Messages in this thread | | | Date | Mon, 30 May 2022 17:47:12 +0200 | Subject | Re: [PATCH AUTOSEL 5.18 074/159] scsi: target: tcmu: Fix possible data corruption | From | Bodo Stroesser <> |
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Sasha,
the below patch introduces a new bug, which is fixed by commit 325d5c5fb216 ("scsi: target: tcmu: Avoid holding XArray lock when calling lock_page") Please consider adding this further fix.
For my understanding: commit 325d5c5fb216 contains a "Fixes:" tag. So I'd expect it to be added automatically. Is there still something missing in the commit?
Thank you, Bodo
On 30.05.22 15:22, Sasha Levin wrote: > From: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com> > > [ Upstream commit bb9b9eb0ae2e9d3f6036f0ad907c3a83dcd43485 ] > > When tcmu_vma_fault() gets a page successfully, before the current context > completes page fault procedure, find_free_blocks() may run and call > unmap_mapping_range() to unmap the page. Assume that when > find_free_blocks() initially completes and the previous page fault > procedure starts to run again and completes, then one truncated page has > been mapped to userspace. But note that tcmu_vma_fault() has gotten a > refcount for the page so any other subsystem won't be able to use the page > unless the userspace address is unmapped later. > > If another command subsequently runs and needs to extend dbi_thresh it may > reuse the corresponding slot for the previous page in data_bitmap. Then > though we'll allocate new page for this slot in data_area, no page fault > will happen because we have a valid map and the real request's data will be > lost. > > Filesystem implementations will also run into this issue but they usually > lock the page when vm_operations_struct->fault gets a page and unlock the > page after finish_fault() completes. For truncate filesystems lock pages in > truncate_inode_pages() to protect against racing wrt. page faults. > > To fix this possible data corruption scenario we can apply a method similar > to the filesystems. For pages that are to be freed, tcmu_blocks_release() > locks and unlocks. Make tcmu_vma_fault() also lock found page under > cmdr_lock. At the same time, since tcmu_vma_fault() gets an extra page > refcount, tcmu_blocks_release() won't free pages if pages are in page fault > procedure, which means it is safe to call tcmu_blocks_release() before > unmap_mapping_range(). > > With these changes tcmu_blocks_release() will wait for all page faults to > be completed before calling unmap_mapping_range(). And later, if > unmap_mapping_range() is called, it will ensure stale mappings are removed. > > Link: https://lore.kernel.org/r/20220421023735.9018-1-xiaoguang.wang@linux.alibaba.com > Reviewed-by: Bodo Stroesser <bostroesser@gmail.com> > Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com> > Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com> > Signed-off-by: Sasha Levin <sashal@kernel.org> > --- > drivers/target/target_core_user.c | 40 ++++++++++++++++++++++++++++--- > 1 file changed, 37 insertions(+), 3 deletions(-) > > diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c > index fd7267baa707..b1fd06edea59 100644 > --- a/drivers/target/target_core_user.c > +++ b/drivers/target/target_core_user.c > @@ -20,6 +20,7 @@ > #include <linux/configfs.h> > #include <linux/mutex.h> > #include <linux/workqueue.h> > +#include <linux/pagemap.h> > #include <net/genetlink.h> > #include <scsi/scsi_common.h> > #include <scsi/scsi_proto.h> > @@ -1667,6 +1668,26 @@ static u32 tcmu_blocks_release(struct tcmu_dev *udev, unsigned long first, > xas_lock(&xas); > xas_for_each(&xas, page, (last + 1) * udev->data_pages_per_blk - 1) { > xas_store(&xas, NULL); > + /* > + * While reaching here there may be page faults occurring on > + * the to-be-released pages. A race condition may occur if > + * unmap_mapping_range() is called before page faults on these > + * pages have completed; a valid but stale map is created. > + * > + * If another command subsequently runs and needs to extend > + * dbi_thresh, it may reuse the slot corresponding to the > + * previous page in data_bitmap. Though we will allocate a new > + * page for the slot in data_area, no page fault will happen > + * because we have a valid map. Therefore the command's data > + * will be lost. > + * > + * We lock and unlock pages that are to be released to ensure > + * all page faults have completed. This way > + * unmap_mapping_range() can ensure stale maps are cleanly > + * removed. > + */ > + lock_page(page); > + unlock_page(page); > __free_page(page); > pages_freed++; > } > @@ -1822,6 +1843,7 @@ static struct page *tcmu_try_get_data_page(struct tcmu_dev *udev, uint32_t dpi) > page = xa_load(&udev->data_pages, dpi); > if (likely(page)) { > get_page(page); > + lock_page(page); > mutex_unlock(&udev->cmdr_lock); > return page; > } > @@ -1863,6 +1885,7 @@ static vm_fault_t tcmu_vma_fault(struct vm_fault *vmf) > struct page *page; > unsigned long offset; > void *addr; > + vm_fault_t ret = 0; > > int mi = tcmu_find_mem_index(vmf->vma); > if (mi < 0) > @@ -1887,10 +1910,11 @@ static vm_fault_t tcmu_vma_fault(struct vm_fault *vmf) > page = tcmu_try_get_data_page(udev, dpi); > if (!page) > return VM_FAULT_SIGBUS; > + ret = VM_FAULT_LOCKED; > } > > vmf->page = page; > - return 0; > + return ret; > } > > static const struct vm_operations_struct tcmu_vm_ops = { > @@ -3205,12 +3229,22 @@ static void find_free_blocks(void) > udev->dbi_max = block; > } > > + /* > + * Release the block pages. > + * > + * Also note that since tcmu_vma_fault() gets an extra page > + * refcount, tcmu_blocks_release() won't free pages if pages > + * are mapped. This means it is safe to call > + * tcmu_blocks_release() before unmap_mapping_range() which > + * drops the refcount of any pages it unmaps and thus releases > + * them. > + */ > + pages_freed = tcmu_blocks_release(udev, start, end - 1); > + > /* Here will truncate the data area from off */ > off = udev->data_off + (loff_t)start * udev->data_blk_size; > unmap_mapping_range(udev->inode->i_mapping, off, 0, 1); > > - /* Release the block pages */ > - pages_freed = tcmu_blocks_release(udev, start, end - 1); > mutex_unlock(&udev->cmdr_lock); > > total_pages_freed += pages_freed;
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