add support for Clang CFI

This change adds support for Clang’s forward-edge Control Flow
Integrity (CFI) checking. With CONFIG_CFI_CLANG, the compiler
injects a runtime check before each indirect function call to ensure
the target is a valid function with the correct static type. This
restricts possible call targets and makes it more difficult for
an attacker to exploit bugs that allow the modification of stored
function pointers. For more details, see:

  https://clang.llvm.org/docs/ControlFlowIntegrity.html

Clang requires CONFIG_LTO_CLANG to be enabled with CFI to gain
visibility to possible call targets. Kernel modules are supported
with Clang’s cross-DSO CFI mode, which allows checking between
independently compiled components.

With CFI enabled, the compiler injects a __cfi_check() function into
the kernel and each module for validating local call targets. For
cross-module calls that cannot be validated locally, the compiler
calls the global __cfi_slowpath_diag() function, which determines
the target module and calls the correct __cfi_check() function. This
patch includes a slowpath implementation that uses __module_address()
to resolve call targets, and with CONFIG_CFI_CLANG_SHADOW enabled, a
shadow map that speeds up module look-ups by ~3x.

Clang implements indirect call checking using jump tables and
offers two methods of generating them. With canonical jump tables,
the compiler renames each address-taken function to <function>.cfi
and points the original symbol to a jump table entry, which passes
__cfi_check() validation. This isn’t compatible with stand-alone
assembly code, which the compiler doesn’t instrument, and would
result in indirect calls to assembly code to fail. Therefore, we
default to using non-canonical jump tables instead, where the compiler
generates a local jump table entry <function>.cfi_jt for each
address-taken function, and replaces all references to the function
with the address of the jump table entry.

Note that because non-canonical jump table addresses are local
to each component, they break cross-module function address
equality. Specifically, the address of a global function will be
different in each module, as it's replaced with the address of a local
jump table entry. If this address is passed to a different module,
it won’t match the address of the same function taken there. This
may break code that relies on comparing addresses passed from other
components.

CFI checking can be disabled in a function with the __nocfi attribute.
Additionally, CFI can be disabled for an entire compilation unit by
filtering out CC_FLAGS_CFI.

By default, CFI failures result in a kernel panic to stop a potential
exploit. CONFIG_CFI_PERMISSIVE enables a permissive mode, where the
kernel prints out a rate-limited warning instead, and allows execution
to continue. This option is helpful for locating type mismatches, but
should only be enabled during development.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210408182843.1754385-2-samitolvanen@google.com

1 files changed, 45 insertions, 0 deletions

diff --git a/arch/Kconfig b/arch/Kconfig
index ecfd3520b676..f6a85ba6cba2 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -692,6 +692,51 @@ config LTO_CLANG_THIN
 	  If unsure, say Y.
 endchoice
 
+config ARCH_SUPPORTS_CFI_CLANG
+	bool
+	help
+	  An architecture should select this option if it can support Clang's
+	  Control-Flow Integrity (CFI) checking.
+
+config CFI_CLANG
+	bool "Use Clang's Control Flow Integrity (CFI)"
+	depends on LTO_CLANG && ARCH_SUPPORTS_CFI_CLANG
+	# Clang >= 12:
+	# - https://bugs.llvm.org/show_bug.cgi?id=46258
+	# - https://bugs.llvm.org/show_bug.cgi?id=47479
+	depends on CLANG_VERSION >= 120000
+	select KALLSYMS
+	help
+	  This option enables Clang’s forward-edge Control Flow Integrity
+	  (CFI) checking, where the compiler injects a runtime check to each
+	  indirect function call to ensure the target is a valid function with
+	  the correct static type. This restricts possible call targets and
+	  makes it more difficult for an attacker to exploit bugs that allow
+	  the modification of stored function pointers. More information can be
+	  found from Clang's documentation:
+
+	    https://clang.llvm.org/docs/ControlFlowIntegrity.html
+
+config CFI_CLANG_SHADOW
+	bool "Use CFI shadow to speed up cross-module checks"
+	default y
+	depends on CFI_CLANG && MODULES
+	help
+	  If you select this option, the kernel builds a fast look-up table of
+	  CFI check functions in loaded modules to reduce performance overhead.
+
+	  If unsure, say Y.
+
+config CFI_PERMISSIVE
+	bool "Use CFI in permissive mode"
+	depends on CFI_CLANG
+	help
+	  When selected, Control Flow Integrity (CFI) violations result in a
+	  warning instead of a kernel panic. This option should only be used
+	  for finding indirect call type mismatches during development.
+
+	  If unsure, say N.
+
 config HAVE_ARCH_WITHIN_STACK_FRAMES
 	bool
 	help