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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id d2sm3142742pju.2.2022.01.28.11.10.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 28 Jan 2022 11:10:21 -0800 (PST) Date: Fri, 28 Jan 2022 11:10:20 -0800 From: Kees Cook To: Marco Elver Cc: Thomas Gleixner , Peter Zijlstra , Ingo Molnar , Nathan Chancellor , Nick Desaulniers , Elena Reshetova , Alexander Potapenko , llvm@lists.linux.dev, kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] stack: Constrain stack offset randomization with Clang builds Message-ID: <202201281058.83EC9565@keescook> References: <20220128114446.740575-1-elver@google.com> <20220128114446.740575-2-elver@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220128114446.740575-2-elver@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 28, 2022 at 12:44:46PM +0100, Marco Elver wrote: > 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 > --- > arch/Kconfig | 1 + > include/linux/randomize_kstack.h | 14 ++++++++++++-- > 2 files changed, 13 insertions(+), 2 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 This makes it _unavailable_ for folks with Clang < 14, which seems too strong, especially since it's run-time off by default. I'd prefer dropping this hunk and adding some language to the _DEFAULT help noting the specific performance impact on Clang < 14. > help > The kernel stack offset can be randomized (after pt_regs) by > roughly 5 bits of entropy, frustrating memory corruption > diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h > index 91f1b990a3c3..5c711d73ed10 100644 > --- a/include/linux/randomize_kstack.h > +++ b/include/linux/randomize_kstack.h > @@ -17,8 +17,18 @@ DECLARE_PER_CPU(u32, kstack_offset); > * alignment. Also, since this use is being explicitly masked to a max of > * 10 bits, stack-clash style attacks are unlikely. For more details see > * "VLAs" in Documentation/process/deprecated.rst > + * > + * The normal alloca() can be initialized with INIT_STACK_ALL. Initializing the > + * unused area on each syscall entry is expensive, and generating an implicit > + * call to memset() may also be problematic (such as in noinstr functions). > + * Therefore, if the compiler provides it, use the "uninitialized" variant. Can you include the note that GCC doesn't initialize its alloca()? Otherwise, yeah, looks good to me. -Kees > */ > -void *__builtin_alloca(size_t size); > +#if __has_builtin(__builtin_alloca_uninitialized) > +#define __kstack_alloca __builtin_alloca_uninitialized > +#else > +#define __kstack_alloca __builtin_alloca > +#endif > + > /* > * Use, at most, 10 bits of entropy. We explicitly cap this to keep the > * "VLA" from being unbounded (see above). 10 bits leaves enough room for > @@ -37,7 +47,7 @@ void *__builtin_alloca(size_t size); > if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ > &randomize_kstack_offset)) { \ > u32 offset = raw_cpu_read(kstack_offset); \ > - u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset)); \ > + u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \ > /* Keep allocation even after "ptr" loses scope. */ \ > asm volatile("" :: "r"(ptr) : "memory"); \ > } \ > -- > 2.35.0.rc0.227.g00780c9af4-goog > -- Kees Cook