Re: [PATCH] locking/refcounts, x86/asm: Use unique .text section for refcount exceptions

Ard Biesheuvel
2017-09-01T21:43:28Z

On 1 September 2017 at 21:22, Kees Cook <keescook@chromium.org> wrote:
> Using .text.unlikely for refcount exceptions isn't safe because gcc may
> move entire functions into .text.unlikely (e.g. in6_dev_get()), which
> would cause any uses of a protected refcount_t function to stay inline
> with the function, triggering the protection unconditionally:
>
> .section .text.unlikely,"ax",@progbits
> .type in6_dev_get, @function
> in6_dev_getx:
> .LFB4673:
> .loc 2 4128 0
> .cfi_startproc
> ...
> lock; incl 480(%rbx)
> js 111f
> .pushsection .text.unlikely
> 111: lea 480(%rbx), %rcx
> 112: .byte 0x0f, 0xff
> .popsection
> 113:
>
> This creates a unique .text section and adds an additional test to the
> exception handler to WARN in the case of having none of OF, SF, nor ZF
> set so we can see things like this more easily in the future.
>
> Reported-by: Mike Galbraith <efault@gmx.de>
> Fixes: 7a46ec0e2f48 ("locking/refcounts, x86/asm: Implement fast refcount overflow protection")
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> arch/x86/Kconfig | 2 +-
> arch/x86/include/asm/refcount.h | 2 +-
> arch/x86/mm/extable.c | 7 ++++++-
> include/asm-generic/vmlinux.lds.h | 1 +
> 4 files changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index eaa8ff41f424..c6acdcdb3fc6 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -56,7 +56,7 @@ config X86
> select ARCH_HAS_MMIO_FLUSH
> select ARCH_HAS_PMEM_API if X86_64
> # Causing hangs/crashes, see the commit that added this change for details.
> - select ARCH_HAS_REFCOUNT if BROKEN
> + select ARCH_HAS_REFCOUNT
> select ARCH_HAS_UACCESS_FLUSHCACHE if X86_64
> select ARCH_HAS_SET_MEMORY
> select ARCH_HAS_SG_CHAIN
> diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
> index ff871210b9f2..4e44250e7d0d 100644
> --- a/arch/x86/include/asm/refcount.h
> +++ b/arch/x86/include/asm/refcount.h
> @@ -15,7 +15,7 @@
> * back to the regular execution flow in .text.
> */
> #define _REFCOUNT_EXCEPTION \
> - ".pushsection .text.unlikely\n" \
> + ".pushsection .text..refcount\n" \

You could also use a .subsection here: those are always emitted out of
line, but into the same section.

> "111:\tlea %[counter], %%" _ASM_CX "\n" \
> "112:\t" ASM_UD0 "\n" \
> ASM_UNREACHABLE \
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index c076f710de4c..cf0d74b47ae0 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -66,12 +66,17 @@ bool ex_handler_refcount(const struct exception_table_entry *fixup,
> * wrapped around) will be set. Additionally, seeing the refcount
> * reach 0 will set ZF (Zero Flag: result was zero). In each of
> * these cases we want a report, since it's a boundary condition.
> - *
> + * The SF case is not reported since it indicates post-boundary
> + * manipulations below zero or above INT_MAX. And if none of the
> + * flags are set, something has gone very wrong, so report it.
> */
> if (regs->flags & (X86_EFLAGS_OF | X86_EFLAGS_ZF)) {
> bool zero = regs->flags & X86_EFLAGS_ZF;
>
> refcount_error_report(regs, zero ? "hit zero" : "overflow");
> + } else if ((regs->flags & X86_EFLAGS_SF) == 0) {
> + /* Report if none of OF, ZF, nor SF are set. */
> + refcount_error_report(regs, "unexpected saturation");
> }
>
> return true;
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 8acfc1e099e1..e549bff87c5b 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -459,6 +459,7 @@
> #define TEXT_TEXT \
> ALIGN_FUNCTION(); \
> *(.text.hot TEXT_MAIN .text.fixup .text.unlikely) \
> + *(.text..refcount) \
> *(.ref.text) \
> MEM_KEEP(init.text) \
> MEM_KEEP(exit.text) \
> --
> 2.7.4
>
>
> --
> Kees Cook
> Pixel Security