Re: [PATCH] arm: pgtable: fix NULL pointer dereference issue

[David Hildenbrand <david@redhat.com>]

On 12.02.25 09:28, Qi Zheng wrote:
> 
> 
> On 2025/2/12 16:20, David Hildenbrand wrote:
>> On 12.02.25 07:40, Qi Zheng wrote:
>>> When update_mmu_cache_range() is called by update_mmu_cache(), the vmf
>>> parameter is NULL, which will cause a NULL pointer dereference issue in
>>> adjust_pte():
>>>
>>> Unable to handle kernel NULL pointer dereference at virtual address
>>> 00000030 when read
>>> Hardware name: Atmel AT91SAM9
>>> PC is at update_mmu_cache_range+0x1e0/0x278
>>> LR is at pte_offset_map_rw_nolock+0x18/0x2c
>>> Call trace:
>>>    update_mmu_cache_range from remove_migration_pte+0x29c/0x2ec
>>>    remove_migration_pte from rmap_walk_file+0xcc/0x130
>>>    rmap_walk_file from remove_migration_ptes+0x90/0xa4
>>>    remove_migration_ptes from migrate_pages_batch+0x6d4/0x858
>>>    migrate_pages_batch from migrate_pages+0x188/0x488
>>>    migrate_pages from compact_zone+0x56c/0x954
>>>    compact_zone from compact_node+0x90/0xf0
>>>    compact_node from kcompactd+0x1d4/0x204
>>>    kcompactd from kthread+0x120/0x12c
>>>    kthread from ret_from_fork+0x14/0x38
>>> Exception stack(0xc0d8bfb0 to 0xc0d8bff8)
>>>
>>> To fix it, do not rely on whether 'ptl' is equal to decide whether to
>>> hold
>>> the pte lock, but decide it by whether CONFIG_SPLIT_PTE_PTLOCKS is
>>> enabled. In addition, if two vmas map to the same PTE page, there is no
>>> need to hold the pte lock again, otherwise a deadlock will occur. Just
>>> add
>>> the need_lock parameter to let adjust_pte() know this information.
>>>
>>> Reported-by: Ezra Buehler <ezra@easyb.ch>
>>> Closes:
>>> https://lore.kernel.org/lkml/CAM1KZSmZ2T_riHvay+7cKEFxoPgeVpHkVFTzVVEQ1BO0cLkHEQ@mail.gmail.com/
>>> Fixes: fc9c45b71f43 ("arm: adjust_pte() use pte_offset_map_rw_nolock()")
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>> ---
>>>    arch/arm/mm/fault-armv.c | 40 ++++++++++++++++++++++++++++------------
>>>    1 file changed, 28 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/arch/arm/mm/fault-armv.c b/arch/arm/mm/fault-armv.c
>>> index 2bec87c3327d2..3627bf0957c75 100644
>>> --- a/arch/arm/mm/fault-armv.c
>>> +++ b/arch/arm/mm/fault-armv.c
>>> @@ -62,7 +62,7 @@ static int do_adjust_pte(struct vm_area_struct *vma,
>>> unsigned long address,
>>>    }
>>>    static int adjust_pte(struct vm_area_struct *vma, unsigned long
>>> address,
>>> -              unsigned long pfn, struct vm_fault *vmf)
>>> +              unsigned long pfn, bool need_lock)
>>>    {
>>>        spinlock_t *ptl;
>>>        pgd_t *pgd;
>>> @@ -99,12 +99,11 @@ static int adjust_pte(struct vm_area_struct *vma,
>>> unsigned long address,
>>>        if (!pte)
>>>            return 0;
>>> -    /*
>>> -     * If we are using split PTE locks, then we need to take the page
>>> -     * lock here.  Otherwise we are using shared mm->page_table_lock
>>> -     * which is already locked, thus cannot take it.
>>> -     */
>>> -    if (ptl != vmf->ptl) {
>>> +    if (need_lock) {
>>> +        /*
>>> +         * Use nested version here to indicate that we are already
>>> +         * holding one similar spinlock.
>>> +         */
>>>            spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
>>>            if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pmd)))) {
>>>                pte_unmap_unlock(pte, ptl);
>>> @@ -114,7 +113,7 @@ static int adjust_pte(struct vm_area_struct *vma,
>>> unsigned long address,
>>>        ret = do_adjust_pte(vma, address, pfn, pte);
>>> -    if (ptl != vmf->ptl)
>>> +    if (need_lock)
>>>            spin_unlock(ptl);
>>>        pte_unmap(pte);
>>> @@ -123,16 +122,17 @@ static int adjust_pte(struct vm_area_struct
>>> *vma, unsigned long address,
>>>    static void
>>>    make_coherent(struct address_space *mapping, struct vm_area_struct
>>> *vma,
>>> -          unsigned long addr, pte_t *ptep, unsigned long pfn,
>>> -          struct vm_fault *vmf)
>>> +          unsigned long addr, pte_t *ptep, unsigned long pfn)
>>>    {
>>>        struct mm_struct *mm = vma->vm_mm;
>>>        struct vm_area_struct *mpnt;
>>>        unsigned long offset;
>>> +    unsigned long start;
>>>        pgoff_t pgoff;
>>>        int aliases = 0;
>>>        pgoff = vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT);
>>> +    start = ALIGN_DOWN(addr, PMD_SIZE);
>>
>> I assume you can come up with a better name than "start" :)
>>
>> aligned_addr ... pmd_start_addr ...
>>
>> Maybe simply
>>
>> pmd_start_addr = ALIGN_DOWN(addr, PMD_SIZE);
>> pmd_end_addr = addr + PMD_SIZE;
> 
> you mean:
> 
> pmd_end_addr = pmd_start_addr + PMD_SIZE;
> 
> Right?

Yes :)

>>
>>> +
>>>            /*
>>>             * If this VMA is not in our MM, we can ignore it.
>>>             * Note that we intentionally mask out the VMA
>>> @@ -151,7 +159,15 @@ make_coherent(struct address_space *mapping,
>>> struct vm_area_struct *vma,
>>>            if (!(mpnt->vm_flags & VM_MAYSHARE))
>>>                continue;
>>>            offset = (pgoff - mpnt->vm_pgoff) << PAGE_SHIFT;
>>> -        aliases += adjust_pte(mpnt, mpnt->vm_start + offset, pfn, vmf);
>>> +        mpnt_addr = mpnt->vm_start + offset;
>>> +        /*
>>> +         * If mpnt_addr and addr are mapped to the same PTE page, there
>>> +         * is no need to hold the pte lock again, otherwise a deadlock
>>> +         * will occur.
>>
>> /*
>>    * Avoid deadlocks by not grabbing the PTE lock if we already hold the
>>    * PTE lock of this PTE table in the caller.
>>    */
> 
> Maybe just:
> 
> /* Avoid deadlocks by not grabbing the same PTE lock again. */
> 

Agreed.

-- 
Cheers,

David / dhildenb