[PATCH 2/3] drm/xe/hmm: Don't dereference struct page pointers without notifier lock
Matthew Auld
matthew.auld at intel.com
Fri Feb 28 12:55:10 UTC 2025
On 28/02/2025 10:44, Thomas Hellström wrote:
> The pnfs that we obtain from hmm_range_fault() point to pages that
> we don't have a reference on, and the guarantee that they are still
> in the cpu page-tables is that the notifier lock must be held and the
> notifier seqno is still valid.
>
> So while building the sg table and marking the pages accesses / dirty
> we need to hold this lock with a validated seqno.
>
> However, the lock is reclaim tainted which makes
> sg_alloc_table_from_pages_segment() unusable, since it internally
> allocates memory.
>
> Instead build the sg-table manually. For the non-iommu case
> this might lead to fewer coalesces, but if that's a problem it can
> be fixed up later in the resource cursor code. For the iommu case,
> the whole sg-table may still be coalesced to a single contigous
> device va region.
>
> This avoids marking pages that we don't own dirty and accessed, and
> it also avoid dereferencing struct pages that we don't own.
>
> Fixes: 81e058a3e7fd ("drm/xe: Introduce helper to populate userptr")
> Cc: Oak Zeng <oak.zeng at intel.com>
> Cc: <stable at vger.kernel.org> # v6.10+
> Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> ---
> drivers/gpu/drm/xe/xe_hmm.c | 115 ++++++++++++++++++++++++++----------
> 1 file changed, 85 insertions(+), 30 deletions(-)
>
> diff --git a/drivers/gpu/drm/xe/xe_hmm.c b/drivers/gpu/drm/xe/xe_hmm.c
> index c56738fa713b..d3b5551496d0 100644
> --- a/drivers/gpu/drm/xe/xe_hmm.c
> +++ b/drivers/gpu/drm/xe/xe_hmm.c
> @@ -42,6 +42,36 @@ static void xe_mark_range_accessed(struct hmm_range *range, bool write)
> }
> }
>
> +static int xe_alloc_sg(struct sg_table *st, struct hmm_range *range,
> + struct rw_semaphore *notifier_sem)
> +{
> + unsigned long i, npages, hmm_pfn;
> + unsigned long num_chunks = 0;
> + int ret;
> +
> + /* HMM docs says this is needed. */
> + ret = down_read_interruptible(notifier_sem);
> + if (ret)
> + return ret;
> +
> + if (mmu_interval_read_retry(range->notifier, range->notifier_seq))
> + return -EAGAIN;
> +
> + npages = xe_npages_in_range(range->start, range->end);
> + for (i = 0; i < npages;) {
> + hmm_pfn = range->hmm_pfns[i];
> + if (!(hmm_pfn & HMM_PFN_VALID)) {
Is this possible? The default_flags are always REQ_FAULT, so that should
ensure PFN_VALID, or the hmm_fault would have returned an error?
From the docs:
"HMM_PFN_REQ_FAULT - The output must have HMM_PFN_VALID or
hmm_range_fault() will fail"
Should this be an assert?
Also probably dumb question, but why do we need to hold the notifier
lock over this loop? What is it protecting?
> + up_read(notifier_sem);
> + return -EFAULT;
> + }
> + num_chunks++;
> + i += 1UL << hmm_pfn_to_map_order(hmm_pfn);
> + }
> + up_read(notifier_sem);
> +
> + return sg_alloc_table(st, num_chunks, GFP_KERNEL);
> +}
> +
> /**
> * xe_build_sg() - build a scatter gather table for all the physical pages/pfn
> * in a hmm_range. dma-map pages if necessary. dma-address is save in sg table
> @@ -50,6 +80,7 @@ static void xe_mark_range_accessed(struct hmm_range *range, bool write)
> * @range: the hmm range that we build the sg table from. range->hmm_pfns[]
> * has the pfn numbers of pages that back up this hmm address range.
> * @st: pointer to the sg table.
> + * @notifier_sem: The xe notifier lock.
> * @write: whether we write to this range. This decides dma map direction
> * for system pages. If write we map it bi-diretional; otherwise
> * DMA_TO_DEVICE
> @@ -76,38 +107,33 @@ static void xe_mark_range_accessed(struct hmm_range *range, bool write)
> * Returns 0 if successful; -ENOMEM if fails to allocate memory
> */
> static int xe_build_sg(struct xe_device *xe, struct hmm_range *range,
> - struct sg_table *st, bool write)
> + struct sg_table *st,
> + struct rw_semaphore *notifier_sem,
> + bool write)
> {
> struct device *dev = xe->drm.dev;
> - struct page **pages;
> - u64 i, npages;
> - int ret;
> -
> - npages = xe_npages_in_range(range->start, range->end);
> - pages = kvmalloc_array(npages, sizeof(*pages), GFP_KERNEL);
> - if (!pages)
> - return -ENOMEM;
> -
> - for (i = 0; i < npages; i++) {
> - pages[i] = hmm_pfn_to_page(range->hmm_pfns[i]);
> - xe_assert(xe, !is_device_private_page(pages[i]));
> - }
> -
> - ret = sg_alloc_table_from_pages_segment(st, pages, npages, 0, npages << PAGE_SHIFT,
> - xe_sg_segment_size(dev), GFP_KERNEL);
> - if (ret)
> - goto free_pages;
> -
> - ret = dma_map_sgtable(dev, st, write ? DMA_BIDIRECTIONAL : DMA_TO_DEVICE,
> - DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_NO_KERNEL_MAPPING);
> - if (ret) {
> - sg_free_table(st);
> - st = NULL;
> + unsigned long hmm_pfn, size;
> + struct scatterlist *sgl;
> + struct page *page;
> + unsigned long i, j;
> +
> + lockdep_assert_held(notifier_sem);
> +
> + i = 0;
> + for_each_sg(st->sgl, sgl, st->nents, j) {
> + hmm_pfn = range->hmm_pfns[i];
> + page = hmm_pfn_to_page(hmm_pfn);
> + xe_assert(xe, !is_device_private_page(page));
> + size = 1UL << hmm_pfn_to_map_order(hmm_pfn);
> + sg_set_page(sgl, page, size << PAGE_SHIFT, 0);
> + if (unlikely(j == st->nents - 1))
> + sg_mark_end(sgl);
> + i += size;
> }
> + xe_assert(xe, i == xe_npages_in_range(range->start, range->end));
>
> -free_pages:
> - kvfree(pages);
> - return ret;
> + return dma_map_sgtable(dev, st, write ? DMA_BIDIRECTIONAL : DMA_TO_DEVICE,
> + DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_NO_KERNEL_MAPPING);
> }
>
> /**
> @@ -235,16 +261,45 @@ int xe_hmm_userptr_populate_range(struct xe_userptr_vma *uvma,
> if (ret)
> goto free_pfns;
>
> - ret = xe_build_sg(vm->xe, &hmm_range, &userptr->sgt, write);
> + if (unlikely(userptr->sg)) {
> + ret = down_write_killable(&vm->userptr.notifier_lock);
> + if (ret)
> + goto free_pfns;
> +
> + xe_hmm_userptr_free_sg(uvma);
> + up_write(&vm->userptr.notifier_lock);
> + }
> +
> + ret = xe_alloc_sg(&userptr->sgt, &hmm_range, &vm->userptr.notifier_lock);
> if (ret)
> goto free_pfns;
>
> + ret = down_read_interruptible(&vm->userptr.notifier_lock);
> + if (ret)
> + goto free_st;
> +
> + if (mmu_interval_read_retry(hmm_range.notifier, hmm_range.notifier_seq)) {
> + ret = -EAGAIN;
> + goto out_unlock;
> + }
> +
> + ret = xe_build_sg(vm->xe, &hmm_range, &userptr->sgt,
> + &vm->userptr.notifier_lock, write);
> + if (ret)
> + goto out_unlock;
> +
> xe_mark_range_accessed(&hmm_range, write);
> userptr->sg = &userptr->sgt;
> userptr->notifier_seq = hmm_range.notifier_seq;
> + up_read(&vm->userptr.notifier_lock);
> + kvfree(pfns);
> + return 0;
>
> +out_unlock:
> + up_read(&vm->userptr.notifier_lock);
> +free_st:
> + sg_free_table(&userptr->sgt);
> free_pfns:
> kvfree(pfns);
> return ret;
> }
> -
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