3.2.55-rt79-rc1 stable review patch. If anyone has any objections, please let me know. ------------------ From: Sebastian Andrzej Siewior where do I start. Let me explain what is going on here. The code sequence | pushf | pop %edx | or $0x1,%dh | push %edx | mov $0xe0,%eax | popf | sysenter triggers the bug. On 64bit kernel we see the double fault (with 32bit and 64bit userland) and on 32bit kernel there is no problem. The reporter said that double fault does not happen on 64bit kernel with 64bit userland and this is because in that case the VDSO uses the "syscall" interface instead of "sysenter". The bug. "popf" loads the flags with the TF bit set which enables "single stepping" and this leads to a debug exception. Usually on 64bit we have a special IST stack for the debug exception. Due to patch [0] we do not use the IST stack but the kernel stack instead. On 64bit the sysenter instruction starts in kernel with the stack address NULL. The code sequence above enters the debug exception (TF flag) after the sysenter instruction was executed which sets the stack pointer to NULL and we have a fault (it seems that the debug exception saves some bytes on the stack). To fix the double fault I'm going to drop patch [0]. It is completely pointless. In do_debug() and do_stack_segment() we disable preemption which means the task can't leave the CPU. So it does not matter if we run on IST or on kernel stack. There is a patch [1] which drops preempt_disable() call for a 32bit kernel but not for 64bit so there should be no regression. And [1] seems valid even for this code sequence. We enter the debug exception with a 256bytes long per cpu stack and migrate to the kernel stack before calling do_debug(). [0] x86-disable-debug-stack.patch [1] fix-rt-int3-x86_32-3.2-rt.patch Cc: stable-rt@vger.kernel.org Reported-by: Brian Silverman Cc: Andi Kleen Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Steven Rostedt --- arch/x86/include/asm/page_64_types.h | 21 ++++++--------------- arch/x86/kernel/cpu/common.c | 2 -- arch/x86/kernel/dumpstack_64.c | 4 ---- 3 files changed, 6 insertions(+), 21 deletions(-) diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h index 0883ecd..7639dbf 100644 --- a/arch/x86/include/asm/page_64_types.h +++ b/arch/x86/include/asm/page_64_types.h @@ -14,21 +14,12 @@ #define IRQ_STACK_ORDER 2 #define IRQ_STACK_SIZE (PAGE_SIZE << IRQ_STACK_ORDER) -#ifdef CONFIG_PREEMPT_RT_FULL -# define STACKFAULT_STACK 0 -# define DOUBLEFAULT_STACK 1 -# define NMI_STACK 2 -# define DEBUG_STACK 0 -# define MCE_STACK 3 -# define N_EXCEPTION_STACKS 3 /* hw limit: 7 */ -#else -# 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 */ -#endif +#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 PUD_PAGE_SIZE (_AC(1, UL) << PUD_SHIFT) #define PUD_PAGE_MASK (~(PUD_PAGE_SIZE-1)) diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index edc013e..ca93cc7 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1050,9 +1050,7 @@ DEFINE_PER_CPU(unsigned int, irq_count) = -1; */ static const unsigned int exception_stack_sizes[N_EXCEPTION_STACKS] = { [0 ... N_EXCEPTION_STACKS - 1] = EXCEPTION_STKSZ, -#if DEBUG_STACK > 0 [DEBUG_STACK - 1] = DEBUG_STKSZ -#endif }; static DEFINE_PER_CPU_PAGE_ALIGNED(char, exception_stacks diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c index 352beb7..6d728d9 100644 --- a/arch/x86/kernel/dumpstack_64.c +++ b/arch/x86/kernel/dumpstack_64.c @@ -21,14 +21,10 @@ (N_EXCEPTION_STACKS + DEBUG_STKSZ/EXCEPTION_STKSZ - 2) static char x86_stack_ids[][8] = { -#if DEBUG_STACK > 0 [ DEBUG_STACK-1 ] = "#DB", -#endif [ NMI_STACK-1 ] = "NMI", [ DOUBLEFAULT_STACK-1 ] = "#DF", -#if STACKFAULT_STACK > 0 [ STACKFAULT_STACK-1 ] = "#SS", -#endif [ MCE_STACK-1 ] = "#MC", #if DEBUG_STKSZ > EXCEPTION_STKSZ [ N_EXCEPTION_STACKS ... -- 1.8.5.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/