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SubjectRe: [PATCH v3 2/2] x86/refcount: Implement fast refcount overflow protection
On Mon, May 08, 2017 at 08:58:29PM -0500, Josh Poimboeuf wrote:
> On Mon, May 08, 2017 at 04:31:11PM -0700, Kees Cook wrote:
> > On Mon, May 8, 2017 at 3:53 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > > On Mon, May 08, 2017 at 12:32:52PM -0700, Kees Cook wrote:
> > >> +#define REFCOUNT_EXCEPTION \
> > >> + "movl $0x7fffffff, %[counter]\n\t" \
> > >> + "int $"__stringify(X86_REFCOUNT_VECTOR)"\n" \
> > >> + "0:\n\t" \
> > >> + _ASM_EXTABLE(0b, 0b)
> > >
> > > Despite the objtool warnings going away, this still uses the exception
> > > table in a new way, which will confuse objtool. I need to do some more
> > > thinking about the best way to fix it, either as a change to your patch
> > > or a change to objtool.
> >
> > In that it's not a "true" exception?
>
> Right. And also that it doesn't need the "fixup" since it would return
> to the same address anyway.

How about the following on top of your patch? It uses #UD (invalid
opcode). Notice it's mostly code deletions :-)


diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index bba6976..50bc269 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -789,15 +789,6 @@ ENTRY(spurious_interrupt_bug)
jmp common_exception
END(spurious_interrupt_bug)

-#ifdef CONFIG_FAST_REFCOUNT
-ENTRY(refcount_error)
- ASM_CLAC
- pushl $0
- pushl $do_refcount_error
- jmp common_exception
-ENDPROC(refcount_error)
-#endif
-
#ifdef CONFIG_XEN
ENTRY(xen_hypervisor_callback)
pushl $-1 /* orig_ax = -1 => not a system call */
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 783045d..607d72c 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -855,9 +855,6 @@ idtentry coprocessor_error do_coprocessor_error has_error_code=0
idtentry alignment_check do_alignment_check has_error_code=1
idtentry simd_coprocessor_error do_simd_coprocessor_error has_error_code=0

-#ifdef CONFIG_FAST_REFCOUNT
-idtentry refcount_error do_refcount_error has_error_code=0
-#endif

/*
* Reload gs selector with exception handling
diff --git a/arch/x86/include/asm/irq_vectors.h b/arch/x86/include/asm/irq_vectors.h
index d117776..6ca9fd6 100644
--- a/arch/x86/include/asm/irq_vectors.h
+++ b/arch/x86/include/asm/irq_vectors.h
@@ -48,9 +48,6 @@

#define IA32_SYSCALL_VECTOR 0x80

-/* Refcount overflow reporting exception. */
-#define X86_REFCOUNT_VECTOR 0x81
-
/*
* Vectors 0x30-0x3f are used for ISA interrupts.
* round up to the next 16-vector boundary
diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
index 6e8bbd7..653a985 100644
--- a/arch/x86/include/asm/refcount.h
+++ b/arch/x86/include/asm/refcount.h
@@ -8,15 +8,16 @@
*/
#include <linux/refcount.h>
#include <asm/irq_vectors.h>
+#include <asm/bug.h>

#define REFCOUNT_EXCEPTION \
"movl $0x7fffffff, %[counter]\n\t" \
- "int $"__stringify(X86_REFCOUNT_VECTOR)"\n" \
- "0:\n\t" \
- _ASM_EXTABLE(0b, 0b)
+ "1:\t" ASM_UD0 "\n" \
+ "2:\n\t" \
+ _ASM_EXTABLE(1b, 2b)

#define REFCOUNT_CHECK \
- "jns 0f\n\t" \
+ "jns 2f\n\t" \
REFCOUNT_EXCEPTION

static __always_inline void refcount_add(unsigned int i, refcount_t *r)
diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index e4d8db7..01fd0a7 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -38,10 +38,6 @@ asmlinkage void machine_check(void);
#endif /* CONFIG_X86_MCE */
asmlinkage void simd_coprocessor_error(void);

-#ifdef CONFIG_FAST_REFCOUNT
-asmlinkage void refcount_error(void);
-#endif
-
#ifdef CONFIG_TRACING
asmlinkage void trace_page_fault(void);
#define trace_stack_segment stack_segment
@@ -58,7 +54,6 @@ asmlinkage void trace_page_fault(void);
#define trace_alignment_check alignment_check
#define trace_simd_coprocessor_error simd_coprocessor_error
#define trace_async_page_fault async_page_fault
-#define trace_refcount_error refcount_error
#endif

dotraplinkage void do_divide_error(struct pt_regs *, long);
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 0b2dbcc..7de95b7 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -220,8 +220,8 @@ do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
if (!user_mode(regs)) {
if (fixup_exception(regs, trapnr)) {
if (IS_ENABLED(CONFIG_FAST_REFCOUNT) &&
- trapnr == X86_REFCOUNT_VECTOR)
- refcount_error_report(regs, str);
+ trapnr == X86_TRAP_UD)
+ refcount_error_report(regs);

return 0;
}
@@ -332,10 +332,6 @@ DO_ERROR(X86_TRAP_NP, SIGBUS, "segment not present", segment_not_present)
DO_ERROR(X86_TRAP_SS, SIGBUS, "stack segment", stack_segment)
DO_ERROR(X86_TRAP_AC, SIGBUS, "alignment check", alignment_check)

-#ifdef CONFIG_FAST_REFCOUNT
-DO_ERROR(X86_REFCOUNT_VECTOR, SIGILL, "refcount overflow", refcount_error)
-#endif
-
#ifdef CONFIG_VMAP_STACK
__visible void __noreturn handle_stack_overflow(const char *message,
struct pt_regs *regs,
@@ -1026,11 +1022,6 @@ void __init trap_init(void)
set_bit(IA32_SYSCALL_VECTOR, used_vectors);
#endif

-#ifdef CONFIG_FAST_REFCOUNT
- set_intr_gate(X86_REFCOUNT_VECTOR, refcount_error);
- set_bit(X86_REFCOUNT_VECTOR, used_vectors);
-#endif
-
/*
* Set the IDT descriptor to a fixed read-only location, so that the
* "sidt" instruction will not leak the location of the kernel, and
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 94f87d5..53c9326 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -276,7 +276,7 @@ extern int oops_may_print(void);
void do_exit(long error_code) __noreturn;
void complete_and_exit(struct completion *, long) __noreturn;

-void refcount_error_report(struct pt_regs *regs, const char *kind);
+void refcount_error_report(struct pt_regs *regs);

/* Internal, do not use. */
int __must_check _kstrtoul(const char *s, unsigned int base, unsigned long *res);
diff --git a/kernel/panic.c b/kernel/panic.c
index c95b919..2c4ce79 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -605,7 +605,7 @@ EXPORT_SYMBOL(__stack_chk_fail);
#ifdef CONFIG_FAST_REFCOUNT
static DEFINE_RATELIMIT_STATE(refcount_ratelimit, 15 * HZ, 3);

-void refcount_error_report(struct pt_regs *regs, const char *kind)
+void refcount_error_report(struct pt_regs *regs)
{
/* Always make sure triggering process will be terminated. */
do_send_sig_info(SIGKILL, SEND_SIG_FORCED, current, true);
@@ -613,8 +613,7 @@ void refcount_error_report(struct pt_regs *regs, const char *kind)
if (!__ratelimit(&refcount_ratelimit))
return;

- pr_emerg("%s detected in: %s:%d, uid/euid: %u/%u\n",
- kind ? kind : "refcount error",
+ pr_emerg("refcount error detected in: %s:%d, uid/euid: %u/%u\n",
current->comm, task_pid_nr(current),
from_kuid_munged(&init_user_ns, current_uid()),
from_kuid_munged(&init_user_ns, current_euid()));
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