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Message-ID: <d5bba68b-1dba-4367-8d4f-103348b80229@redhat.com>
Date: Wed, 12 Feb 2025 09:20:19 +0100
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.com>, linux@...linux.org.uk,
ezra@...yb.ch, hughd@...gle.com, ryan.roberts@....com,
akpm@...ux-foundation.org, muchun.song@...ux.dev
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] arm: pgtable: fix NULL pointer dereference issue
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@...yb.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@...r.kernel.org
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.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;
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.
> +
> /*
> * 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.
*/
?
> + */
> + if (mpnt_addr >= start && mpnt_addr - start < PMD_SIZE)
> + need_lock = false;
--
Cheers,
David / dhildenb
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