From 2e04bc76560decd9270be2a805927316f617ef56 Mon Sep 17 00:00:00 2001 From: Alexander van Heukelum Date: Thu, 18 Jun 2009 00:35:57 +0200 Subject: [PATCH] i386: fix return to 16-bit stack from NMI handler Returning to a task with a 16-bit stack requires special care: the iret instruction does not restore the high word of esp in that case. The espfix code fixes this, but currently is not invoked on NMIs. This means that a running task gets the upper word of esp clobbered due intervening NMIs. To reproduce, compile and run the following program with the nmi watchdog enabled (nmi_watchdog=2 on the command line). Using gdb you can see that the high bits of esp contain garbage, while the low bits are still correct. This patch puts the espfix code back into the NMI code path. The patch is slightly complicated due to the irqtrace infrastructure not being NMI-safe. The NMI return path cannot call TRACE_IRQS_IRET. Otherwise, the tail of the normal iret-code is correct for the nmi code path too. To be able to share this code-path, the TRACE_IRQS_IRET was move up a bit. The espfix code exists after the TRACE_IRQS_IRET, but this code explicitly disables interrupts. This short interrupts-off section is now not traced anymore. The return-to-kernel path now always includes the preliminary test to decide if the espfix code should be called. This is never the case, but doing it this way keeps the patch as simple as possible and the few extra instructions should not affect timing in any significant way. #define _GNU_SOURCE #include #include #include #include #include #include int modify_ldt(int func, void *ptr, unsigned long bytecount) { return syscall(SYS_modify_ldt, func, ptr, bytecount); } /* this is assumed to be usable */ #define SEGBASEADDR 0x10000 #define SEGLIMIT 0x20000 /* 16-bit segment */ struct user_desc desc = { .entry_number = 0, .base_addr = SEGBASEADDR, .limit = SEGLIMIT, .seg_32bit = 0, .contents = 0, /* ??? */ .read_exec_only = 0, .limit_in_pages = 0, .seg_not_present = 0, .useable = 1 }; int main(void) { setvbuf(stdout, NULL, _IONBF, 0); /* map a 64 kb segment */ char *pointer = mmap((void *)SEGBASEADDR, SEGLIMIT+1, PROT_EXEC|PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0); if (pointer == NULL) { printf("could not map space\n"); return 0; } /* write ldt, new mode */ int err = modify_ldt(0x11, &desc, sizeof(desc)); if (err) { printf("error modifying ldt: %i\n", err); return 0; } for (int i=0; i<1000; i++) { asm volatile ( "pusha\n\t" "mov %ss, %eax\n\t" /* preserve ss:esp */ "mov %esp, %ebp\n\t" "push $7\n\t" /* index 0, ldt, user mode */ "push $65536-4096\n\t" /* esp */ "lss (%esp), %esp\n\t" /* switch to new stack */ "push %eax\n\t" /* save old ss:esp on new stack */ "push %ebp\n\t" "add $17*65536, %esp\n\t" /* set high bits */ "mov %esp, %edx\n\t" "mov $10000000, %ecx\n\t" /* wait... */ "1: loop 1b\n\t" /* ... a bit */ "cmp %esp, %edx\n\t" "je 1f\n\t" "ud2\n\t" /* esp changed inexplicably! */ "1:\n\t" "sub $17*65536, %esp\n\t" /* restore high bits */ "lss (%esp), %esp\n\t" /* restore old ss:esp */ "popa\n\t"); printf("\rx%ix", i); } return 0; } Signed-off-by: Alexander van Heukelum Acked-by: Stas Sergeev Signed-off-by: H. Peter Anvin --- arch/x86/kernel/entry_32.S | 14 ++++++++------ 1 file changed, 8 insertions(+), 6 deletions(-) diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S index c929add475c9..d7d1c7d20e4e 100644 --- a/arch/x86/kernel/entry_32.S +++ b/arch/x86/kernel/entry_32.S @@ -84,7 +84,7 @@ #define preempt_stop(clobbers) DISABLE_INTERRUPTS(clobbers); TRACE_IRQS_OFF #else #define preempt_stop(clobbers) -#define resume_kernel restore_nocheck +#define resume_kernel restore_all #endif .macro TRACE_IRQS_IRET @@ -372,7 +372,7 @@ END(ret_from_exception) ENTRY(resume_kernel) DISABLE_INTERRUPTS(CLBR_ANY) cmpl $0,TI_preempt_count(%ebp) # non-zero preempt_count ? - jnz restore_nocheck + jnz restore_all need_resched: movl TI_flags(%ebp), %ecx # need_resched set ? testb $_TIF_NEED_RESCHED, %cl @@ -540,6 +540,8 @@ syscall_exit: jne syscall_exit_work restore_all: + TRACE_IRQS_IRET +restore_all_notrace: movl PT_EFLAGS(%esp), %eax # mix EFLAGS, SS and CS # Warning: PT_OLDSS(%esp) contains the wrong/random values if we # are returning to the kernel. @@ -551,8 +553,6 @@ restore_all: CFI_REMEMBER_STATE je ldt_ss # returning to user-space with LDT SS restore_nocheck: - TRACE_IRQS_IRET -restore_nocheck_notrace: RESTORE_REGS 4 # skip orig_eax/error_code CFI_ADJUST_CFA_OFFSET -4 irq_return: @@ -601,8 +601,10 @@ ldt_ss: CFI_ADJUST_CFA_OFFSET 4 pushl %eax CFI_ADJUST_CFA_OFFSET 4 + /* Disable interrupts, but do not irqtrace this section: we + * will soon execute iret and the tracer was already set to + * the irqstate after the iret */ DISABLE_INTERRUPTS(CLBR_EAX) - TRACE_IRQS_OFF lss (%esp), %esp CFI_ADJUST_CFA_OFFSET -8 jmp restore_nocheck @@ -1329,7 +1331,7 @@ nmi_stack_correct: xorl %edx,%edx # zero error code movl %esp,%eax # pt_regs pointer call do_nmi - jmp restore_nocheck_notrace + jmp restore_all_notrace CFI_ENDPROC nmi_stack_fixup: -- 2.39.5