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mm: fix race in kunmap_atomic()

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Christoph reported a nice splat which illustrated a race in the new stack
based kmap_atomic implementation.

The problem is that we pop our stack slot before we're completely done
resetting its state -- in particular clearing the PTE (sometimes that's
CONFIG_DEBUG_HIGHMEM).  If an interrupt happens before we actually clear
the PTE used for the last slot, that interrupt can reuse the slot in a
dirty state, which triggers a BUG in kmap_atomic().

Fix this by introducing kmap_atomic_idx() which reports the current slot
index without actually releasing it and use that to find the PTE and delay
the _pop() until after we're completely done.

Signed-off-by: Peter Zijlstra <[email protected]>
Reported-by: Christoph Hellwig <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Peter Zijlstra 2010-10-27 15:32:58 -07:00 • committed by Linus Torvalds
commit 20273941f2
10 changed files with 26 additions and 9 deletions
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3
arch/frv/mm/highmem.c
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@ -68,7 +68,7 @@ EXPORT_SYMBOL(__kmap_atomic);
void __kunmap_atomic(void *kvaddr)
{
int type = kmap_atomic_idx_pop();
int type = kmap_atomic_idx();
switch (type) {
case 0: __kunmap_atomic_primary(4, 6); break;
case 1: __kunmap_atomic_primary(5, 7); break;
@ -83,6 +83,7 @@ void __kunmap_atomic(void *kvaddr)
default:
BUG();
}
kmap_atomic_idx_pop();
pagefault_enable();
}
EXPORT_SYMBOL(__kunmap_atomic);