Re: [mm] 9343f6818b: BUG:kernel_NULL_pointer_dereference,address

Steven Price
2019-10-31T17:22:54Z

On Thu, Oct 31, 2019 at 03:15:10PM +0000, kernel test robot wrote:
> FYI, we noticed the following commit (built with gcc-7):
>
> commit: 9343f6818bb98cf0c982bfff6ed89b2c7176bcf9 ("[PATCH v14 12/22] mm: pagewalk: Allow walking without vma")
> url: https://github.com/0day-ci/linux/commits/Steven-Price/Generic-page-walk-and-ptdump/20191030-085205
>
[...]
>
> [ 36.010874] BUG: kernel NULL pointer dereference, address: 0000000000000053
> [ 36.012644] #PF: supervisor read access in kernel mode
> [ 36.014074] #PF: error_code(0x0000) - not-present page
> [ 36.015481] PGD 0 P4D 0
> [ 36.016433] Oops: 0000 [#1] SMP PTI
> [ 36.017561] CPU: 1 PID: 2376 Comm: mmap12 Not tainted 5.4.0-rc5-00046-g9343f6818bb98 #1
> [ 36.019340] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
> [ 36.021250] RIP: 0010:pagemap_pmd_range+0x5ae/0x7b0

So it looks like this has broken /proc/<pid>/pagemap because we can now
call the callbacks with a NULL vma if the region passed into
walk_page_range is (partially) outside the VMA range.

Somehow, in this situation, there is a region which has a PMD entry but
no corresponding VMA. So the pmd_entry callback is called but with
walk->vma==NULL.

The options for fixing this seem to be:
a) Make the pagemap callback robust against a PMD entry without a VMA.
For example treating it as a hole (as it would have been before this
patch):

---8<---
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 9442631fd4af..b6d819c4bbb2 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1369,6 +1369,9 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
pte_t *pte, *orig_pte;
int err = 0;

+ if (!vma)
+ return pagemap_pte_hole(addr, end, walk);
+
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
ptl = pmd_trans_huge_lock(pmdp, vma);
if (ptl) {
---8<---

b) Provide a flag (or another function) for walk_page_range() which
restores the previous behaviour. Only those users that want to walk
ranges without VMAs would then need to deal with NULL-vma returns.

---8<---
diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index 12004b097eae..519258e8fffa 100644
--- a/include/linux/pagewalk.h
+++ b/include/linux/pagewalk.h
@@ -61,6 +61,7 @@ struct mm_walk {
const struct mm_walk_ops *ops;
struct mm_struct *mm;
struct vm_area_struct *vma;
+ bool ignore_vma;
void *private;
};

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index 4139e9163aee..f2fccbc3cba8 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -38,7 +38,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
do {
again:
next = pmd_addr_end(addr, end);
- if (pmd_none(*pmd)) {
+ if (pmd_none(*pmd) || (!walk->vma && walk->ignore_vma)) {
if (ops->pte_hole)
err = ops->pte_hole(addr, next, walk);
if (err)
@@ -89,7 +89,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
do {
again:
next = pud_addr_end(addr, end);
- if (pud_none(*pud)) {
+ if (pud_none(*pud) || (!walk->vma && !walk->ignore_vma)) {
if (ops->pte_hole)
err = ops->pte_hole(addr, next, walk);
if (err)
---8<---

I'm currently inclined towards the latter because I don't want to have
to try to audit all existing users in case there's anything similar
lurking in another user of walk_page_range().

Steve