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

[Qi Zheng <zhengqi.arch@bytedance.com>]

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?

> 
> Then the comparison below also becomes easier to read.
> 
>>       /*
>>        * If we have any shared mappings that are in the same mm
>> @@ -141,6 +141,14 @@ make_coherent(struct address_space *mapping, 
>> struct vm_area_struct *vma,
>>        */
>>       flush_dcache_mmap_lock(mapping);
>>       vma_interval_tree_foreach(mpnt, &mapping->i_mmap, pgoff, pgoff) {
>> +        unsigned long mpnt_addr;
>> +        /*
>> +         * If we are using split PTE locks, then we need to take the pte
>> +         * lock. Otherwise we are using shared mm->page_table_lock which
>> +         * is already locked, thus cannot take it.
>> +         */
>> +        bool need_lock = IS_ENABLED(CONFIG_SPLIT_PTE_PTLOCKS);
> 
> Nit: move "unsigned long mpnt_addr;" below this longer variable+init.

OK, will do.

> 
>> +
>>           /*
>>            * 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. */

Thanks,
Qi

> 
> ?
> 
>> +         */
>> +        if (mpnt_addr >= start && mpnt_addr - start < PMD_SIZE)
>> +            need_lock = false;
> 
> 
>