On Tue, Feb 10, 2015 at 10:42:31PM +0400, Konstantin Khlebnikov wrote: > On Mon, Dec 22, 2014 at 10:14 PM, Kirill A. Shutemov > <kirill@xxxxxxxxxxxxx> wrote: > > On Mon, Dec 22, 2014 at 01:05:13PM -0500, Sasha Levin wrote: > >> On 12/22/2014 01:01 PM, Kirill A. Shutemov wrote: > >> > On Mon, Dec 22, 2014 at 10:04:02AM -0500, Sasha Levin wrote: > >> >> > Hi all, > >> >> > > >> >> > While fuzzing with trinity inside a KVM tools guest running the latest -next > >> >> > kernel, I've stumbled on the following spew: > >> >> > > >> >> > [ 432.376425] BUG: unable to handle kernel NULL pointer dereference at 0000000000000038 > >> >> > [ 432.378876] IP: down_write (./arch/x86/include/asm/rwsem.h:105 ./arch/x86/include/asm/rwsem.h:121 kernel/locking/rwsem.c:71) > >> > Looks like vma->vm_file->mapping is NULL. Somebody freed ->vm_file from > >> > under us? > >> > > >> > I suspect Davidlohr's patchset on i_mmap_lock, but I cannot find any code > >> > path which could lead to the crash. > >> > >> I've reported a different issue which that patchset: https://lkml.org/lkml/2014/12/9/741 > >> > >> I guess it could be related? > > > > Maybe. > > > > Other thing: > > > > unmap_mapping_range() > > i_mmap_lock_read(mapping); > > unmap_mapping_range_tree() > > unmap_mapping_range_vma() > > zap_page_range_single() > > unmap_single_vma() > > untrack_pfn() > > vma->vm_flags &= ~VM_PAT; > > > > It seems we modify ->vm_flags without mmap_sem taken, means we can corrupt > > them. > > > > Sasha could you check if you hit untrack_pfn()? > > > > The problem probably was hidden by exclusive i_mmap_lock on > > unmap_mapping_range(), but it's not exclusive anymore afrer Dave's > > patchset. > > > > Konstantin, you've modified untrack_pfn() back in 2012 to change > > ->vm_flags. Any coments? > > Hmm. I don't really understand how > unmap_mapping_range() could be used for VM_PFNMAP mappings > except unmap() or exit_mmap() where mm is locked anyway. > Somebody truncates memory mapped device and unmaps mapped PFNs? Hm. Probably not. But it's not obvious to me what would stop this. Should we at least have assert on mmap_sem locked in untrack_pfn()? > If it's a problem then I think VM_PAT could be tuned into hint which > means PAT tracking was here and we pat should check internal structure > for details and take actions if pat tracking is still here. As I see > pat tracking probably also have problems if somebody unmaps that vma > partially. IIUC, we only mark a vma with VM_PAT if whole vma is subject for remap_pfn_range(). I don't see a point in cleaning VM_PAT -- just let it die with vma. Or do I miss something? -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>