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| author | Marco Elver <elver@google.com> | 2022-01-31 10:05:21 +0100 |
|---|---|---|
| committer | Kees Cook <keescook@chromium.org> | 2022-02-14 11:07:12 -0800 |
| commit | efa90c11f62e6b7252fb75efe2787056872a627c (patch) | |
| tree | 0946c60d170df43a0197200c713ecb0d7374c1cc /arch/Kconfig | |
| parent | 8cb37a5974a48569aab8a1736d21399fddbdbdb2 (diff) | |
| download | lwn-efa90c11f62e6b7252fb75efe2787056872a627c.tar.gz lwn-efa90c11f62e6b7252fb75efe2787056872a627c.zip | |
stack: Constrain and fix stack offset randomization with Clang builds
All supported versions of Clang perform auto-init of __builtin_alloca()
when stack auto-init is on (CONFIG_INIT_STACK_ALL_{ZERO,PATTERN}).
add_random_kstack_offset() uses __builtin_alloca() to add a stack
offset. This means, when CONFIG_INIT_STACK_ALL_{ZERO,PATTERN} is
enabled, add_random_kstack_offset() will auto-init that unused portion
of the stack used to add an offset.
There are several problems with this:
1. These offsets can be as large as 1023 bytes. Performing
memset() on them isn't exactly cheap, and this is done on
every syscall entry.
2. Architectures adding add_random_kstack_offset() to syscall
entry implemented in C require them to be 'noinstr' (e.g. see
x86 and s390). The potential problem here is that a call to
memset may occur, which is not noinstr.
A x86_64 defconfig kernel with Clang 11 and CONFIG_VMLINUX_VALIDATION shows:
| vmlinux.o: warning: objtool: do_syscall_64()+0x9d: call to memset() leaves .noinstr.text section
| vmlinux.o: warning: objtool: do_int80_syscall_32()+0xab: call to memset() leaves .noinstr.text section
| vmlinux.o: warning: objtool: __do_fast_syscall_32()+0xe2: call to memset() leaves .noinstr.text section
| vmlinux.o: warning: objtool: fixup_bad_iret()+0x2f: call to memset() leaves .noinstr.text section
Clang 14 (unreleased) will introduce a way to skip alloca initialization
via __builtin_alloca_uninitialized() (https://reviews.llvm.org/D115440).
Constrain RANDOMIZE_KSTACK_OFFSET to only be enabled if no stack
auto-init is enabled, the compiler is GCC, or Clang is version 14+. Use
__builtin_alloca_uninitialized() if the compiler provides it, as is done
by Clang 14.
Link: https://lkml.kernel.org/r/YbHTKUjEejZCLyhX@elver.google.com
Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220131090521.1947110-2-elver@google.com
Diffstat (limited to 'arch/Kconfig')
| -rw-r--r-- | arch/Kconfig | 1 |
1 files changed, 1 insertions, 0 deletions
diff --git a/arch/Kconfig b/arch/Kconfig index 2cde48d9b77c..c5b50bfe31c1 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -1163,6 +1163,7 @@ config RANDOMIZE_KSTACK_OFFSET bool "Support for randomizing kernel stack offset on syscall entry" if EXPERT default y depends on HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET + depends on INIT_STACK_NONE || !CC_IS_CLANG || CLANG_VERSION >= 140000 help The kernel stack offset can be randomized (after pt_regs) by roughly 5 bits of entropy, frustrating memory corruption |