Re: kvm/arm64: use-after-free in kvm_vm_ioctl/vmacache_update

From: Suzuki K Poulose
Date: Tue Mar 14 2017 - 07:03:35 EST

On 13/03/17 09:58, Marc Zyngier wrote:
On 10/03/17 18:37, Suzuki K Poulose wrote:
On 10/03/17 15:50, Andrey Konovalov wrote:
On Fri, Mar 10, 2017 at 2:38 PM, Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
Hi,

I've got the following error report while fuzzing the kernel with syzkaller.

On linux-next commit 56b8bad5e066c23e8fa273ef5fba50bd3da2ace8 (Mar 8).

Unfortunately I can't reproduce it.

==================================================================
BUG: KASAN: use-after-free in vmacache_update+0x114/0x118 mm/vmacache.c:63
Read of size 8 at addr ffff80003b9a2040 by task syz-executor/26615

CPU: 1 PID: 26615 Comm: syz-executor Not tainted
4.11.0-rc1-next-20170308-xc2-dirty #3
Hardware name: Hardkernel ODROID-C2 (DT)
Call trace:
[<ffff20000808fbb0>] dump_backtrace+0x0/0x440 arch/arm64/kernel/traps.c:505
[<ffff200008090010>] show_stack+0x20/0x30 arch/arm64/kernel/traps.c:228
[<ffff2000088e9578>] __dump_stack lib/dump_stack.c:16 [inline]
[<ffff2000088e9578>] dump_stack+0x110/0x168 lib/dump_stack.c:52
[<ffff200008414018>] print_address_description+0x60/0x248 mm/kasan/report.c:250
[<ffff2000084142e8>] kasan_report_error+0xe8/0x250 mm/kasan/report.c:349
[<ffff200008414564>] kasan_report mm/kasan/report.c:372 [inline]
[<ffff200008414564>] __asan_report_load8_noabort+0x3c/0x48 mm/kasan/report.c:393
[<ffff200008383f64>] vmacache_update+0x114/0x118 mm/vmacache.c:63
[<ffff2000083a9000>] find_vma+0xf8/0x150 mm/mmap.c:2124
[<ffff2000080dc19c>] kvm_arch_prepare_memory_region+0x2ac/0x488
arch/arm64/kvm/../../../arch/arm/kvm/mmu.c:1817
[<ffff2000080c2920>] __kvm_set_memory_region+0x3d8/0x12b8
arch/arm64/kvm/../../../virt/kvm/kvm_main.c:1026
[<ffff2000080c3838>] kvm_set_memory_region+0x38/0x58
arch/arm64/kvm/../../../virt/kvm/kvm_main.c:1075
[<ffff2000080c747c>] kvm_vm_ioctl_set_memory_region
arch/arm64/kvm/../../../virt/kvm/kvm_main.c:1087 [inline]
[<ffff2000080c747c>] kvm_vm_ioctl+0xb94/0x1308
arch/arm64/kvm/../../../virt/kvm/kvm_main.c:2960
[<ffff20000848f928>] vfs_ioctl fs/ioctl.c:45 [inline]
[<ffff20000848f928>] do_vfs_ioctl+0x128/0xfc0 fs/ioctl.c:685
[<ffff200008490868>] SYSC_ioctl fs/ioctl.c:700 [inline]
[<ffff200008490868>] SyS_ioctl+0xa8/0xb8 fs/ioctl.c:691
[<ffff200008083f70>] el0_svc_naked+0x24/0x28

Allocated by task 26657:
save_stack_trace_tsk+0x0/0x330 arch/arm64/kernel/stacktrace.c:133
save_stack_trace+0x20/0x30 arch/arm64/kernel/stacktrace.c:216
save_stack mm/kasan/kasan.c:515 [inline]
set_track mm/kasan/kasan.c:527 [inline]
kasan_kmalloc+0xd4/0x180 mm/kasan/kasan.c:619
kasan_slab_alloc+0x14/0x20 mm/kasan/kasan.c:557
slab_post_alloc_hook mm/slab.h:456 [inline]
slab_alloc_node mm/slub.c:2718 [inline]
slab_alloc mm/slub.c:2726 [inline]
kmem_cache_alloc+0x144/0x230 mm/slub.c:2731
__split_vma+0x118/0x608 mm/mmap.c:2515
do_munmap+0x194/0x9b0 mm/mmap.c:2636
Freed by task 26657:
save_stack_trace_tsk+0x0/0x330 arch/arm64/kernel/stacktrace.c:133
save_stack_trace+0x20/0x30 arch/arm64/kernel/stacktrace.c:216
save_stack mm/kasan/kasan.c:515 [inline]
set_track mm/kasan/kasan.c:527 [inline]
kasan_slab_free+0x84/0x198 mm/kasan/kasan.c:592
slab_free_hook mm/slub.c:1357 [inline]
slab_free_freelist_hook mm/slub.c:1379 [inline]
slab_free mm/slub.c:2961 [inline]
kmem_cache_free+0x80/0x258 mm/slub.c:2983
__vma_adjust+0x6b0/0xf mm/mmap.c:890] el0_svc_naked+0x24/0x28

The buggy address belongs to the object at ffff80003b9a2000
which belongs to the cache vm_area_struct(647:session-6.scope) of size 184
The buggy address is located 64 bytes inside of
184-byte region [ffff80003b9a2000, ffff80003b9a20b8)
The buggy address belongs to the page:
page:ffff7e0000ee6880 count:1 mapcount:0 mapping: (null) index:0x0
flags: 0xfffc00000000100(slab)
raw: 0fffc00000000100 0000000000000000 0000000000000000 0000000180100010
raw: 0000000000000000 0000000c00000001 ffff80005a5cc600 ffff80005ac99980
page dumped because: kasan: bad access detected
page->mem_cgroup:ffff80005ac99980

Memory state around the buggy address:
ffff80003b9a1f00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff80003b9a1f80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff80003b9a2000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff80003b9a2080: fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc fb
ffff80003b9a2100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

Another one that looks related and doesn't have parts of stack traces missing:

==================================================================
BUG: KASAN: use-after-free in find_vma+0x140/0x150 mm/mmap.c:2114
Read of size 8 at addr ffff800031a03e90 by task syz-executor/4360

CPU: 2 PID: 4360 Comm: syz-executor Not tainted
4.11.0-rc1-next-20170308-xc2-dirty #3
Hardware name: Hardkernel ODROID-C2 (DT)
Call trace:
[<ffff20000808fbb0>] dump_backtrace+0x0/0x440 arch/arm64/kernel/traps.c:505
[<ffff200008090010>] show_stack+0x20/0x30 arch/arm64/kernel/traps.c:228
[<ffff2000088e9578>] __dump_stack lib/dump_stack.c:16 [inline]
[<ffff2000088e9578>] dump_stack+0x110/0x168 lib/dump_stack.c:52
[<ffff200008414018>] print_address_description+0x60/0x248 mm/kasan/report.c:250
[<ffff2000084142e8>] kasan_report_error+0xe8/0x250 mm/kasan/report.c:349
[<ffff200008414564>] kasan_report mm/kasan/report.c:372 [inline]
[<ffff200008414564>] __asan_report_load8_noabort+0x3c/0x48 mm/kasan/report.c:393
[<ffff2000083a9048>] find_vma+0x140/0x150 mm/mmap.c:2114
[<ffff2000080dc19c>] kvm_arch_prepare_memory_region+0x2ac/0x488
arch/arm64/kvm/../../../arch/arm/kvm/mmu.c:1817

It looks like we don't take the mmap_sem before calling find_vma() in
stage2_unmap_memslot() and in kvm_arch_prepare_memory_region(), which is causing
the race, with probably the test trying to unmap ranges in between.

That indeed seems like a possible failure mode. The annoying thing is
that we're not exactly in a position to take mmap_sem in
stage2_unmap_memslot, since we hold the kvm->mmu_lock spinlock. We may
have to hold mmap_sem while iterating over all the memslots.

How about the following (very lightly tested):

diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index 962616fd4ddd..2006a79d5912 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -803,6 +803,7 @@ void stage2_unmap_vm(struct kvm *kvm)
int idx;

idx = srcu_read_lock(&kvm->srcu);
+ down_read(&current->mm->mmap_sem);
spin_lock(&kvm->mmu_lock);

slots = kvm_memslots(kvm);
@@ -810,6 +811,7 @@ void stage2_unmap_vm(struct kvm *kvm)
stage2_unmap_memslot(kvm, memslot);

spin_unlock(&kvm->mmu_lock);
+ up_read(&current->mm->mmap_sem);
srcu_read_unlock(&kvm->srcu, idx);
}

@@ -1813,6 +1815,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
* | memory region |
* +--------------------------------------------+
*/
+ down_read(&current->mm->mmap_sem);
do {
struct vm_area_struct *vma = find_vma(current->mm, hva);
hva_t vm_start, vm_end;

I have added the following hunk :

@@ -1844,8 +1847,10 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
pa += vm_start - vma->vm_start;
/* IO region dirty page logging not allowed */
- if (memslot->flags & KVM_MEM_LOG_DIRTY_PAGES)
- return -EINVAL;
+ if (memslot->flags & KVM_MEM_LOG_DIRTY_PAGES) {
+ ret = -EINVAL;
+ goto out;
+ }
ret = kvm_phys_addr_ioremap(kvm, gpa, pa,
vm_end - vm_start,


I'm much more worried about the other report, as I don't really see yet
how it happens. Coffee required.

I have something about that one. Will soon post a patch for it.

Suzuki