On Thu 14-12-17 18:55:27, Yan, Zheng wrote:
> 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
>
> ceph_read_iter() uses current->journal_info to pass context info to
> ceph_readpages(). Because ceph_readpages() needs to know if its caller
> has already gotten capability of using page cache (distinguish read
> from readahead/fadvise). ceph_read_iter() set current->journal_info,
> then calls generic_file_read_iter().
>
> In above Oops, page fault happened when copying data to userspace.
> Page fault handler called ext4_page_mkwrite(). Ext4 code read
> current->journal_info and assumed it is journal handle.
>
> I checked other filesystems, btrfs probably suffers similar problem
> for its readpage. (page fault happens when write() copies data from
> userspace memory and the memory is mapped to a file in btrfs.
> verify_parent_transid() can be called during readpage)
>
> Cc: sta...@vger.kernel.org
> Signed-off-by: "Yan, Zheng" <z...@redhat.com>
I am not an FS expert so (ab)using journal_info for unrelated purposes
might be acceptable in general but hooking into the generic PF path like
this is just too ugly to live. Can this be limited to a FS code so that
not everybody has to pay additional cycles? With a big fat warning that
(ab)users might want to find a better way to comunicate their internal
stuff.
> ---
> mm/memory.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index a728bed16c20..db2a50233c49 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4044,6 +4044,7 @@ int handle_mm_fault(struct vm_area_struct *vma,
> unsigned long address,
> unsigned int flags)
> {
> int ret;
> + void *old_journal_info;
>
> __set_current_state(TASK_RUNNING);
>
> @@ -4065,11 +4066,24 @@ int handle_mm_fault(struct vm_area_struct *vma,
> unsigned long address,
> if (flags & FAULT_FLAG_USER)
> mem_cgroup_oom_enable();
>
> + /*
> + * Fault can happen when filesystem A's read_iter()/write_iter()
> + * copies data to/from userspace. Filesystem A may have set
> + * current->journal_info. If the userspace memory is MAP_SHARED
> + * mapped to a file in filesystem B, we later may call filesystem
> + * B's vm operation. Filesystem B may also want to read/set
> + * current->journal_info.
> + */
> + old_journal_info = current->journal_info;
> + current->journal_info = NULL;
> +
> if (unlikely(is_vm_hugetlb_page(vma)))
> ret = hugetlb_fault(vma->vm_mm, vma, address, flags);
> else
> ret = __handle_mm_fault(vma, address, flags);
>
> + current->journal_info = old_journal_info;
> +
> if (flags & FAULT_FLAG_USER) {
> mem_cgroup_oom_disable();
> /*
> --
> 2.13.6
>
> --
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--
Michal Hocko
SUSE Labs
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