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SubjectRe: 2.5.52-mm2
Andrew Morton wrote:
>
> Andrew Morton wrote:
> >
> > ...
> > slab-poisoning.patch
> > more informative slab poisoning
>
> This patch has exposed a quite long-standing use-after-free bug in
> mremap(). It make the machine go BUG when starting the X server if
> memory debugging is turned on.
>
> The bug might be present in 2.4 as well..

here's a 2.4 patch:




move_vma() calls do_munmap() and then uses the memory at *new_vma.

But when starting X11 it just happens that the memory which do_munmap
unmapped had the same start address and the range at *new_vma. So
new_vma is freed by do_munmap().

This was never noticed before because (vm_flags & VM_LOCKED) evaluates
false when vm_flags is 0x5a5a5a5a. But I just changed that to
0x6b6b6b6b and we call make_pages_present() with start == end ==
0x6b6b6b6b and it goes BUG.

The fix is for move_vma() to not inspect the values of any vma's after
it has called do_munmap().

The patch does that, for `vma' and `new_vma'.

It also removes three redundant tests of !vma->vm_file. can_vma_merge()
already tested that.


mm/mremap.c | 31 +++++++++++++++++++++++--------
1 files changed, 23 insertions(+), 8 deletions(-)
--- 24/mm/mremap.c~move_vma-use-after-free Thu Dec 19 01:29:52 2002
+++ 24-akpm/mm/mremap.c Thu Dec 19 01:31:43 2002
@@ -134,14 +134,16 @@ static inline unsigned long move_vma(str
next = find_vma_prev(mm, new_addr, &prev);
if (next) {
if (prev && prev->vm_end == new_addr &&
- can_vma_merge(prev, vma->vm_flags) && !vma->vm_file && !(vma->vm_flags & VM_SHARED)) {
+ can_vma_merge(prev, vma->vm_flags) &&
+ !(vma->vm_flags & VM_SHARED)) {
spin_lock(&mm->page_table_lock);
prev->vm_end = new_addr + new_len;
spin_unlock(&mm->page_table_lock);
new_vma = prev;
if (next != prev->vm_next)
BUG();
- if (prev->vm_end == next->vm_start && can_vma_merge(next, prev->vm_flags)) {
+ if (prev->vm_end == next->vm_start &&
+ can_vma_merge(next, prev->vm_flags)) {
spin_lock(&mm->page_table_lock);
prev->vm_end = next->vm_end;
__vma_unlink(mm, next, prev);
@@ -151,7 +153,8 @@ static inline unsigned long move_vma(str
kmem_cache_free(vm_area_cachep, next);
}
} else if (next->vm_start == new_addr + new_len &&
- can_vma_merge(next, vma->vm_flags) && !vma->vm_file && !(vma->vm_flags & VM_SHARED)) {
+ can_vma_merge(next, vma->vm_flags) &&
+ !(vma->vm_flags & VM_SHARED)) {
spin_lock(&mm->page_table_lock);
next->vm_start = new_addr;
spin_unlock(&mm->page_table_lock);
@@ -160,7 +163,8 @@ static inline unsigned long move_vma(str
} else {
prev = find_vma(mm, new_addr-1);
if (prev && prev->vm_end == new_addr &&
- can_vma_merge(prev, vma->vm_flags) && !vma->vm_file && !(vma->vm_flags & VM_SHARED)) {
+ can_vma_merge(prev, vma->vm_flags) &&
+ !(vma->vm_flags & VM_SHARED)) {
spin_lock(&mm->page_table_lock);
prev->vm_end = new_addr + new_len;
spin_unlock(&mm->page_table_lock);
@@ -177,11 +181,15 @@ static inline unsigned long move_vma(str
}

if (!move_page_tables(current->mm, new_addr, addr, old_len)) {
+ unsigned long must_fault_in;
+ unsigned long fault_in_start;
+ unsigned long fault_in_end;
+
if (allocated_vma) {
*new_vma = *vma;
new_vma->vm_start = new_addr;
new_vma->vm_end = new_addr+new_len;
- new_vma->vm_pgoff += (addr - vma->vm_start) >> PAGE_SHIFT;
+ new_vma->vm_pgoff += (addr-vma->vm_start) >> PAGE_SHIFT;
new_vma->vm_raend = 0;
if (new_vma->vm_file)
get_file(new_vma->vm_file);
@@ -189,12 +197,19 @@ static inline unsigned long move_vma(str
new_vma->vm_ops->open(new_vma);
insert_vm_struct(current->mm, new_vma);
}
+
+ must_fault_in = new_vma->vm_flags & VM_LOCKED;
+ fault_in_start = new_vma->vm_start;
+ fault_in_end = new_vma->vm_end;
+
do_munmap(current->mm, addr, old_len);
+
+ /* new_vma could have been invalidated by do_munmap */
+
current->mm->total_vm += new_len >> PAGE_SHIFT;
- if (new_vma->vm_flags & VM_LOCKED) {
+ if (must_fault_in) {
current->mm->locked_vm += new_len >> PAGE_SHIFT;
- make_pages_present(new_vma->vm_start,
- new_vma->vm_end);
+ make_pages_present(fault_in_start, fault_in_end);
}
return new_addr;
}
_
-
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