x86_64, traps: Stop using IST for #SS

On a 32-bit kernel, this has no effect, since there are no IST stacks.

On a 64-bit kernel, #SS can only happen in user code, on a failed iret
to user space, a canonical violation on access via RSP or RBP, or a
genuine stack segment violation in 32-bit kernel code.  The first two
cases don't need IST, and the latter two cases are unlikely fatal bugs,
and promoting them to double faults would be fine.

This fixes a bug in which the espfix64 code mishandles a stack segment
violation.

This saves 4k of memory per CPU and a tiny bit of code.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

6 files changed, 8 insertions, 26 deletions

diff --git a/arch/x86/include/asm/page_32_types.h b/arch/x86/include/asm/page_32_types.h
index f48b17df4224..3a52ee0e726d 100644
--- a/arch/x86/include/asm/page_32_types.h
+++ b/arch/x86/include/asm/page_32_types.h
@@ -20,7 +20,6 @@
 #define THREAD_SIZE_ORDER       1
 #define THREAD_SIZE             (PAGE_SIZE << THREAD_SIZE_ORDER)
 
-#define STACKFAULT_STACK 0
 #define DOUBLEFAULT_STACK 1
 #define NMI_STACK 0
 #define DEBUG_STACK 0
diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
index 678205195ae1..75450b2c7be4 100644
--- a/arch/x86/include/asm/page_64_types.h
+++ b/arch/x86/include/asm/page_64_types.h
@@ -14,12 +14,11 @@
 #define IRQ_STACK_ORDER 2
 #define IRQ_STACK_SIZE (PAGE_SIZE << IRQ_STACK_ORDER)
 
-#define STACKFAULT_STACK 1
-#define DOUBLEFAULT_STACK 2
-#define NMI_STACK 3
-#define DEBUG_STACK 4
-#define MCE_STACK 5
-#define N_EXCEPTION_STACKS 5  /* hw limit: 7 */
+#define DOUBLEFAULT_STACK 1
+#define NMI_STACK 2
+#define DEBUG_STACK 3
+#define MCE_STACK 4
+#define N_EXCEPTION_STACKS 4  /* hw limit: 7 */
 
 #define PUD_PAGE_SIZE           (_AC(1, UL) << PUD_SHIFT)
 #define PUD_PAGE_MASK           (~(PUD_PAGE_SIZE-1))
diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index bc8352e7010a..707adc6549d8 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -39,6 +39,7 @@ asmlinkage void simd_coprocessor_error(void);
 
 #ifdef CONFIG_TRACING
 asmlinkage void trace_page_fault(void);
+#define trace_stack_segment stack_segment
 #define trace_divide_error divide_error
 #define trace_bounds bounds
 #define trace_invalid_op invalid_op
diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
index 1abcb50b48ae..ff86f19b5758 100644
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -24,7 +24,6 @@ static char x86_stack_ids[][8] = {
                 [ DEBUG_STACK-1                 ]       = "#DB",
                 [ NMI_STACK-1                   ]       = "NMI",
                 [ DOUBLEFAULT_STACK-1           ]       = "#DF",
-                [ STACKFAULT_STACK-1            ]       = "#SS",
                 [ MCE_STACK-1                   ]       = "#MC",
 #if DEBUG_STKSZ > EXCEPTION_STKSZ
                 [ N_EXCEPTION_STACKS ...
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index a4dc8de7c4be..49a0c1781253 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1259,7 +1259,7 @@ apicinterrupt3 HYPERVISOR_CALLBACK_VECTOR \
 
 idtentry debug do_debug has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
 idtentry int3 do_int3 has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
-idtentry stack_segment do_stack_segment has_error_code=1 paranoid=1
+idtentry stack_segment do_stack_segment has_error_code=1
 #ifdef CONFIG_XEN
 idtentry xen_debug do_debug has_error_code=0
 idtentry xen_int3 do_int3 has_error_code=0
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 819662746e23..48035e9cdde9 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -233,27 +233,11 @@ DO_ERROR(X86_TRAP_UD,     SIGILL,  "invalid opcode",		invalid_op)
 DO_ERROR(X86_TRAP_OLD_MF, SIGFPE,  "coprocessor segment overrun",coprocessor_segment_overrun)
 DO_ERROR(X86_TRAP_TS,     SIGSEGV, "invalid TSS",               invalid_TSS)
 DO_ERROR(X86_TRAP_NP,     SIGBUS,  "segment not present",       segment_not_present)
-#ifdef CONFIG_X86_32
 DO_ERROR(X86_TRAP_SS,     SIGBUS,  "stack segment",             stack_segment)
-#endif
 DO_ERROR(X86_TRAP_AC,     SIGBUS,  "alignment check",           alignment_check)
 
 #ifdef CONFIG_X86_64
 /* Runs on IST stack */
-dotraplinkage void do_stack_segment(struct pt_regs *regs, long error_code)
-{
-        enum ctx_state prev_state;
-
-        prev_state = exception_enter();
-        if (notify_die(DIE_TRAP, "stack segment", regs, error_code,
-                       X86_TRAP_SS, SIGBUS) != NOTIFY_STOP) {
-                preempt_conditional_sti(regs);
-                do_trap(X86_TRAP_SS, SIGBUS, "stack segment", regs, error_code, NULL);
-                preempt_conditional_cli(regs);
-        }
-        exception_exit(prev_state);
-}
-
 dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
 {
         static const char str[] = "double fault";
@@ -802,7 +786,7 @@ void __init trap_init(void)
         set_intr_gate(X86_TRAP_OLD_MF, coprocessor_segment_overrun);
         set_intr_gate(X86_TRAP_TS, invalid_TSS);
         set_intr_gate(X86_TRAP_NP, segment_not_present);
-        set_intr_gate_ist(X86_TRAP_SS, &stack_segment, STACKFAULT_STACK);
+        set_intr_gate(X86_TRAP_SS, stack_segment);
         set_intr_gate(X86_TRAP_GP, general_protection);
         set_intr_gate(X86_TRAP_SPURIOUS, spurious_interrupt_bug);
         set_intr_gate(X86_TRAP_MF, coprocessor_error);