From: "Yan, Zheng" Subject: [PATCH] mm: save current->journal_info before calling fault/page_mkwrite Date: Wed, 13 Dec 2017 11:58:36 +0800 Message-ID: <20171213035836.916-1-zyan@redhat.com> Cc: akpm@linux-foundation.org, viro@zeniv.linux.org.uk, jlayton@redhat.com, "Yan, Zheng" To: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, ceph-devel@vger.kernel.org, linux-ext4@vger.kernel.org Return-path: Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org We recently got an Oops report: BUG: unable to handle kernel NULL pointer dereference at (null) IP: jbd2__journal_start+0x38/0x1a2 [...] Call Trace: ext4_page_mkwrite+0x307/0x52b _ext4_get_block+0xd8/0xd8 do_page_mkwrite+0x6e/0xd8 handle_mm_fault+0x686/0xf9b mntput_no_expire+0x1f/0x21e __do_page_fault+0x21d/0x465 dput+0x4a/0x2f7 page_fault+0x22/0x30 copy_user_generic_string+0x2c/0x40 copy_page_to_iter+0x8c/0x2b8 generic_file_read_iter+0x26e/0x845 timerqueue_del+0x31/0x90 ceph_read_iter+0x697/0xa33 [ceph] hrtimer_cancel+0x23/0x41 futex_wait+0x1c8/0x24d get_futex_key+0x32c/0x39a __vfs_read+0xe0/0x130 vfs_read.part.1+0x6c/0x123 handle_mm_fault+0x831/0xf9b __fget+0x7e/0xbf SyS_read+0x4d/0xb5 The reason is that page fault can happen when one filesystem copies data from/to userspace, the filesystem may set current->journal_info. If the userspace memory is mapped to a file on another filesystem, the later filesystem may also want to use current->journal_info. Signed-off-by: "Yan, Zheng" --- mm/memory.c | 23 ++++++++++++++++++++++- 1 file changed, 22 insertions(+), 1 deletion(-) diff --git a/mm/memory.c b/mm/memory.c index 5eb3d2524bdc..e51383cd49bf 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2347,12 +2347,22 @@ static int do_page_mkwrite(struct vm_fault *vmf) { int ret; struct page *page = vmf->page; + void *old_journal_info = current->journal_info; unsigned int old_flags = vmf->flags; + /* + * If the fault happens during read_iter() copies data to + * userspace, filesystem may have set current->journal_info. + * If the userspace memory is mapped to a file on another + * filesystem, page_mkwrite() of the later filesystem may + * want to access/modify current->journal_info. + */ + current->journal_info = NULL; vmf->flags = FAULT_FLAG_WRITE|FAULT_FLAG_MKWRITE; ret = vmf->vma->vm_ops->page_mkwrite(vmf); - /* Restore original flags so that caller is not surprised */ + /* Restore original journal_info and flags */ + current->journal_info = old_journal_info; vmf->flags = old_flags; if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))) return ret; @@ -3191,9 +3201,20 @@ static int do_anonymous_page(struct vm_fault *vmf) static int __do_fault(struct vm_fault *vmf) { struct vm_area_struct *vma = vmf->vma; + void *old_journal_info = current->journal_info; int ret; + /* + * If the fault happens during write_iter() copies data from + * userspace, filesystem may have set current->journal_info. + * If the userspace memory is mapped to a file on another + * filesystem, fault handler of the later filesystem may want + * to access/modify current->journal_info. + */ + current->journal_info = NULL; ret = vma->vm_ops->fault(vmf); + /* Restore original journal_info */ + current->journal_info = old_journal_info; if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY | VM_FAULT_DONE_COW))) return ret; -- 2.13.6