| Message ID | 6ed0c50c-78ef-0719-b3c5-60c0c010431c@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: fix oops when filemap_map_pmd() without prealloc_pte | expand |
On 17.11.23 09:49, Hugh Dickins wrote: > syzbot reports oops in lockdep's __lock_acquire(), called from > __pte_offset_map_lock() called from filemap_map_pages(); or when I run > the repro, the oops comes in pmd_install(), called from filemap_map_pmd() > called from filemap_map_pages(), just before the __pte_offset_map_lock(). > > The problem is that filemap_map_pmd() has been assuming that when it > finds pmd_none(), a page table has already been prepared in prealloc_pte; > and indeed do_fault_around() has been careful to preallocate one there, > when it finds pmd_none(): but what if *pmd became none in between? > > My 6.6 mods in mm/khugepaged.c, avoiding mmap_lock for write, have made > it easy for *pmd to be cleared while servicing a page fault; but even > before those, a huge *pmd might be zapped while a fault is serviced. > > The difference in symptomatic stack traces comes from the "memory model" > in use: pmd_install() uses pmd_populate() uses page_to_pfn(): in some > models that is strict, and will oops on the NULL prealloc_pte; in other > models, it will construct a bogus value to be populated into *pmd, then > __pte_offset_map_lock() oops when trying to access split ptlock pointer > (or some other symptom in normal case of ptlock embedded not pointer). > > Reported-and-tested-by: syzbot+89edd67979b52675ddec@syzkaller.appspotmail.com > Closes: https://lore.kernel.org/linux-mm/0000000000005e44550608a0806c@google.com/ > Link: https://lore.kernel.org/linux-mm/20231115065506.19780-1-jose.pekkarinen@foxhound.fi/ > Fixes: f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths") Quite an old one Reviewed-by: David Hildenbrand <david@redhat.com>
diff --git a/mm/filemap.c b/mm/filemap.c index 9710f43a89ac..3d4dae9d1070 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3371,7 +3371,7 @@ static bool filemap_map_pmd(struct vm_fault *vmf, struct folio *folio, } } - if (pmd_none(*vmf->pmd)) + if (pmd_none(*vmf->pmd) && vmf->prealloc_pte) pmd_install(mm, vmf->pmd, &vmf->prealloc_pte); return false;
syzbot reports oops in lockdep's __lock_acquire(), called from __pte_offset_map_lock() called from filemap_map_pages(); or when I run the repro, the oops comes in pmd_install(), called from filemap_map_pmd() called from filemap_map_pages(), just before the __pte_offset_map_lock(). The problem is that filemap_map_pmd() has been assuming that when it finds pmd_none(), a page table has already been prepared in prealloc_pte; and indeed do_fault_around() has been careful to preallocate one there, when it finds pmd_none(): but what if *pmd became none in between? My 6.6 mods in mm/khugepaged.c, avoiding mmap_lock for write, have made it easy for *pmd to be cleared while servicing a page fault; but even before those, a huge *pmd might be zapped while a fault is serviced. The difference in symptomatic stack traces comes from the "memory model" in use: pmd_install() uses pmd_populate() uses page_to_pfn(): in some models that is strict, and will oops on the NULL prealloc_pte; in other models, it will construct a bogus value to be populated into *pmd, then __pte_offset_map_lock() oops when trying to access split ptlock pointer (or some other symptom in normal case of ptlock embedded not pointer). Reported-and-tested-by: syzbot+89edd67979b52675ddec@syzkaller.appspotmail.com Closes: https://lore.kernel.org/linux-mm/0000000000005e44550608a0806c@google.com/ Link: https://lore.kernel.org/linux-mm/20231115065506.19780-1-jose.pekkarinen@foxhound.fi/ Fixes: f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths") Signed-off-by: Hugh Dickins <hughd@google.com> Cc: <stable@vger.kernel.org> [5.12+] --- mm/filemap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)