On 12/27/18 3:44 AM, Colin Ian King wrote:
> Hi,
>
> Static analysis with CoverityScan on linux-next detected a potential
> null pointer dereference with the following commit:
>
> From d8a1051ed4ba55679ef24e838a1942c9c40f0a14 Mon Sep 17 00:00:00 2001
> From: Mike Kravetz <[email protected]>
> Date: Sat, 22 Dec 2018 10:55:57 +1100
> Subject: [PATCH] hugetlbfs: use i_mmap_rwsem for more pmd sharing
>
> The earlier check implies that "mapping" may be a null pointer:
>
> var_compare_op: Comparing mapping to null implies that mapping might be
> null.
>
> 1008 if (!(flags & MF_MUST_KILL) && !PageDirty(hpage) && mapping &&
> 1009 mapping_cap_writeback_dirty(mapping)) {
>
> ..however later "mapper" is dereferenced when it may be potentially null:
>
> 1034 /*
> 1035 * For hugetlb pages, try_to_unmap could potentially
> call
> 1036 * huge_pmd_unshare. Because of this, take semaphore in
> 1037 * write mode here and set TTU_RMAP_LOCKED to
> indicate we
> 1038 * have taken the lock at this higer level.
> 1039 */
> CID 1476097 (#1 of 1): Dereference after null check (FORWARD_NULL)
>
> var_deref_model: Passing null pointer mapping to
> i_mmap_lock_write, which dereferences it.
>
> 1040 i_mmap_lock_write(mapping);
> 1041 unmap_success = try_to_unmap(hpage,
> ttu|TTU_RMAP_LOCKED);
> 1042 i_mmap_unlock_write(mapping);
>
Thanks for the report.
The 'good news' is that mapping can not be null in the code path above.
The reasons are:
- The page is locked upon entry to the routine
- Earlier in the routine there is the check:
if (!page_mapped(hpage))
return true;
For huge pages (which are processed in the else clause above), page_mapped
implies page->mapping != null.
However, the routine hwpoison_user_mappings handles all page types. The
page_mapped check is actually there to check for pages in the swap cache.
It is just coincidence that it also implies mapping != null for huge pages.
It would be better to make an explicit check for mapping != null before
calling i_mmap_lock_write/try_to_unmap. In this way, unrelated changes to
code above will not potentially lead to the possibility of mapping == null.
I'm not sure what is the best way to handle this. Below is an updated version
of the patch sent to Andrew. I can also provide a simple patch to the patch
if that is easier.
From: Mike Kravetz <[email protected]>
hugetlbfs: use i_mmap_rwsem for more pmd sharing synchronization
While looking at BUGs associated with invalid huge page map counts,
it was discovered and observed that a huge pte pointer could become
'invalid' and point to another task's page table. Consider the
following:
A task takes a page fault on a shared hugetlbfs file and calls
huge_pte_alloc to get a ptep. Suppose the returned ptep points to a
shared pmd.
Now, another task truncates the hugetlbfs file. As part of truncation,
it unmaps everyone who has the file mapped. If the range being
truncated is covered by a shared pmd, huge_pmd_unshare will be called.
For all but the last user of the shared pmd, huge_pmd_unshare will
clear the pud pointing to the pmd. If the task in the middle of the
page fault is not the last user, the ptep returned by huge_pte_alloc
now points to another task's page table or worse. This leads to bad
things such as incorrect page map/reference counts or invalid memory
references.
To fix, expand the use of i_mmap_rwsem as follows:
- i_mmap_rwsem is held in read mode whenever huge_pmd_share is called.
huge_pmd_share is only called via huge_pte_alloc, so callers of
huge_pte_alloc take i_mmap_rwsem before calling. In addition, callers
of huge_pte_alloc continue to hold the semaphore until finished with
the ptep.
- i_mmap_rwsem is held in write mode whenever huge_pmd_unshare is called.
Cc: <[email protected]>
Fixes: 39dde65c9940 ("shared page table for hugetlb page")
Signed-off-by: Mike Kravetz <[email protected]>
---
mm/hugetlb.c | 67 ++++++++++++++++++++++++++++++++++-----------
mm/memory-failure.c | 16 +++++++++--
mm/migrate.c | 13 ++++++++-
mm/rmap.c | 4 +++
mm/userfaultfd.c | 11 ++++++--
5 files changed, 90 insertions(+), 21 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 309fb8c969af..2a3162030167 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3239,6 +3239,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct
mm_struct *src,
int cow;
struct hstate *h = hstate_vma(vma);
unsigned long sz = huge_page_size(h);
+ struct address_space *mapping = vma->vm_file->f_mapping;
unsigned long mmun_start; /* For mmu_notifiers */
unsigned long mmun_end; /* For mmu_notifiers */
int ret = 0;
@@ -3247,14 +3248,25 @@ int copy_hugetlb_page_range(struct mm_struct *dst,
struct mm_struct *src,
mmun_start = vma->vm_start;
mmun_end = vma->vm_end;
- if (cow)
+ if (cow) {
mmu_notifier_invalidate_range_start(src, mmun_start, mmun_end);
+ } else {
+ /*
+ * For shared mappings i_mmap_rwsem must be held to call
+ * huge_pte_alloc, otherwise the returned ptep could go
+ * away if part of a shared pmd and another thread calls
+ * huge_pmd_unshare.
+ */
+ i_mmap_lock_read(mapping);
+ }
for (addr = vma->vm_start; addr < vma->vm_end; addr += sz) {
spinlock_t *src_ptl, *dst_ptl;
+
src_pte = huge_pte_offset(src, addr, sz);
if (!src_pte)
continue;
+
dst_pte = huge_pte_alloc(dst, addr, sz);
if (!dst_pte) {
ret = -ENOMEM;
@@ -3325,6 +3337,8 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct
mm_struct *src,
if (cow)
mmu_notifier_invalidate_range_end(src, mmun_start, mmun_end);
+ else
+ i_mmap_unlock_read(mapping);
return ret;
}
@@ -3772,14 +3786,18 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
};
/*
- * hugetlb_fault_mutex must be dropped before
- * handling userfault. Reacquire after handling
- * fault to make calling code simpler.
+ * hugetlb_fault_mutex and i_mmap_rwsem must be
+ * dropped before handling userfault. Reacquire
+ * after handling fault to make calling code simpler.
*/
hash = hugetlb_fault_mutex_hash(h, mm, vma, mapping,
idx, haddr);
mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+ i_mmap_unlock_read(mapping);
+
ret = handle_userfault(&vmf, VM_UFFD_MISSING);
+
+ i_mmap_lock_read(mapping);
mutex_lock(&hugetlb_fault_mutex_table[hash]);
goto out;
}
@@ -3927,6 +3945,11 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct
vm_area_struct *vma,
ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
if (ptep) {
+ /*
+ * Since we hold no locks, ptep could be stale. That is
+ * OK as we are only making decisions based on content and
+ * not actually modifying content here.
+ */
entry = huge_ptep_get(ptep);
if (unlikely(is_hugetlb_entry_migration(entry))) {
migration_entry_wait_huge(vma, mm, ptep);
@@ -3934,20 +3957,31 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct
vm_area_struct *vma,
} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
return VM_FAULT_HWPOISON_LARGE |
VM_FAULT_SET_HINDEX(hstate_index(h));
- } else {
- ptep = huge_pte_alloc(mm, haddr, huge_page_size(h));
- if (!ptep)
- return VM_FAULT_OOM;
}
+ /*
+ * Acquire i_mmap_rwsem before calling huge_pte_alloc and hold
+ * until finished with ptep. This prevents huge_pmd_unshare from
+ * being called elsewhere and making the ptep no longer valid.
+ *
+ * ptep could have already be assigned via huge_pte_offset. That
+ * is OK, as huge_pte_alloc will return the same value unless
+ * something changed.
+ */
mapping = vma->vm_file->f_mapping;
- idx = vma_hugecache_offset(h, vma, haddr);
+ i_mmap_lock_read(mapping);
+ ptep = huge_pte_alloc(mm, haddr, huge_page_size(h));
+ if (!ptep) {
+ i_mmap_unlock_read(mapping);
+ return VM_FAULT_OOM;
+ }
/*
* Serialize hugepage allocation and instantiation, so that we don't
* get spurious allocation failures if two CPUs race to instantiate
* the same page in the page cache.
*/
+ idx = vma_hugecache_offset(h, vma, haddr);
hash = hugetlb_fault_mutex_hash(h, mm, vma, mapping, idx, haddr);
mutex_lock(&hugetlb_fault_mutex_table[hash]);
@@ -4035,6 +4069,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct
vm_area_struct *vma,
}
out_mutex:
mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+ i_mmap_unlock_read(mapping);
/*
* Generally it's safe to hold refcount during waiting page lock. But
* here we just wait to defer the next page fault to avoid busy loop and
@@ -4639,10 +4674,12 @@ void adjust_range_if_pmd_sharing_possible(struct
vm_area_struct *vma,
* Search for a shareable pmd page for hugetlb. In any case calls pmd_alloc()
* and returns the corresponding pte. While this is not necessary for the
* !shared pmd case because we can allocate the pmd later as well, it makes the
- * code much cleaner. pmd allocation is essential for the shared case because
- * pud has to be populated inside the same i_mmap_rwsem section - otherwise
- * racing tasks could either miss the sharing (see huge_pte_offset) or select a
- * bad pmd for sharing.
+ * code much cleaner.
+ *
+ * This routine must be called with i_mmap_rwsem held in at least read mode.
+ * For hugetlbfs, this prevents removal of any page table entries associated
+ * with the address space. This is important as we are setting up sharing
+ * based on existing page table entries (mappings).
*/
pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
{
@@ -4659,7 +4696,6 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long
addr, pud_t *pud)
if (!vma_shareable(vma, addr))
return (pte_t *)pmd_alloc(mm, pud, addr);
- i_mmap_lock_write(mapping);
vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
if (svma == vma)
continue;
@@ -4689,7 +4725,6 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long
addr, pud_t *pud)
spin_unlock(ptl);
out:
pte = (pte_t *)pmd_alloc(mm, pud, addr);
- i_mmap_unlock_write(mapping);
return pte;
}
@@ -4700,7 +4735,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long
addr, pud_t *pud)
* indicated by page_count > 1, unmap is achieved by clearing pud and
* decrementing the ref count. If count == 1, the pte page is not shared.
*
- * called with page table lock held.
+ * Called with page table lock held and i_mmap_rwsem held in write mode.
*
* returns: 1 successfully unmapped a shared pte page
* 0 the underlying pte page is not shared, or it is the last user
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 0cd3de3550f0..93558fb981fb 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -966,7 +966,7 @@ static bool hwpoison_user_mappings(struct page *p, unsigned
long pfn,
enum ttu_flags ttu = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS;
struct address_space *mapping;
LIST_HEAD(tokill);
- bool unmap_success;
+ bool unmap_success = true;
int kill = 1, forcekill;
struct page *hpage = *hpagep;
bool mlocked = PageMlocked(hpage);
@@ -1028,7 +1028,19 @@ static bool hwpoison_user_mappings(struct page *p,
unsigned long pfn,
if (kill)
collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);
- unmap_success = try_to_unmap(hpage, ttu);
+ if (!PageHuge(hpage)) {
+ unmap_success = try_to_unmap(hpage, ttu);
+ } else if (mapping) {
+ /*
+ * For hugetlb pages, try_to_unmap could potentially call
+ * huge_pmd_unshare. Because of this, take semaphore in
+ * write mode here and set TTU_RMAP_LOCKED to indicate we
+ * have taken the lock at this higer level.
+ */
+ i_mmap_lock_write(mapping);
+ unmap_success = try_to_unmap(hpage, ttu|TTU_RMAP_LOCKED);
+ i_mmap_unlock_write(mapping);
+ }
if (!unmap_success)
pr_err("Memory failure: %#lx: failed to unmap page
(mapcount=%d)\n",
pfn, page_mapcount(hpage));
diff --git a/mm/migrate.c b/mm/migrate.c
index 84381b55b2bd..725edaef238a 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1307,8 +1307,19 @@ static int unmap_and_move_huge_page(new_page_t
get_new_page,
goto put_anon;
if (page_mapped(hpage)) {
+ struct address_space *mapping = page_mapping(hpage);
+
+ /*
+ * try_to_unmap could potentially call huge_pmd_unshare.
+ * Because of this, take semaphore in write mode here and
+ * set TTU_RMAP_LOCKED to let lower levels know we have
+ * taken the lock.
+ */
+ i_mmap_lock_write(mapping);
try_to_unmap(hpage,
- TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
+ TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS|
+ TTU_RMAP_LOCKED);
+ i_mmap_unlock_write(mapping);
page_was_mapped = 1;
}
diff --git a/mm/rmap.c b/mm/rmap.c
index 85b7f9423352..c566bd552535 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -25,6 +25,7 @@
* page->flags PG_locked (lock_page)
* hugetlbfs_i_mmap_rwsem_key (in huge_pmd_share)
* mapping->i_mmap_rwsem
+ * hugetlb_fault_mutex (hugetlbfs specific page fault mutex)
* anon_vma->rwsem
* mm->page_table_lock or pte_lock
* zone_lru_lock (in mark_page_accessed, isolate_lru_page)
@@ -1374,6 +1375,9 @@ static bool try_to_unmap_one(struct page *page, struct
vm_area_struct *vma,
/*
* If sharing is possible, start and end will be adjusted
* accordingly.
+ *
+ * If called for a huge page, caller must hold i_mmap_rwsem
+ * in write mode as it is possible to call huge_pmd_unshare.
*/
adjust_range_if_pmd_sharing_possible(vma, &start, &end);
}
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 458acda96f20..48368589f519 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -267,10 +267,14 @@ static __always_inline ssize_t
__mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
VM_BUG_ON(dst_addr & ~huge_page_mask(h));
/*
- * Serialize via hugetlb_fault_mutex
+ * Serialize via i_mmap_rwsem and hugetlb_fault_mutex.
+ * i_mmap_rwsem ensures the dst_pte remains valid even
+ * in the case of shared pmds. fault mutex prevents
+ * races with other faulting threads.
*/
- idx = linear_page_index(dst_vma, dst_addr);
mapping = dst_vma->vm_file->f_mapping;
+ i_mmap_lock_read(mapping);
+ idx = linear_page_index(dst_vma, dst_addr);
hash = hugetlb_fault_mutex_hash(h, dst_mm, dst_vma, mapping,
idx, dst_addr);
mutex_lock(&hugetlb_fault_mutex_table[hash]);
@@ -279,6 +283,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct
mm_struct *dst_mm,
dst_pte = huge_pte_alloc(dst_mm, dst_addr, huge_page_size(h));
if (!dst_pte) {
mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+ i_mmap_unlock_read(mapping);
goto out_unlock;
}
@@ -286,6 +291,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct
mm_struct *dst_mm,
dst_pteval = huge_ptep_get(dst_pte);
if (!huge_pte_none(dst_pteval)) {
mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+ i_mmap_unlock_read(mapping);
goto out_unlock;
}
@@ -293,6 +299,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct
mm_struct *dst_mm,
dst_addr, src_addr, &page);
mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+ i_mmap_unlock_read(mapping);
vm_alloc_shared = vm_shared;
cond_resched();
--
2.17.2
