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Subject[patch] ldt-fix-2.5.32-A3

this is an updated version of the LDT fixes. It fixes the following kinds
of problems:

- fix a possible gcc optimization causing a race causing the loading of a
corrupt LDT descriptor upon context switch. [this fix got simplified
over previous versions.]

- remove an unconditional OOM printk, and there's no need to set ->size
in the OOM path.

- fix preemption bugs, load_LDT()/clear_LDT() was not preemption-safe,
when it was used outside of spinlocks.

the context-switch race is the following. 'LDT modification' is the
following operation: the seg->ldt pointer is modified, then seg->size is
modified. In theory gcc is free to reschedule the two modifications, and
first modify ->size, then ->ldt. Thus if this modification is not
synchronized with context-switches, another thread might see a temporary
state of the new ->size [which was increased], but still the old pointer.
Ie.:

CPU0 CPU1

pc->size = newsize;
load_LDT(); // (oldptr, newsize)
pc->ldt = newptr;

the corrupt LDT is loaded until the SMP cross-call is sent, leaving the
window open for many usecs.

the fix is to put a wmb() after ->ldt modifications. [this is also in
preparation of not-write-ordered SMP x86 designs.]

i have tested the patch on 2.5.32-BK, SMP and UP as well.

Ingo

--- linux/arch/i386/kernel/ldt.c.orig Fri Aug 30 08:57:38 2002
+++ linux/arch/i386/kernel/ldt.c Fri Aug 30 15:30:34 2002
@@ -49,17 +49,20 @@
memcpy(newldt, pc->ldt, oldsize*LDT_ENTRY_SIZE);
oldldt = pc->ldt;
memset(newldt+oldsize*LDT_ENTRY_SIZE, 0, (mincount-oldsize)*LDT_ENTRY_SIZE);
- wmb();
pc->ldt = newldt;
+ wmb();
pc->size = mincount;
+ wmb();
+
if (reload) {
load_LDT(pc);
#ifdef CONFIG_SMP
- if (current->mm->cpu_vm_mask != (1<<smp_processor_id()))
+ preempt_disable();
+ if (current->mm->cpu_vm_mask != (1 << smp_processor_id()))
smp_call_function(flush_ldt, 0, 1, 1);
+ preempt_enable();
#endif
}
- wmb();
if (oldsize) {
if (oldsize*LDT_ENTRY_SIZE > PAGE_SIZE)
vfree(oldldt);
@@ -72,11 +75,8 @@
static inline int copy_ldt(mm_context_t *new, mm_context_t *old)
{
int err = alloc_ldt(new, old->size, 0);
- if (err < 0) {
- printk(KERN_WARNING "ldt allocation failed\n");
- new->size = 0;
+ if (err < 0)
return err;
- }
memcpy(new->ldt, old->ldt, old->size*LDT_ENTRY_SIZE);
return 0;
}
--- linux/include/asm-i386/mmu.h.orig Sun Jun 9 07:30:36 2002
+++ linux/include/asm-i386/mmu.h Fri Aug 30 15:29:57 2002
@@ -10,7 +10,7 @@
typedef struct {
int size;
struct semaphore sem;
- void * ldt;
+ void *ldt;
} mm_context_t;

#endif
--- linux/include/asm-i386/desc.h.orig Fri Aug 30 08:57:40 2002
+++ linux/include/asm-i386/desc.h Fri Aug 30 15:28:27 2002
@@ -86,14 +86,17 @@

static inline void clear_LDT(void)
{
- set_ldt_desc(smp_processor_id(), &default_ldt[0], 5);
+ int cpu = get_cpu();
+
+ set_ldt_desc(cpu, &default_ldt[0], 5);
load_LDT_desc();
+ put_cpu();
}

/*
* load one particular LDT into the current CPU
*/
-static inline void load_LDT (mm_context_t *pc)
+static inline void load_LDT_nolock(mm_context_t *pc, int cpu)
{
void *segments = pc->ldt;
int count = pc->size;
@@ -103,8 +106,15 @@
count = 5;
}

- set_ldt_desc(smp_processor_id(), segments, count);
+ set_ldt_desc(cpu, segments, count);
load_LDT_desc();
+}
+
+static inline void load_LDT(mm_context_t *pc)
+{
+ int cpu = get_cpu();
+ load_LDT_nolock(pc, cpu);
+ put_cpu();
}

#endif /* !__ASSEMBLY__ */
--- linux/include/asm-i386/mmu_context.h.orig Fri Aug 30 08:57:31 2002
+++ linux/include/asm-i386/mmu_context.h Fri Aug 30 15:32:20 2002
@@ -44,7 +44,7 @@
* load the LDT, if the LDT is different:
*/
if (unlikely(prev->context.ldt != next->context.ldt))
- load_LDT(&next->context);
+ load_LDT_nolock(&next->context, cpu);
}
#ifdef CONFIG_SMP
else {
@@ -56,7 +56,7 @@
* tlb flush IPI delivery. We must reload %cr3.
*/
load_cr3(next->pgd);
- load_LDT(&next->context);
+ load_LDT_nolock(&next->context, cpu);
}
}
#endif
-
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