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

[Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>]

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@xxxxxxxx>
> > Closes: 
> > https://lore.kernel.org/lkml/CAM1KZSmZ2T_riHvay+7cKEFxoPgeVpHkVFTzVVEQ1BO0cLkHEQ@xxxxxxxxxxxxxx/
> > Fixes: fc9c45b71f43 ("arm: adjust_pte() use pte_offset_map_rw_nolock()")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > ---
> >   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;