Re: [PATCH] powerpc: Fix random segfault when freeing hugetlb range

From: Aneesh Kumar K.V
Date: Wed Sep 02 2020 - 04:16:22 EST


On 9/2/20 1:41 PM, Christophe Leroy wrote:


Le 02/09/2020 à 05:23, Aneesh Kumar K.V a écrit :
Christophe Leroy <christophe.leroy@xxxxxxxxxx> writes:

The following random segfault is observed from time to time with
map_hugetlb selftest:

root@localhost:~# ./map_hugetlb 1 19
524288 kB hugepages
Mapping 1 Mbytes
Segmentation fault

[   31.219972] map_hugetlb[365]: segfault (11) at 117 nip 77974f8c lr 779a6834 code 1 in ld-2.23.so[77966000+21000]
[   31.220192] map_hugetlb[365]: code: 9421ffc0 480318d1 93410028 90010044 9361002c 93810030 93a10034 93c10038
[   31.220307] map_hugetlb[365]: code: 93e1003c 93210024 8123007c 81430038 <80e90004> 814a0004 7f443a14 813a0004
[   31.221911] BUG: Bad rss-counter state mm:(ptrval) type:MM_FILEPAGES val:33
[   31.229362] BUG: Bad rss-counter state mm:(ptrval) type:MM_ANONPAGES val:5

This fault is due to hugetlb_free_pgd_range() freeing page tables
that are also used by regular pages.

As explain in the comment at the beginning of
hugetlb_free_pgd_range(), the verification done in free_pgd_range()
on floor and ceiling is not done here, which means
hugetlb_free_pte_range() can free outside the expected range.

As the verification cannot be done in hugetlb_free_pgd_range(), it
must be done in hugetlb_free_pte_range().


Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>

Fixes: b250c8c08c79 ("powerpc/8xx: Manage 512k huge pages as standard pages.")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---
  arch/powerpc/mm/hugetlbpage.c | 18 ++++++++++++++++--
  1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
index 26292544630f..e7ae2a2c4545 100644
--- a/arch/powerpc/mm/hugetlbpage.c
+++ b/arch/powerpc/mm/hugetlbpage.c
@@ -330,10 +330,24 @@ static void free_hugepd_range(struct mmu_gather *tlb, hugepd_t *hpdp, int pdshif
                   get_hugepd_cache_index(pdshift - shift));
  }
-static void hugetlb_free_pte_range(struct mmu_gather *tlb, pmd_t *pmd, unsigned long addr)
+static void hugetlb_free_pte_range(struct mmu_gather *tlb, pmd_t *pmd,
+                   unsigned long addr, unsigned long end,
+                   unsigned long floor, unsigned long ceiling)
  {
+    unsigned long start = addr;
      pgtable_t token = pmd_pgtable(*pmd);
+    start &= PMD_MASK;
+    if (start < floor)
+        return;
+    if (ceiling) {
+        ceiling &= PMD_MASK;
+        if (!ceiling)
+            return;
+    }
+    if (end - 1 > ceiling - 1)
+        return;
+

We do repeat that for pud/pmd/pte hugetlb_free_range. Can we consolidate
that with comment explaining we are checking if the pgtable entry is
mapping outside the range?

I was thinking about refactoring that into a helper and add all the necessary comments to explain what it does.

Will do that in a followup series if you are OK. This patch is a bug fix and also have to go through stable.


agreed.

Thanks.
-aneesh