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SubjectRe: ESP corruption bug - what CPUs are affected?
Hi Petr et al.

I coded up something that remotely looks like the
discussed patch.
I have deviated from your proposals at some important
points:
1. I am not allocating the ring1 stack separately,
I am allocating it on a ring0 stack. Much simpler
code, although non-reentrant/preempt-unsafe.
2. I am disabling the interrupts after all. That's
because of the preempt-unsafeness. I pass up the
IOPL=1 when necessary, to avoid problems.
But I guess also with your technique the interrupts
had to be disabled, unless the ring1 stack is
per-thread.
3. I am using LAR. Do you really think it can be
slower than the whole thing of locating LDT?

Let me know if I did something stupid.
The patch is attached.
I tested (pretty much) everything in it, except
probably the "popl %esp" restartability. But that
one looks fairly simple and should work.

Does this patch look good?

diff -urN linux-2.6.8-pcsp/arch/i386/kernel/entry.S linux-2.6.8-stacks/arch/i386/kernel/entry.S
--- linux-2.6.8-pcsp/arch/i386/kernel/entry.S 2004-06-10 13:28:35.000000000 +0400
+++ linux-2.6.8-stacks/arch/i386/kernel/entry.S 2004-09-22 15:39:50.000000000 +0400
@@ -70,9 +70,16 @@
CF_MASK = 0x00000001
TF_MASK = 0x00000100
IF_MASK = 0x00000200
-DF_MASK = 0x00000400
+DF_MASK = 0x00000400
+IOPL1_MASK = 0x00001000
+IOPL2_MASK = 0x00002000
+IOPL_MASK = 0x00003000
NT_MASK = 0x00004000
+RF_MASK = 0x00010000
VM_MASK = 0x00020000
+AC_MASK = 0x00040000
+VIF_MASK = 0x00080000
+VIP_MASK = 0x00100000

#ifdef CONFIG_PREEMPT
#define preempt_stop cli
@@ -81,6 +88,49 @@
#define resume_kernel restore_all
#endif

+/*
+ * First, reserve the 0x400 bytes on stack for the NMI
+ * handler - it uses the same stack.
+ * Second, push the additional iret frame to switch to
+ * the Ring-1 trampoline. The correct value of ESP
+ * higher word is pushed here and popped on Ring-1.
+ * espfix_trampoline is working on a "small" stack.
+ * We patch its base in GDT here to allow the Ring-1 code
+ * to access the right place on stack. This is preempt-unsafe
+ * and therefore must be done with the interrupts disabled.
+ * To allow Ring-1 code to re-enable interrupts, we make
+ * sure it will be executing with at least IOPL1.
+ */
+#define NMI_STACK_RESERVE 0x400
+#define MAKE_ESPFIX \
+ cli; \
+ subl $NMI_STACK_RESERVE, %esp; \
+ .rept 5; \
+ pushl NMI_STACK_RESERVE+0x10(%esp); \
+ .endr; \
+ pushl %eax; \
+ movl %esp, %eax; \
+ pushl %ebp; \
+ GET_THREAD_INFO(%ebp); \
+ movl TI_cpu(%ebp), %ebp; \
+ shll $GDT_SIZE_SHIFT, %ebp; \
+ addl $cpu_gdt_table, %ebp; \
+ movw %ax, ((GDT_ENTRY_ESPFIX_SS << GDT_ENTRY_SHIFT) + 2)(%ebp); \
+ shrl $16, %eax; \
+ movb %al, ((GDT_ENTRY_ESPFIX_SS << GDT_ENTRY_SHIFT) + 4)(%ebp); \
+ movb %ah, ((GDT_ENTRY_ESPFIX_SS << GDT_ENTRY_SHIFT) + 7)(%ebp); \
+ popl %ebp; \
+ popl %eax; \
+ pushw 14(%esp); \
+ pushw $4; \
+ pushl $__ESPFIX_SS; \
+ pushl $0; \
+ pushl 20(%esp); \
+ andl $~(IF_MASK | TF_MASK | RF_MASK | NT_MASK | AC_MASK), (%esp); \
+ orl $IOPL1_MASK, (%esp); \
+ pushl $__ESPFIX_CS; \
+ pushl $espfix_trampoline;
+
#define SAVE_ALL \
cld; \
pushl %es; \
@@ -122,9 +172,24 @@
.previous


+/* If returning to Ring-3, not to V86, and with
+ * the small stack, try to fix the higher word of
+ * ESP, as the CPU won't restore it from stack.
+ * This is an "official" bug of all the x86-compatible
+ * CPUs, which we can try to work around to make
+ * dosemu happy. */
#define RESTORE_ALL \
- RESTORE_REGS \
- addl $4, %esp; \
+ movl EFLAGS(%esp), %eax; \
+ movb CS(%esp), %al; \
+ andl $(VM_MASK | 2), %eax; \
+ cmpl $2, %eax; \
+ jne 3f; \
+ larl OLDSS(%esp), %eax; \
+ testl $0x00400000, %eax; \
+3: RESTORE_REGS \
+ leal 4(%esp), %esp; \
+ jnz 1f; \
+ MAKE_ESPFIX \
1: iret; \
.section .fixup,"ax"; \
2: sti; \
@@ -174,6 +239,23 @@
jmp do_lcall


+ENTRY(espfix_trampoline)
+ popl %esp
+espfix_past_esp:
+1: iret
+.section .fixup,"ax"
+2: sti
+ movl $(__USER_DS), %edx
+ movl %edx, %ds
+ movl %edx, %es
+ pushl $11
+ call do_exit
+.previous
+.section __ex_table,"a"
+ .align 4
+ .long 1b,2b
+.previous
+
ENTRY(ret_from_fork)
pushl %eax
call schedule_tail
@@ -196,7 +278,7 @@
GET_THREAD_INFO(%ebp)
movl EFLAGS(%esp), %eax # mix EFLAGS and CS
movb CS(%esp), %al
- testl $(VM_MASK | 3), %eax
+ testl $(VM_MASK | 2), %eax
jz resume_kernel # returning to kernel or vm86-space
ENTRY(resume_userspace)
cli # make sure we don't miss an interrupt
@@ -504,7 +586,12 @@
ENTRY(nmi)
cmpl $sysenter_entry,(%esp)
je nmi_stack_fixup
- pushl %eax
+ cmpl $espfix_past_esp,(%esp)
+ jne 1f
+ /* restart the "popl %esp" */
+ decl (%esp)
+ subw $4, 12(%esp)
+1: pushl %eax
movl %esp,%eax
/* Do not access memory above the end of our stack page,
* it might not exist.
diff -urN linux-2.6.8-pcsp/arch/i386/kernel/head.S linux-2.6.8-stacks/arch/i386/kernel/head.S
--- linux-2.6.8-pcsp/arch/i386/kernel/head.S 2004-08-10 11:02:36.000000000 +0400
+++ linux-2.6.8-stacks/arch/i386/kernel/head.S 2004-09-21 14:58:16.000000000 +0400
@@ -514,8 +514,8 @@
.quad 0x00009a0000000000 /* 0xc0 APM CS 16 code (16 bit) */
.quad 0x0040920000000000 /* 0xc8 APM DS data */

- .quad 0x0000000000000000 /* 0xd0 - unused */
- .quad 0x0000000000000000 /* 0xd8 - unused */
+ .quad 0x00cfba000000ffff /* 0xd0 - ESPfix CS */
+ .quad 0x0000b2000000ffff /* 0xd8 - ESPfix SS */
.quad 0x0000000000000000 /* 0xe0 - unused */
.quad 0x0000000000000000 /* 0xe8 - unused */
.quad 0x0000000000000000 /* 0xf0 - unused */
diff -urN linux-2.6.8-pcsp/arch/i386/kernel/process.c linux-2.6.8-stacks/arch/i386/kernel/process.c
--- linux-2.6.8-pcsp/arch/i386/kernel/process.c 2004-06-21 09:19:07.000000000 +0400
+++ linux-2.6.8-stacks/arch/i386/kernel/process.c 2004-09-21 10:45:55.000000000 +0400
@@ -225,7 +225,7 @@
printk("EIP: %04x:[<%08lx>] CPU: %d\n",0xffff & regs->xcs,regs->eip, smp_processor_id());
print_symbol("EIP is at %s\n", regs->eip);

- if (regs->xcs & 3)
+ if (regs->xcs & 2)
printk(" ESP: %04x:%08lx",0xffff & regs->xss,regs->esp);
printk(" EFLAGS: %08lx %s (%s)\n",regs->eflags, print_tainted(),UTS_RELEASE);
printk("EAX: %08lx EBX: %08lx ECX: %08lx EDX: %08lx\n",
diff -urN linux-2.6.8-pcsp/arch/i386/kernel/signal.c linux-2.6.8-stacks/arch/i386/kernel/signal.c
--- linux-2.6.8-pcsp/arch/i386/kernel/signal.c 2004-08-10 11:02:36.000000000 +0400
+++ linux-2.6.8-stacks/arch/i386/kernel/signal.c 2004-09-21 10:48:38.000000000 +0400
@@ -558,7 +558,7 @@
* kernel mode. Just return without doing anything
* if so.
*/
- if ((regs->xcs & 3) != 3)
+ if (!(regs->xcs & 2))
return 1;

if (current->flags & PF_FREEZE) {
diff -urN linux-2.6.8-pcsp/arch/i386/kernel/traps.c linux-2.6.8-stacks/arch/i386/kernel/traps.c
--- linux-2.6.8-pcsp/arch/i386/kernel/traps.c 2004-08-10 11:02:36.000000000 +0400
+++ linux-2.6.8-stacks/arch/i386/kernel/traps.c 2004-09-21 10:47:37.000000000 +0400
@@ -210,7 +210,7 @@

esp = (unsigned long) (&regs->esp);
ss = __KERNEL_DS;
- if (regs->xcs & 3) {
+ if (regs->xcs & 2) {
in_kernel = 0;
esp = regs->esp;
ss = regs->xss & 0xffff;
@@ -264,7 +264,7 @@
char c;
unsigned long eip;

- if (regs->xcs & 3)
+ if (regs->xcs & 2)
goto no_bug; /* Not in kernel */

eip = regs->eip;
@@ -335,7 +335,7 @@

static inline void die_if_kernel(const char * str, struct pt_regs * regs, long err)
{
- if (!(regs->eflags & VM_MASK) && !(3 & regs->xcs))
+ if (!(regs->eflags & VM_MASK) && !(2 & regs->xcs))
die(str, regs, err);
}

@@ -357,7 +357,7 @@
goto trap_signal;
}

- if (!(regs->xcs & 3))
+ if (!(regs->xcs & 2))
goto kernel_trap;

trap_signal: {
@@ -437,7 +437,7 @@
if (regs->eflags & VM_MASK)
goto gp_in_vm86;

- if (!(regs->xcs & 3))
+ if (!(regs->xcs & 2))
goto gp_in_kernel;

current->thread.error_code = error_code;
@@ -614,7 +614,7 @@
* allowing programs to debug themselves without the ptrace()
* interface.
*/
- if ((regs->xcs & 3) == 0)
+ if ((regs->xcs & 2) == 0)
goto clear_TF_reenable;
if ((tsk->ptrace & (PT_DTRACE|PT_PTRACED)) == PT_DTRACE)
goto clear_TF;
@@ -630,7 +630,7 @@
/* If this is a kernel mode trap, save the user PC on entry to
* the kernel, that's what the debugger can make sense of.
*/
- info.si_addr = ((regs->xcs & 3) == 0) ? (void *)tsk->thread.eip :
+ info.si_addr = ((regs->xcs & 2) == 0) ? (void *)tsk->thread.eip :
(void *)regs->eip;
force_sig_info(SIGTRAP, &info, tsk);

diff -urN linux-2.6.8-pcsp/arch/i386/mm/extable.c linux-2.6.8-stacks/arch/i386/mm/extable.c
--- linux-2.6.8-pcsp/arch/i386/mm/extable.c 2004-06-09 15:44:19.000000000 +0400
+++ linux-2.6.8-stacks/arch/i386/mm/extable.c 2004-09-21 17:17:05.000000000 +0400
@@ -26,6 +26,11 @@
}
#endif

+ if (unlikely(regs->xcs == __ESPFIX_CS))
+ {
+ regs->xcs = __KERNEL_CS;
+ }
+
fixup = search_exception_tables(regs->eip);
if (fixup) {
regs->eip = fixup->fixup;
diff -urN linux-2.6.8-pcsp/arch/i386/mm/fault.c linux-2.6.8-stacks/arch/i386/mm/fault.c
--- linux-2.6.8-pcsp/arch/i386/mm/fault.c 2004-08-10 11:02:37.000000000 +0400
+++ linux-2.6.8-stacks/arch/i386/mm/fault.c 2004-09-21 16:03:28.000000000 +0400
@@ -77,7 +77,7 @@
u32 seg_ar, seg_limit, base, *desc;

/* The standard kernel/user address space limit. */
- *eip_limit = (seg & 3) ? USER_DS.seg : KERNEL_DS.seg;
+ *eip_limit = (seg & 2) ? USER_DS.seg : KERNEL_DS.seg;

/* Unlikely, but must come before segment checks. */
if (unlikely((regs->eflags & VM_MASK) != 0))
diff -urN linux-2.6.8-pcsp/include/asm-i386/segment.h linux-2.6.8-stacks/include/asm-i386/segment.h
--- linux-2.6.8-pcsp/include/asm-i386/segment.h 2004-01-09 09:59:19.000000000 +0300
+++ linux-2.6.8-stacks/include/asm-i386/segment.h 2004-09-21 14:57:44.000000000 +0400
@@ -38,8 +38,8 @@
* 24 - APM BIOS support
* 25 - APM BIOS support
*
- * 26 - unused
- * 27 - unused
+ * 26 - ESPfix Ring-1 trampoline CS
+ * 27 - ESPfix Ring-1 small SS
* 28 - unused
* 29 - unused
* 30 - unused
@@ -71,14 +71,21 @@
#define GDT_ENTRY_PNPBIOS_BASE (GDT_ENTRY_KERNEL_BASE + 6)
#define GDT_ENTRY_APMBIOS_BASE (GDT_ENTRY_KERNEL_BASE + 11)

+#define GDT_ENTRY_ESPFIX_CS (GDT_ENTRY_KERNEL_BASE + 14)
+#define GDT_ENTRY_ESPFIX_SS (GDT_ENTRY_KERNEL_BASE + 15)
+#define __ESPFIX_CS (GDT_ENTRY_ESPFIX_CS * 8 + 1)
+#define __ESPFIX_SS (GDT_ENTRY_ESPFIX_SS * 8 + 1)
+
#define GDT_ENTRY_DOUBLEFAULT_TSS 31

/*
* The GDT has 32 entries
*/
-#define GDT_ENTRIES 32
-
-#define GDT_SIZE (GDT_ENTRIES * 8)
+#define GDT_ENTRIES_SHIFT 5
+#define GDT_ENTRIES (1 << GDT_ENTRIES_SHIFT)
+#define GDT_ENTRY_SHIFT 3
+#define GDT_SIZE_SHIFT (GDT_ENTRIES_SHIFT + GDT_ENTRY_SHIFT)
+#define GDT_SIZE (1 << GDT_SIZE_SHIFT)

/* Simple and small GDT entries for booting only */
\
 
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