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: stable@vger.kernel.org Signed-off-by: "Yan, Zheng" zyan@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
-- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>