diff mbox series

[1/1] kbuild: Fix CFI hash randomization with KASAN

Message ID 20230112224948.1479453-2-samitolvanen@google.com
State New
Headers
Series Fix CFI hash randomization with KASAN |

Commit Message

Sami Tolvanen Jan. 12, 2023, 10:49 p.m. UTC 
  Clang emits a asan.module_ctor constructor to each object file
when KASAN is enabled, and these functions are indirectly called
in do_ctors. With CONFIG_CFI_CLANG, the compiler also emits a CFI
type hash before each address-taken global function so they can
pass indirect call checks.

However, in commit 0c3e806ec0f9 ("x86/cfi: Add boot time hash
randomization"), x86 implemented boot time hash randomization,
which relies on the .cfi_sites section generated by objtool. As
objtool is run against vmlinux.o instead of individual object
files with X86_KERNEL_IBT (enabled by default), CFI types in
object files that are not part of vmlinux.o end up not being
included in .cfi_sites, and thus won't get randomized and trip
CFI when called.

Only .vmlinux.export.o and init/version-timestamp.o are linked
into vmlinux separately from vmlinux.o. As these files don't
contain any functions, disable KASAN for both of them to avoid
breaking hash randomization.

Link: https://github.com/ClangBuiltLinux/linux/issues/1742
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
 init/Makefile            | 1 +
 scripts/Makefile.vmlinux | 1 +
 2 files changed, 2 insertions(+)

Comments

Kees Cook Jan. 12, 2023, 10:52 p.m. UTC | #1 
On Thu, Jan 12, 2023 at 10:49:48PM +0000, Sami Tolvanen wrote:
> Clang emits a asan.module_ctor constructor to each object file
> when KASAN is enabled, and these functions are indirectly called
> in do_ctors. With CONFIG_CFI_CLANG, the compiler also emits a CFI
> type hash before each address-taken global function so they can
> pass indirect call checks.
> 
> However, in commit 0c3e806ec0f9 ("x86/cfi: Add boot time hash
> randomization"), x86 implemented boot time hash randomization,
> which relies on the .cfi_sites section generated by objtool. As
> objtool is run against vmlinux.o instead of individual object
> files with X86_KERNEL_IBT (enabled by default), CFI types in
> object files that are not part of vmlinux.o end up not being
> included in .cfi_sites, and thus won't get randomized and trip
> CFI when called.
> 
> Only .vmlinux.export.o and init/version-timestamp.o are linked
> into vmlinux separately from vmlinux.o. As these files don't
> contain any functions, disable KASAN for both of them to avoid
> breaking hash randomization.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1742
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

Reviewed-by: Kees Cook <keescook@chromium.org>
Peter Zijlstra Jan. 13, 2023, 9:27 a.m. UTC | #2 
On Thu, Jan 12, 2023 at 10:49:48PM +0000, Sami Tolvanen wrote:
> Clang emits a asan.module_ctor constructor to each object file
> when KASAN is enabled, and these functions are indirectly called
> in do_ctors. With CONFIG_CFI_CLANG, the compiler also emits a CFI
> type hash before each address-taken global function so they can
> pass indirect call checks.
> 
> However, in commit 0c3e806ec0f9 ("x86/cfi: Add boot time hash
> randomization"), x86 implemented boot time hash randomization,
> which relies on the .cfi_sites section generated by objtool. As
> objtool is run against vmlinux.o instead of individual object
> files with X86_KERNEL_IBT (enabled by default), CFI types in
> object files that are not part of vmlinux.o end up not being
> included in .cfi_sites, and thus won't get randomized and trip
> CFI when called.
> 
> Only .vmlinux.export.o and init/version-timestamp.o are linked
> into vmlinux separately from vmlinux.o. As these files don't
> contain any functions, disable KASAN for both of them to avoid
> breaking hash randomization.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1742
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

Must've been 'fun' to figure out, Thanks!

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
diff mbox series

Patch

diff --git a/init/Makefile b/init/Makefile
index 8316c23bead2..26de459006c4 100644
--- a/init/Makefile
+++ b/init/Makefile
@@ -59,3 +59,4 @@  include/generated/utsversion.h: FORCE
 
 $(obj)/version-timestamp.o: include/generated/utsversion.h
 CFLAGS_version-timestamp.o := -include include/generated/utsversion.h
+KASAN_SANITIZE_version-timestamp.o := n
diff --git a/scripts/Makefile.vmlinux b/scripts/Makefile.vmlinux
index 49946cb96844..10176dec97ea 100644
--- a/scripts/Makefile.vmlinux
+++ b/scripts/Makefile.vmlinux
@@ -18,6 +18,7 @@  quiet_cmd_cc_o_c = CC      $@
 	$(call if_changed_dep,cc_o_c)
 
 ifdef CONFIG_MODULES
+KASAN_SANITIZE_.vmlinux.export.o := n
 targets += .vmlinux.export.o
 vmlinux: .vmlinux.export.o
 endif