FAILED: patch "[PATCH] mm/userfaultfd: do not access vma->vm_mm after calling" failed to apply to 4.9-stable tree

PATCH New [email protected] 2 participants 2 messages Nov 23, 2020

Subsystems (3)

Stable branch Memory management THP

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author maintainer

2020-11-23 09:46 UTC #0 lore raw

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From bfe8cc1db02ab243c62780f17fc57f65bde0afe1 Mon Sep 17 00:00:00 2001
From: Gerald Schaefer <[email protected]>
Date: Sat, 21 Nov 2020 22:17:15 -0800
Subject: [PATCH] mm/userfaultfd: do not access vma->vm_mm after calling
handle_userfault()

Alexander reported a syzkaller / KASAN finding on s390, see below for complete output.

In do_huge_pmd_anonymous_page(), the pre-allocated pagetable will be freed in some cases. In the case of userfaultfd_missing(), this will happen after calling handle_userfault(), which might have released the mmap_lock. Therefore, the following pte_free(vma->vm_mm, pgtable) will access an unstable vma->vm_mm, which could have been freed or re-used already.

For all architectures other than s390 this will go w/o any negative impact, because pte_free() simply frees the page and ignores the passed-in mm. The implementation for SPARC32 would also access mm->page_table_lock for pte_free(), but there is no THP support in SPARC32, so the buggy code path will not be used there.

For s390, the mm->context.pgtable_list is being used to maintain the 2K pagetable fragments, and operating on an already freed or even re-used mm could result in various more or less subtle bugs due to list / pagetable corruption.

Fix this by calling pte_free() before handle_userfault(), similar to how it is already done in __do_huge_pmd_anonymous_page() for the WRITE / non-huge_zero_page case.

Commit 6b251fc96cf2c ("userfaultfd: call handle_userfault() for userfaultfd_missing() faults") actually introduced both, the do_huge_pmd_anonymous_page() and also __do_huge_pmd_anonymous_page() changes wrt to calling handle_userfault(), but only in the latter case it put the pte_free() before calling handle_userfault().

  BUG: KASAN: use-after-free in do_huge_pmd_anonymous_page+0xcda/0xd90 mm/huge_memory.c:744
  Read of size 8 at addr 00000000962d6988 by task syz-executor.0/9334

  CPU: 1 PID: 9334 Comm: syz-executor.0 Not tainted 5.10.0-rc1-syzkaller-07083-g4c9720875573 #0
  Hardware name: IBM 3906 M04 701 (KVM/Linux)
  Call Trace:
    do_huge_pmd_anonymous_page+0xcda/0xd90 mm/huge_memory.c:744
    create_huge_pmd mm/memory.c:4256 [inline]
    __handle_mm_fault+0xe6e/0x1068 mm/memory.c:4480
    handle_mm_fault+0x288/0x748 mm/memory.c:4607
    do_exception+0x394/0xae0 arch/s390/mm/fault.c:479
    do_dat_exception+0x34/0x80 arch/s390/mm/fault.c:567
    pgm_check_handler+0x1da/0x22c arch/s390/kernel/entry.S:706
    copy_from_user_mvcos arch/s390/lib/uaccess.c:111 [inline]
    raw_copy_from_user+0x3a/0x88 arch/s390/lib/uaccess.c:174
    _copy_from_user+0x48/0xa8 lib/usercopy.c:16
    copy_from_user include/linux/uaccess.h:192 [inline]
    __do_sys_sigaltstack kernel/signal.c:4064 [inline]
    __s390x_sys_sigaltstack+0xc8/0x240 kernel/signal.c:4060
    system_call+0xe0/0x28c arch/s390/kernel/entry.S:415

  Allocated by task 9334:
    slab_alloc_node mm/slub.c:2891 [inline]
    slab_alloc mm/slub.c:2899 [inline]
    kmem_cache_alloc+0x118/0x348 mm/slub.c:2904
    vm_area_dup+0x9c/0x2b8 kernel/fork.c:356
    __split_vma+0xba/0x560 mm/mmap.c:2742
    split_vma+0xca/0x108 mm/mmap.c:2800
    mlock_fixup+0x4ae/0x600 mm/mlock.c:550
    apply_vma_lock_flags+0x2c6/0x398 mm/mlock.c:619
    do_mlock+0x1aa/0x718 mm/mlock.c:711
    __do_sys_mlock2 mm/mlock.c:738 [inline]
    __s390x_sys_mlock2+0x86/0xa8 mm/mlock.c:728
    system_call+0xe0/0x28c arch/s390/kernel/entry.S:415

  Freed by task 9333:
    slab_free mm/slub.c:3142 [inline]
    kmem_cache_free+0x7c/0x4b8 mm/slub.c:3158
    __vma_adjust+0x7b2/0x2508 mm/mmap.c:960
    vma_merge+0x87e/0xce0 mm/mmap.c:1209
    userfaultfd_release+0x412/0x6b8 fs/userfaultfd.c:868
    __fput+0x22c/0x7a8 fs/file_table.c:281
    task_work_run+0x200/0x320 kernel/task_work.c:151
    tracehook_notify_resume include/linux/tracehook.h:188 [inline]
    do_notify_resume+0x100/0x148 arch/s390/kernel/signal.c:538
    system_call+0xe6/0x28c arch/s390/kernel/entry.S:416

  The buggy address belongs to the object at 00000000962d6948 which belongs to the cache vm_area_struct of size 200
  The buggy address is located 64 bytes inside of 200-byte region [00000000962d6948, 00000000962d6a10)
  The buggy address belongs to the page: page:00000000313a09fe refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x962d6 flags: 0x3ffff00000000200(slab)
  raw: 3ffff00000000200 000040000257e080 0000000c0000000c 000000008020ba00
  raw: 0000000000000000 000f001e00000000 ffffffff00000001 0000000096959501
  page dumped because: kasan: bad access detected
  page->mem_cgroup:0000000096959501

  Memory state around the buggy address:
   00000000962d6880: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   00000000962d6900: 00 fc fc fc fc fc fc fc fc fa fb fb fb fb fb fb
  >00000000962d6980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                        ^
   00000000962d6a00: fb fb fc fc fc fc fc fc fc fc 00 00 00 00 00 00
   00000000962d6a80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  ==================================================================

Fixes 6b251fc96cf2c ("userfaultfd: call handle_userfault() for userfaultfd_missing() faults")

Reported-by Alexander Egorenkov <[email protected]>

Signed-off-by Gerald Schaefer <[email protected]>

Signed-off-by Andrew Morton <[email protected]>

Cc Andrea Arcangeli <[email protected]>

Cc Heiko Carstens <[email protected]>

Cc <[email protected]> [4.3+]

Link https://lkml.kernel.org/r/[email protected] open here

Signed-off-by Linus Torvalds <[email protected]>

mm/huge_memory.c +4 -5

index 9474dbc150ed..ec2bb93f7431 100644
@@ -710,7 +710,6 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
 			transparent_hugepage_use_zero_page()) {
 		pgtable_t pgtable;
 		struct page *zero_page;
-		bool set;
 		vm_fault_t ret;
 		pgtable = pte_alloc_one(vma->vm_mm);
 		if (unlikely(!pgtable))

@@ -723,25 +722,25 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
 		}
 		vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
 		ret = 0;
-		set = false;
 		if (pmd_none(*vmf->pmd)) {
 			ret = check_stable_address_space(vma->vm_mm);
 			if (ret) {
 				spin_unlock(vmf->ptl);
+				pte_free(vma->vm_mm, pgtable);
 			} else if (userfaultfd_missing(vma)) {
 				spin_unlock(vmf->ptl);
+				pte_free(vma->vm_mm, pgtable);
 				ret = handle_userfault(vmf, VM_UFFD_MISSING);
 				VM_BUG_ON(ret & VM_FAULT_FALLBACK);
 			} else {
 				set_huge_zero_page(pgtable, vma->vm_mm, vma,
 						   haddr, vmf->pmd, zero_page);
 				spin_unlock(vmf->ptl);
-				set = true;
 			}
-		} else
+		} else {
 			spin_unlock(vmf->ptl);
-		if (!set)
 			pte_free(vma->vm_mm, pgtable);
+		}
 		return ret;
 	}
 	gfp = alloc_hugepage_direct_gfpmask(vma);

Gerald Schaefer

2020-11-23 17:06 UTC #1 lore raw

PATCH 1/1 mm/userfaultfd: do not access vma->vm_mm after calling handle_userfault()

commit bfe8cc1db02ab243c62780f17fc57f65bde0afe1 upstream.