Re: [PATCH] mm/hugetlb: fix null ptr defer in hugetlb_vma_lock_write

Mike Kravetz
2023-11-03T02:24:59Z

On 11/02/23 20:58, Edward Adam Davis wrote:
> When obtaining resv_map from vma, it is necessary to simultaneously determine
> the flag HPAGE_RESV_OWNER of vm_private_data.
> Only when they are met simultaneously, resv_map is valid.

Thanks for looking into this!

The check for HPAGE_RESV_OWNER does 'work'. However, I believe root
cause is this block of code in __unmap_hugepage_range().

/*
* If a reference page is supplied, it is because a specific
* page is being unmapped, not a range. Ensure the page we
* are about to unmap is the actual page of interest.
*/
if (ref_page) {
if (page != ref_page) {
spin_unlock(ptl);
continue;
}
/*
* Mark the VMA as having unmapped its page so that
* future faults in this VMA will fail rather than
* looking like data was lost
*/
set_vma_resv_flags(vma, HPAGE_RESV_UNMAPPED);
}

In the specific case causing the null-ptr-deref, the resv_map pointer
(vm_private_data) is NULL. So, set_vma_resv_flags() just sets the lower bit.
Because of this, __vma_private_lock returns true.

As mentioned, the check for HPAGE_RESV_OWNER in this patch 'works' because
only the HPAGE_RESV_UNMAPPED bit is set in vm_private_data.

I was thinking a more explicit check for this 'NULL pointer' with lower
bits set could be made in __vma_private_lock. Below is something I put
together. I just open coded the check for 'NULL pointer' instead of
moving a bunch of code to the header file. In addition, I changed the
#defines from HPAGE_* to HUGETLB_* to avoid any confusion as the header
file defines are in the global name space. I thought about also adding
an explicit check for the HPAGE_RESV_OWNER as done in this patch. This
would catch the case where the pointer is not NULL. I do not believe
that is possible in the code today, but might make the check more future
proof.

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 47d25a5e1933..7b472432708e 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1260,6 +1260,15 @@ bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr);
#define flush_hugetlb_tlb_range(vma, addr, end) flush_tlb_range(vma, addr, end)
#endif

+/*
+ * Flags for MAP_PRIVATE reservations. These are stored in the bottom
+ * bits of the reservation map pointer, which are always clear due to
+ * alignment.
+ */
+#define HUGETLB_RESV_OWNER (1UL << 0)
+#define HUGETLB_RESV_UNMAPPED (1UL << 1)
+#define HUGETLB_RESV_MASK (HUGETLB_RESV_OWNER | HUGETLB_RESV_UNMAPPED)
+
static inline bool __vma_shareable_lock(struct vm_area_struct *vma)
{
return (vma->vm_flags & VM_MAYSHARE) && vma->vm_private_data;
@@ -1267,7 +1276,9 @@ static inline bool __vma_shareable_lock(struct vm_area_struct *vma)

static inline bool __vma_private_lock(struct vm_area_struct *vma)
{
- return (!(vma->vm_flags & VM_MAYSHARE)) && vma->vm_private_data;
+ /* Careful - flags may be set in lower bits of pointer */
+ return (!(vma->vm_flags & VM_MAYSHARE)) &&
+ (unsigned long)vma->vm_private_data & ~HUGETLB_RESV_MASK;
}

/*
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 1301ba7b2c9a..d2215f7647b1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1028,15 +1028,6 @@ __weak unsigned long vma_mmu_pagesize(struct vm_area_struct *vma)
return vma_kernel_pagesize(vma);
}

-/*
- * Flags for MAP_PRIVATE reservations. These are stored in the bottom
- * bits of the reservation map pointer, which are always clear due to
- * alignment.
- */
-#define HPAGE_RESV_OWNER (1UL << 0)
-#define HPAGE_RESV_UNMAPPED (1UL << 1)
-#define HPAGE_RESV_MASK (HPAGE_RESV_OWNER | HPAGE_RESV_UNMAPPED)
-
/*
* These helpers are used to track how many pages are reserved for
* faults in a MAP_PRIVATE mapping. Only the process that called mmap()
@@ -1162,7 +1153,7 @@ static struct resv_map *vma_resv_map(struct vm_area_struct *vma)

} else {
return (struct resv_map *)(get_vma_private_data(vma) &
- ~HPAGE_RESV_MASK);
+ ~HUGETLB_RESV_MASK);
}
}

@@ -1236,7 +1227,7 @@ void clear_vma_resv_huge_pages(struct vm_area_struct *vma)
*/
struct resv_map *reservations = vma_resv_map(vma);

- if (reservations && is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
+ if (reservations && is_vma_resv_set(vma, HUGETLB_RESV_OWNER)) {
resv_map_put_hugetlb_cgroup_uncharge_info(reservations);
kref_put(&reservations->refs, resv_map_release);
}
@@ -1282,7 +1273,7 @@ static bool vma_has_reserves(struct vm_area_struct *vma, long chg)
* Only the process that called mmap() has reserves for
* private mappings.
*/
- if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
+ if (is_vma_resv_set(vma, HUGETLB_RESV_OWNER)) {
/*
* Like the shared case above, a hole punch or truncate
* could have been performed on the private mapping.
@@ -2763,7 +2754,7 @@ static long __vma_reservation_common(struct hstate *h,
if (vma->vm_flags & VM_MAYSHARE || mode == VMA_DEL_RESV)
return ret;
/*
- * We know private mapping must have HPAGE_RESV_OWNER set.
+ * We know private mapping must have HUGETLB_RESV_OWNER set.
*
* In most cases, reserves always exist for private mappings.
* However, a file associated with mapping could have been
@@ -4833,7 +4824,7 @@ static void hugetlb_vm_op_open(struct vm_area_struct *vma)
struct resv_map *resv = vma_resv_map(vma);

/*
- * HPAGE_RESV_OWNER indicates a private mapping.
+ * HUGETLB_RESV_OWNER indicates a private mapping.
* This new VMA should share its siblings reservation map if present.
* The VMA will only ever have a valid reservation map pointer where
* it is being copied for another still existing VMA. As that VMA
@@ -4841,7 +4832,7 @@ static void hugetlb_vm_op_open(struct vm_area_struct *vma)
* after this open call completes. It is therefore safe to take a
* new reference here without additional locking.
*/
- if (resv && is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
+ if (resv && is_vma_resv_set(vma, HUGETLB_RESV_OWNER)) {
resv_map_dup_hugetlb_cgroup_uncharge_info(resv);
kref_get(&resv->refs);
}
@@ -4877,7 +4868,7 @@ static void hugetlb_vm_op_close(struct vm_area_struct *vma)
hugetlb_vma_lock_free(vma);

resv = vma_resv_map(vma);
- if (!resv || !is_vma_resv_set(vma, HPAGE_RESV_OWNER))
+ if (!resv || !is_vma_resv_set(vma, HUGETLB_RESV_OWNER))
return;

start = vma_hugecache_offset(h, vma, vma->vm_start);
@@ -5394,7 +5385,7 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
* future faults in this VMA will fail rather than
* looking like data was lost
*/
- set_vma_resv_flags(vma, HPAGE_RESV_UNMAPPED);
+ set_vma_resv_flags(vma, HUGETLB_RESV_UNMAPPED);
}

pte = huge_ptep_get_and_clear(mm, address, ptep);
@@ -5544,7 +5535,7 @@ static void unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
* could insert a zeroed page instead of the data existing
* from the time of fork. This would look like data corruption
*/
- if (!is_vma_resv_set(iter_vma, HPAGE_RESV_OWNER))
+ if (!is_vma_resv_set(iter_vma, HUGETLB_RESV_OWNER))
unmap_hugepage_range(iter_vma, address,
address + huge_page_size(h), page, 0);
}
@@ -5625,7 +5616,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
* at the time of fork() could consume its reserves on COW instead
* of the full address range.
*/
- if (is_vma_resv_set(vma, HPAGE_RESV_OWNER) &&
+ if (is_vma_resv_set(vma, HUGETLB_RESV_OWNER) &&
old_folio != pagecache_folio)
outside_reserve = 1;

@@ -5865,7 +5856,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
* COW/unsharing. Warn that such a situation has occurred as it may not
* be obvious.
*/
- if (is_vma_resv_set(vma, HPAGE_RESV_UNMAPPED)) {
+ if (is_vma_resv_set(vma, HUGETLB_RESV_UNMAPPED)) {
pr_warn_ratelimited("PID %d killed due to inadequate hugepage pool\n",
current->pid);
goto out;
@@ -6756,7 +6747,7 @@ bool hugetlb_reserve_pages(struct inode *inode,
chg = to - from;

set_vma_resv_map(vma, resv_map);
- set_vma_resv_flags(vma, HPAGE_RESV_OWNER);
+ set_vma_resv_flags(vma, HUGETLB_RESV_OWNER);
}

if (chg < 0)
@@ -6853,7 +6844,7 @@ bool hugetlb_reserve_pages(struct inode *inode,
*/
if (chg >= 0 && add < 0)
region_abort(resv_map, from, to, regions_needed);
- if (vma && is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
+ if (vma && is_vma_resv_set(vma, HUGETLB_RESV_OWNER)) {
kref_put(&resv_map->refs, resv_map_release);
set_vma_resv_map(vma, NULL);
}