From: Kees Cook keescook@chromium.org
[ Upstream commit 564c9cc84e2adf8a6671c1937f0a9fe3da2a4b0e ]
Using .text.unlikely for refcount exceptions isn't safe because gcc may move entire functions into .text.unlikely (e.g. in6_dev_dev()), 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..refcount 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.
The double dot for the section name keeps it out of the TEXT_MAIN macro namespace, to avoid collisions and so it can be put at the end with text.unlikely to keep the cold code together.
See commit:
cb87481ee89db ("kbuild: linker script do not match C names unless LD_DEAD_CODE_DATA_ELIMINATION is configured")
... which matches C names: [a-zA-Z0-9_] but not ".".
Reported-by: Mike Galbraith efault@gmx.de Signed-off-by: Kees Cook keescook@chromium.org Cc: Ard Biesheuvel ard.biesheuvel@linaro.org Cc: Elena elena.reshetova@intel.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Cc: linux-arch linux-arch@vger.kernel.org Fixes: 7a46ec0e2f48 ("locking/refcounts, x86/asm: Implement fast refcount overflow protection") Link: http://lkml.kernel.org/r/1504382986-49301-2-git-send-email-keescook@chromium... Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Sasha Levin alexander.levin@verizon.com --- arch/x86/include/asm/refcount.h | 2 +- arch/x86/mm/extable.c | 7 ++++++- include/asm-generic/vmlinux.lds.h | 1 + 3 files changed, 8 insertions(+), 2 deletions(-)
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" \ "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 c3521e2be396..3321b446b66c 100644 --- a/arch/x86/mm/extable.c +++ b/arch/x86/mm/extable.c @@ -67,12 +67,17 @@ 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) \