Re: [PATCH 3.12 14/66] x86_64, traps: Stop using IST for #SS

From: Borislav Petkov
Date: Wed Dec 17 2014 - 10:56:31 EST

On Sat, Dec 06, 2014 at 04:07:06PM +0100, Jiri Slaby wrote:
> From: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>
> 3.12-stable review patch. If anyone has any objections, please let me know.
>
> ===============
>
> commit 6f442be2fb22be02cafa606f1769fa1e6f894441 upstream.
>
> 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@xxxxxxxxxxxxxx>
> Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
> ---
> arch/x86/include/asm/page_32_types.h | 1 -
> arch/x86/include/asm/page_64_types.h | 11 +++++------
> arch/x86/kernel/dumpstack_64.c | 1 -
> arch/x86/kernel/entry_64.S | 2 +-
> arch/x86/kernel/traps.c | 18 +-----------------
> 5 files changed, 7 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 43dcd804ebd5..d1d2972a54db 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/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
> index addb207dab92..66e274a3d968 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 c2ab43484b5d..e310e82de5e6 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -1510,7 +1510,7 @@ apicinterrupt3 HYPERVISOR_CALLBACK_VECTOR \
>
> paranoidzeroentry_ist debug do_debug DEBUG_STACK
> paranoidzeroentry_ist int3 do_int3 DEBUG_STACK
> -paranoiderrorentry stack_segment do_stack_segment
> +errorentry stack_segment do_stack_segment
> #ifdef CONFIG_XEN
> zeroentry xen_debug do_debug
> zeroentry xen_int3 do_int3
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 8a2684f938b9..b2d6c58b28b0 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -221,28 +221,12 @@ 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_INFO(X86_TRAP_AC, SIGBUS, "alignment check", alignment_check,
> BUS_ADRALN, 0)
>
> #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";
> @@ -776,7 +760,7 @@ 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);

I guess this should say "&stack_segment" in order for it to be
consistent with the rest of the set_intr_gate() calls here.

Fortunately, this is not an issue as function designators are
automatically promoted to function pointers, as the identical asm shows:

.loc 1 792 0
movq $stack_segment, %rsi #,
movl $12, %edi #,
call set_intr_gate #

.loc 1 792 0
movq $stack_segment, %rsi #,
movl $12, %edi #,
call set_intr_gate #

--
Regards/Gruss,
Boris.

Sent from a fat crate under my desk. Formatting is fine.
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