[PATCH 3.14 42/42] proc/pagemap: walk page tables under pte lock

From: Greg Kroah-Hartman
Date: Sun Apr 26 2015 - 10:06:41 EST

3.14-stable review patch. If anyone has any objections, please let me know.


From: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>

commit 05fbf357d94152171bc50f8a369390f1f16efd89 upstream.

Lockless access to pte in pagemap_pte_range() might race with page
migration and trigger BUG_ON(!PageLocked()) in migration_entry_to_page():

CPU A (pagemap) CPU B (migration)
try_to_unmap(page, TTU_MIGRATION...)
<read *pte>

Also lockless read might be non-atomic if pte is larger than wordsize.
Other pte walkers (smaps, numa_maps, clear_refs) already lock ptes.

Fixes: 052fb0d635df ("proc: report file/anon bit in /proc/pid/pagemap")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Reported-by: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>
Reviewed-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Acked-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [3.5+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

fs/proc/task_mmu.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -993,7 +993,7 @@ static int pagemap_pte_range(pmd_t *pmd,
struct vm_area_struct *vma;
struct pagemapread *pm = walk->private;
spinlock_t *ptl;
- pte_t *pte;
+ pte_t *pte, *orig_pte;
int err = 0;

/* find the first VMA at or above 'addr' */
@@ -1054,15 +1054,19 @@ static int pagemap_pte_range(pmd_t *pmd,

/* Addresses in the VMA. */
- for (; addr < min(end, vma->vm_end); addr += PAGE_SIZE) {
+ orig_pte = pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
+ for (; addr < min(end, vma->vm_end); pte++, addr += PAGE_SIZE) {
pagemap_entry_t pme;
- pte = pte_offset_map(pmd, addr);
pte_to_pagemap_entry(&pme, pm, vma, addr, *pte);
- pte_unmap(pte);
err = add_to_pagemap(addr, &pme, pm);
if (err)
- return err;
+ break;
+ pte_unmap_unlock(orig_pte, ptl);
+ if (err)
+ return err;

if (addr == end)

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