_FG_KASLR_SHIFT) $(filter %.o, $^) \
+ < $(filter %.lds, $^) > $@
+
+%.ko.lds: %$(mod-prelink-ext).o scripts/module.lds FORCE
+ $(call if_changed,gen_modules_lds)
+endif
+
ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)

quiet_cmd_ld_ko_o = LD [M] $@
cmd_ld_ko_o += \
$(LD) -r $(KBUILD_LDFLAGS) \
$(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE) \
- -T scripts/module.lds -o $@ $(filter %.o, $^); \
+ -T $(filter %.lds, $^) -o $@ $(filter %.o, $^); \
$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)

quiet_cmd_btf_ko = BTF [M] $@
@@ -56,13 +68,15 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check), \

# Re-generate module BTFs if either module's .ko or vmlinux changed
-$(modules): %.ko: %$(mod-prelink-ext).o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE
+$(modules): %.ko: %$(mod-prelink-ext).o %.mod.o
+$(modules): %.ko: $(if $(CONFIG_MODULE_FG_KASLR),%.ko.lds,scripts/module.lds)
+$(modules): %.ko: $(if $(KBUILD_BUILTIN),vmlinux) FORCE
+$(call if_changed_except,ld_ko_o,vmlinux)
ifdef CONFIG_DEBUG_INFO_BTF_MODULES
+$(if $(newer-prereqs),$(call cmd,btf_ko))
endif

-targets += $(modules) $(modules:.ko=.mod.o)
+targets += $(modules) $(modules:.ko=.mod.o) $(modules:.ko=.ko.lds)

# Add FORCE to the prequisites of a target to force it to be always rebuilt.
# ---------------------------------------------------------------------------
diff --git a/scripts/generate_text_sections.pl b/scripts/generate_text_sections.pl
index c95b9be28920..79a37a3c079f 100755
--- a/scripts/generate_text_sections.pl
+++ b/scripts/generate_text_sections.pl
@@ -48,6 +48,7 @@ my $readelf = $ENV{'READELF'} || die "$0: ERROR: READELF not set?";
## text sections array
my @sections = ();
my $has_ccf = 0;
+my $vmlinux = 0;

## max alignment found to reserve some space. It would probably be
## better to start from 64, but CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B
@@ -78,6 +79,12 @@ sub read_sections {
$has_ccf = 1;
}

+ ## If we're processing a module, don't reserve any space
+ ## at the end as its sections are being allocated separately.
+ if ($name eq ".sched.text") {
+ $vmlinux = 1;
+ }
+
if (!($name =~ /^\.text(\.(?!hot\.|unknown\.|unlikely\.|.san\.)[0-9a-zA-Z_]*){1,2}((\.constprop|\.isra|\.part)\.[0-9]){0,2}(|\.[0-9cfi]*)$/)) {
next;
}
@@ -141,7 +148,7 @@ sub print_reserve {
## If we have text sections aligned with 64 bytes or more, make
## sure we reserve some space for them to not overlap _etext
## while shuffling sections.
- if (!$count) {
+ if (!$vmlinux or !$count) {
return;
}

diff --git a/scripts/module.lds.S b/scripts/module.lds.S
index 1d0e1e4dc3d2..6e957aa614b1 100644
--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -3,6 +3,11 @@
* Archs are free to supply their own linker scripts. ld will
* combine them automatically.
*/
+
+#include <asm-generic/vmlinux.lds.h>
+
+#undef SANITIZER_DISCARDS
+
#ifdef CONFIG_CFI_CLANG
# include <asm/page.h>
# define ALIGN_CFI ALIGN(PAGE_SIZE)
@@ -58,9 +63,16 @@ SECTIONS {
*/
.text : ALIGN_CFI {
*(.text.__cfi_check)
- *(.text .text.[0-9a-zA-Z_]* .text..L.cfi*)
+ *(TEXT_MAIN_MODULE)
+ *(.text..L.cfi.jumptable .text..L.cfi.jumptable.*)
+ }
+#elif defined(CONFIG_MODULE_FG_KASLR)
+ .text : {
+ *(TEXT_MAIN_MODULE)
}
#endif
+
+ TEXT_FG_KASLR
}

/* bring in arch-specific sections */
--
2.33.1