Re: ESP corruption bug - what CPUs are affected?

[Stas Sergeev <stsp@aknet.ru>]

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Hi.

I've been pointed out that my prev ring-0
patch was not completely safe. I was
doing "popl %es". I thought it is save
because RESTORE_REGS also does that
with the fixup in place, but the anonymous
guy thinks that if %es refers to LDT and
the thread on another CPU changes that LDT
entry in a mean time, my "popl %es" can GPF.
So I have to avoid popping any segregs.
I moved my recovery code to error_code,
right after %es is used last time and before
the %esp is used directly (I am lucky such
a place exist there!).
New patch is attached. Does it look safe
this time?



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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	2004-10-09 20:55:36.772613096 +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 +274,100 @@
 	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 + 40 bytes
+ * for 10 pushes */
+#define FRAME_SIZE_ESTIM (16 + 10 * 4)
+/* 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)
+	/* 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 bytes below the iret frame */
+	leal -FRAME_SIZE_ESTIM+8(%esp), %eax
+	movl %eax, ESPFIX_SWITCH32_OFFS+8(%esp)
+	/* %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
@@ -396,6 +469,34 @@
 	call do_IRQ
 	jmp ret_from_intr
 
+/* 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; \
+	cmpw $__ESPFIX_SS, %ax; \
+	jne label;
+/* using %es here because NMI can change %ss */
+#define UNWIND_ESPFIX_STACK \
+	CHECK_ESPFIX_STACK(28f) \
+	movl %eax, %es; \
+	lss %es:ESPFIX_SWITCH32, %esp; \
+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;
+
 #define BUILD_INTERRUPT(name, nr)	\
 ENTRY(name)				\
 	pushl $nr-256;			\
@@ -423,6 +524,7 @@
 	pushl %ebx
 	cld
 	movl %es, %ecx
+	UNWIND_ESPFIX_STACK		# this corrupts %es
 	movl ORIG_EAX(%esp), %esi	# get the error code
 	movl ES(%esp), %edi		# get the function address
 	movl %eax, ORIG_EAX(%esp)
@@ -502,6 +604,7 @@
  * fault happened on the sysenter path.
  */
 ENTRY(nmi)
+	UNWIND_ESPFIX_STACK_NMI
 	cmpl $sysenter_entry,(%esp)
 	je nmi_stack_fixup
 	pushl %eax
@@ -523,7 +626,7 @@
 	pushl %edx
 	call do_nmi
 	addl $8, %esp
-	RESTORE_ALL
+	jmp restore_all
 
 nmi_stack_fixup:
 	FIX_STACK(12,nmi_stack_correct, 1)
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 21:58:04.000000000 +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 */