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SubjectRe: ESP corruption bug - what CPUs are affected? (patch att
Hi.

Petr Vandrovec wrote:
> CPL0 solution is certainly more localized, but I have hard problems
> to convice myself that it is actually safe.
I spent 2 days convincing myself the same way:)
The most problematic part was to make sure that
the stack is properly unwinded even if NMI comes
before the exception handler managed to switch
out of 16bit. But I think this is now handled.

> I would appreciate if you could add comments what values are set
OK, I did. But in fact that makes the patch
only even more obfuscated:( It doesn't look
possible to explain all the magic pattern and
its constraints in a comments. But I tried:)
The patch is attached.

> by ESPFIX_SWITCH_16 + 8 + 4 and simillar moves, and what they actually
> do.
Fortunately there are no such moves.
In an attempt to make the patch a little
self-explanatory, I #define'd all the meaningfull
values. So if there is a move to
ESPFIX_SWITCH16_OFFS+some_value, it is safe to
assume that the move is intended to load the
ESPFIX_SWITCH16, and "some_value" is just the
correction constant. So besides the 2 magic
pointers, there are really no moves above the
iret frame.

> And convicing myself that ESPFIX_SWITCH_32 has just right value so
> pushl %eax
> pushl %es
> lss ESPFIX_SWITCH_32,%esp
> popl %es
> popl %eax
> actually works took almost an hour...
I realize that and thats really the big problem
of that patch. It is very obfuscated and difficult
to understand.
I guess if you see the first version of that
patch, which was before I found the way to use
the fixed offsets for locating the switches,
you might just get sick:)

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 2004-06-26 15:20:23.000000000 +0400
+++ linux-2.6.8-stacks/arch/i386/kernel/entry.S 2004-10-06 22:39:43.084860592 +0400
@@ -78,7 +78,7 @@
#define preempt_stop cli
#else
#define preempt_stop
-#define resume_kernel restore_all
+#define resume_kernel restore_context
#endif

#define SAVE_ALL \
@@ -122,25 +122,6 @@
.previous


-#define RESTORE_ALL \
- RESTORE_REGS \
- addl $4, %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(lcall7)
pushfl # We get a different stack layout with call
# gates, which has to be cleaned up later..
@@ -197,7 +178,7 @@
movl EFLAGS(%esp), %eax # mix EFLAGS and CS
movb CS(%esp), %al
testl $(VM_MASK | 3), %eax
- jz resume_kernel # returning to kernel or vm86-space
+ jz resume_kernel
ENTRY(resume_userspace)
cli # make sure we don't miss an interrupt
# setting need_resched or sigpending
@@ -211,7 +192,7 @@
#ifdef CONFIG_PREEMPT
ENTRY(resume_kernel)
cmpl $0,TI_preempt_count(%ebp) # non-zero preempt_count ?
- jnz restore_all
+ jnz restore_context
need_resched:
movl TI_flags(%ebp), %ecx # need_resched set ?
testb $_TIF_NEED_RESCHED, %cl
@@ -293,8 +274,102 @@
movl TI_flags(%ebp), %ecx
testw $_TIF_ALLWORK_MASK, %cx # current->work
jne syscall_exit_work
+
restore_all:
- RESTORE_ALL
+ /* 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. */
+ movl EFLAGS(%esp), %eax
+ movb CS(%esp), %al
+ andl $(VM_MASK | 3), %eax
+ /* returning to user-space and not to v86? */
+ cmpl $3, %eax
+ jne restore_context
+ larl OLDSS(%esp), %eax
+ /* returning to "small" stack? */
+ testl $0x00400000, %eax
+ jz 8f # go fixing ESP instead of just to return :(
+restore_context:
+ RESTORE_REGS
+ addl $4, %esp
+1: iret
+.section .fixup,"ax"
+iret_exc:
+ 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,iret_exc
+.previous
+ /* preparing the ESPfix here */
+/* FRAME_SIZE_ESTIM is the size of an exception stack frame + 8 bytes
+ * for 2 pushes - need to save 2 registers on stack */
+#define FRAME_SIZE_ESTIM 24
+/* ESPFIX_STACK_RESERVE is the value that goes into SP after switch to 16bit */
+#define ESPFIX_STACK_RESERVE (FRAME_SIZE_ESTIM * 3)
+#define SP_ESTIMATE (ESPFIX_STACK_RESERVE - FRAME_SIZE_ESTIM)
+/* iret frame takes 20 bytes.
+ * We hide 2 magic pointers above it, 8 bytes each (seg:off). */
+#define ESPFIX_SWITCH16_OFFS 20
+#define ESPFIX_SWITCH32_OFFS 28
+#define ESPFIX_SWITCH16 (ESPFIX_STACK_RESERVE + ESPFIX_SWITCH16_OFFS)
+#define ESPFIX_SWITCH32 (ESPFIX_STACK_RESERVE + ESPFIX_SWITCH32_OFFS)
+8: RESTORE_REGS
+ addl $4, %esp
+ /* reserve the room for 2 magic pointers (16 bytes) and 4 bytes wasted. */
+ .rept 5
+ pushl 16(%esp)
+ .endr
+ /* Preserve some registers. That makes all the +8 offsets below. */
+ pushl %eax
+ pushl %ebp
+ /* Set up the SWITCH16 magic pointer. */
+ movl $__ESPFIX_SS, ESPFIX_SWITCH16_OFFS+8+4(%esp)
+ /* Get the "old" ESP */
+ movl 8+12(%esp), %eax
+ /* replace the lower word of "old" ESP with our magic value */
+ movw $ESPFIX_STACK_RESERVE, %ax
+ movl %eax, ESPFIX_SWITCH16_OFFS+8(%esp)
+ /* SWITCH16 ready */
+ /* Now set up the SWITCH32 magic pointer. */
+ movl $__KERNEL_DS, ESPFIX_SWITCH32_OFFS+8+4(%esp)
+ /* ESP must match the SP_ESTIMATE,
+ * which is FRAME_SIZE_ESTIM below the iret frame */
+ leal -FRAME_SIZE_ESTIM+8(%esp), %eax
+ movl %eax, ESPFIX_SWITCH32_OFFS+8(%esp)
+ /* SWITCH32 ready */
+ /* %eax will make the base of __ESPFIX_SS.
+ * It have to be ESPFIX_STACK_RESERVE bytes below the iret frame. */
+ subl $(ESPFIX_STACK_RESERVE-FRAME_SIZE_ESTIM), %eax
+ cli
+ GET_THREAD_INFO(%ebp)
+ movl TI_cpu(%ebp), %ebp
+ shll $GDT_SIZE_SHIFT, %ebp
+ /* find GDT of the proper CPU */
+ addl $cpu_gdt_table, %ebp
+ /* patch the base of the 16bit stack */
+ 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
+espfix_before_lss:
+ lss ESPFIX_SWITCH16_OFFS(%esp), %esp
+espfix_after_lss:
+ iret
+.section __ex_table,"a"
+ .align 4
+ .long espfix_after_lss,iret_exc
+.previous

# perform work that needs to be done immediately before resumption
ALIGN
@@ -493,6 +568,36 @@
pushl $do_debug
jmp error_code

+/* Check if we are on 16bit stack. Can happen either on iret of ESPFIX,
+ * or in an exception handler after that iret... */
+#define CHECK_ESPFIX_STACK(label) \
+ pushl %eax; \
+ movl %ss, %eax; \
+ cmpl $__ESPFIX_SS, %eax; \
+ jne label;
+/* using %es here because NMI can change %ss */
+#define UNWIND_ESPFIX_STACK \
+ CHECK_ESPFIX_STACK(28f) \
+ pushl %es; \
+ movl %eax, %es; \
+ lss %es:ESPFIX_SWITCH32, %esp; \
+ popl %es; \
+28: popl %eax;
+/* have to compensate the difference between real SP and estimated. */
+#define UNWIND_ESPFIX_STACK_NMI \
+ CHECK_ESPFIX_STACK(28f) \
+ movw $SP_ESTIMATE, %ax; \
+ subw %sp, %ax; \
+ movswl %ax, %eax; \
+ lss %ss:ESPFIX_SWITCH32, %esp; \
+ subl %eax, %esp; \
+ popl %eax; \
+ cmpl $espfix_after_lss, (%esp); \
+ jne nmi_stack_correct; \
+ movl $espfix_before_lss, (%esp); \
+ jmp nmi_stack_correct; \
+28: popl %eax;
+
/*
* NMI is doubly nasty. It can happen _while_ we're handling
* a debug fault, and the debug fault hasn't yet been able to
@@ -502,6 +607,7 @@
* fault happened on the sysenter path.
*/
ENTRY(nmi)
+ UNWIND_ESPFIX_STACK_NMI
cmpl $sysenter_entry,(%esp)
je nmi_stack_fixup
pushl %eax
@@ -523,7 +629,7 @@
pushl %edx
call do_nmi
addl $8, %esp
- RESTORE_ALL
+ jmp restore_all

nmi_stack_fixup:
FIX_STACK(12,nmi_stack_correct, 1)
@@ -569,14 +675,17 @@
jmp error_code

ENTRY(segment_not_present)
+ UNWIND_ESPFIX_STACK
pushl $do_segment_not_present
jmp error_code

ENTRY(stack_segment)
+ UNWIND_ESPFIX_STACK
pushl $do_stack_segment
jmp error_code

ENTRY(general_protection)
+ UNWIND_ESPFIX_STACK
pushl $do_general_protection
jmp error_code

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 2004-07-17 23:31:24.000000000 +0400
+++ linux-2.6.8-stacks/arch/i386/kernel/head.S 2004-10-06 21:58:04.873646424 +0400
@@ -514,7 +514,7 @@
.quad 0x00009a0000000000 /* 0xc0 APM CS 16 code (16 bit) */
.quad 0x0040920000000000 /* 0xc8 APM DS data */

- .quad 0x0000000000000000 /* 0xd0 - unused */
+ .quad 0x000092000000ffff /* 0xd0 - ESPfix 16-bit SS */
.quad 0x0000000000000000 /* 0xd8 - unused */
.quad 0x0000000000000000 /* 0xe0 - unused */
.quad 0x0000000000000000 /* 0xe8 - unused */
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 2004-10-06 21:58:04.000000000 +0400
@@ -38,7 +38,7 @@
* 24 - APM BIOS support
* 25 - APM BIOS support
*
- * 26 - unused
+ * 26 - ESPfix small SS
* 27 - unused
* 28 - unused
* 29 - unused
@@ -71,14 +71,19 @@
#define GDT_ENTRY_PNPBIOS_BASE (GDT_ENTRY_KERNEL_BASE + 6)
#define GDT_ENTRY_APMBIOS_BASE (GDT_ENTRY_KERNEL_BASE + 11)

+#define GDT_ENTRY_ESPFIX_SS (GDT_ENTRY_KERNEL_BASE + 14)
+#define __ESPFIX_SS (GDT_ENTRY_ESPFIX_SS * 8)
+
#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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