From: Hugh Dickins Date: Fri, 17 Nov 2023 08:49:18 +0000 (-0800) Subject: mm: fix oops when filemap_map_pmd() without prealloc_pte X-Git-Tag: v6.1.68~71 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=686cc4de099fdb3b3935896e21583803bad0fbf0;p=platform%2Fkernel%2Flinux-starfive.git mm: fix oops when filemap_map_pmd() without prealloc_pte commit 9aa1345d66b8132745ffb99b348b1492088da9e2 upstream. 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). Link: https://lore.kernel.org/linux-mm/20231115065506.19780-1-jose.pekkarinen@foxhound.fi/ Link: https://lkml.kernel.org/r/6ed0c50c-78ef-0719-b3c5-60c0c010431c@google.com Fixes: f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths") Signed-off-by: Hugh Dickins Reported-and-tested-by: syzbot+89edd67979b52675ddec@syzkaller.appspotmail.com Closes: https://lore.kernel.org/linux-mm/0000000000005e44550608a0806c@google.com/ Reviewed-by: David Hildenbrand Cc: Jann Horn , Cc: José Pekkarinen Cc: Kirill A. Shutemov Cc: Matthew Wilcox (Oracle) Cc: [5.12+] Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman --- diff --git a/mm/filemap.c b/mm/filemap.c index 2d93047..d633ab8 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3319,7 +3319,7 @@ static bool filemap_map_pmd(struct vm_fault *vmf, struct page *page) } } - if (pmd_none(*vmf->pmd)) + if (pmd_none(*vmf->pmd) && vmf->prealloc_pte) pmd_install(mm, vmf->pmd, &vmf->prealloc_pte); /* See comment in handle_pte_fault() */