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=============================================================================
FreeBSD-SA-97:06 Security Advisory
FreeBSD, Inc.
Topic: Pentium processors have flaw allowing unpriviledged crashes
Category: core
Module: kern
Announced: 1997-12-09
Affects: FreeBSD 2.1.*, FreeBSD 2.2.*,
FreeBSD-stable and FreeBSD-current
Corrected: FreeBSD-current as of 1997-12-04
FreeBSD-stable as of 1997-12-04
FreeBSD only: no
Patches: ftp://freebsd.org/pub/CERT/patches/SA-97:06/
=============================================================================
I. Background
Intel processors have instruction combiniations that, when
executed, produce illegal instruction traps. This is a normal
part of every cpu manufactured and is how new instructions are
generally emulated on older hardware.
II. Problem Description
A specific sequence of instructions, starting with the byte codes
F0 0F (hex) cause Pentium processors to lock up. This lockup
wedges the entire system, requiring a hard reset to correct.
Systems that allow users to run arbitrary code are vulnerable to
this attack.
III. Impact
An unpriviledged user can crash your system.
IV. Workaround
None is available.
V. Solution
The following patch corrects the problem for FreeBSD-current
systems before 1997-12-04, for FreeBSD 2.2-stable before
1997-12-04 and for FreeBSD 2.2.5.
We urge users of FreeBSD 2.1.* to upgrade to the more stable and
more powerfull FreeBSD 2.2.5 release.
Index: identcpu.c
===================================================================
RCS file: /home/cvsup/freebsd/CVS/src/sys/i386/i386/identcpu.c,v
retrieving revision 1.33
retrieving revision 1.35
diff -u -r1.33 -r1.35
--- identcpu.c 1997/11/07 08:52:27 1.33
+++ identcpu.c 1997/12/04 14:35:38 1.35
@@ -107,6 +107,10 @@
);
}
+#if defined(I586_CPU) && !defined(NO_F00F_HACK)
+int has_f00f_bug = 0;
+#endif
+
void
printcpuinfo(void)
{
@@ -136,6 +140,14 @@
break;
case 0x500:
strcat(cpu_model, "Pentium"); /* nb no space */
+#if defined(I586_CPU) && !defined(NO_F00F_HACK)
+ /*
+ * XXX - If/when Intel fixes the bug, this
+ * should also check the version of the
+ * CPU, not just that it's a Pentium.
+ */
+ has_f00f_bug = 1;
+#endif
break;
case 0x600:
strcat(cpu_model, "Pentium Pro");
Index: machdep.c
===================================================================
RCS file: /home/cvsup/freebsd/CVS/src/sys/i386/i386/machdep.c,v
retrieving revision 1.274
retrieving revision 1.278
diff -u -r1.274 -r1.278
--- machdep.c 1997/11/24 18:35:11 1.274
+++ machdep.c 1997/12/04 21:21:24 1.278
@@ -866,6 +867,11 @@
#endif /* VM86 */
#endif
+#if defined(I586_CPU) && !defined(NO_F00F_HACK)
+struct gate_descriptor *t_idt;
+extern int has_f00f_bug;
+#endif
+
static struct i386tss dblfault_tss;
static char dblfault_stack[PAGE_SIZE];
@@ -1533,6 +1539,40 @@
proc0.p_addr->u_pcb.pcb_mpnest = 1;
proc0.p_addr->u_pcb.pcb_ext = 0;
}
+
+#if defined(I586_CPU) && !defined(NO_F00F_HACK)
+void f00f_hack(void);
+SYSINIT(f00f_hack, SI_SUB_INTRINSIC, SI_ORDER_FIRST, f00f_hack, NULL);
+
+void
+f00f_hack(void) {
+ struct region_descriptor r_idt;
+ unsigned char *tmp;
+ int i;
+
+ if (!has_f00f_bug)
+ return;
+
+ printf("Intel Pentium F00F detected, installing workaround\n");
+
+ r_idt.rd_limit = sizeof(idt) - 1;
+
+ tmp = kmem_alloc(kernel_map, PAGE_SIZE * 2);
+ if (tmp == 0)
+ panic("kmem_alloc returned 0");
+ if (((unsigned int)tmp & (PAGE_SIZE-1)) != 0)
+ panic("kmem_alloc returned non-page-aligned memory");
+ /* Put the first seven entries in the lower page */
+ t_idt = (struct gate_descriptor*)(tmp + PAGE_SIZE - (7*8));
+ bcopy(idt, t_idt, sizeof(idt));
+ r_idt.rd_base = (int)t_idt;
+ lidt(&r_idt);
+ if (vm_map_protect(kernel_map, tmp, tmp + PAGE_SIZE,
+ VM_PROT_READ, FALSE) != KERN_SUCCESS)
+ panic("vm_map_protect failed");
+ return;
+}
+#endif /* defined(I586_CPU) && !NO_F00F_HACK */
int
ptrace_set_pc(p, addr)
Index: trap.c
===================================================================
RCS file: /home/cvsup/freebsd/CVS/src/sys/i386/i386/trap.c,v
retrieving revision 1.115
retrieving revision 1.118
diff -u -r1.115 -r1.118
--- trap.c 1997/11/24 13:25:37 1.115
+++ trap.c 1997/12/04 21:21:26 1.118
@@ -142,6 +143,11 @@
static void userret __P((struct proc *p, struct trapframe *frame,
u_quad_t oticks));
+#if defined(I586_CPU) && !defined(NO_F00F_HACK)
+extern struct gate_descriptor *t_idt;
+extern int has_f00f_bug;
+#endif
+
static inline void
userret(p, frame, oticks)
struct proc *p;
@@ -211,6 +217,9 @@
u_long eva;
#endif
+#if defined(I586_CPU) && !defined(NO_F00F_HACK)
+restart:
+#endif
type = frame.tf_trapno;
code = frame.tf_err;
@@ -276,6 +285,10 @@
i = trap_pfault(&frame, TRUE);
if (i == -1)
return;
+#if defined(I586_CPU) && !defined(NO_F00F_HACK)
+ if (i == -2)
+ goto restart;
+#endif
if (i == 0)
goto out;
@@ -642,7 +655,18 @@
if (va >= KERNBASE) {
/*
* Don't allow user-mode faults in kernel address space.
+ * An exception: if the faulting address is the invalid
+ * instruction entry in the IDT, then the Intel Pentium
+ * F00F bug workaround was triggered, and we need to
+ * treat it is as an illegal instruction, and not a page
+ * fault.
*/
+#if defined(I586_CPU) && !defined(NO_F00F_HACK)
+ if ((eva == (unsigned int)&t_idt[6]) && has_f00f_bug) {
+ frame->tf_trapno = T_PRIVINFLT;
+ return -2;
+ }
+#endif
if (usermode)
goto nogo;
=============================================================================
FreeBSD, Inc.
Web Site: http://www.freebsd.org/
Confidential contacts: security-officerat_private
PGP Key: ftp://freebsd.org/pub/CERT/public_key.asc
Security notifications: security-notificationsat_private
Security public discussion: securityat_private
Notice: Any patches in this document may not apply cleanly due to
modifications caused by digital signature or mailer software.
Please reference the URL listed at the top of this document
for original copies of all patches if necessary.
=============================================================================
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