Re: [PATCH v2] mm/alloc_tag: fix vm_module_tags_populate's KASAN poisoning logic

[Hao Ge <hao.ge@linux.dev>]

Hi Suren


On 12/5/24 03:33, Suren Baghdasaryan wrote:
> On Wed, Dec 4, 2024 at 7:08 AM Hao Ge <hao.ge@linux.dev> wrote:
>> Hi Suren
>>
>>
>> Thank you for your review.
>>
>>
>> On 12/4/24 22:39, Suren Baghdasaryan wrote:
>>> On Wed, Dec 4, 2024 at 12:35 AM Hao Ge <hao.ge@linux.dev> wrote:
>>>> From: Hao Ge <gehao@kylinos.cn>
>>>>
>>>> After merge commit 233e89322cbe ("alloc_tag:
>>>> fix module allocation tags populated area calculation"),
>>>> We still encountered a KASAN bug.
>>>>
>>>> This is because we have only actually performed
>>>> page allocation and address mapping here.
>>>> we need to unpoisoned portions of underlying memory.
>>>>
>>>> Because we have a change in the size here,we need to
>>>> re-annotate poisoned and unpoisoned portions of underlying memory
>>>> according to the new size.
>>>>
>>>> Here is the log for KASAN:
>>>>
>>>> [    5.041171][    T1] ==================================================================
>>>> [    5.042047][    T1] BUG: KASAN: vmalloc-out-of-bounds in move_module+0x2c0/0x708
>>>> [    5.042723][    T1] Write of size 240 at addr ffff80007e510000 by task systemd/1
>>>> [    5.043412][    T1]
>>>> [    5.043523][   T72] input: QEMU QEMU USB Tablet as /devices/pci0000:00/0000:00:01.1/0000:02:001
>>>> [    5.043614][    T1] CPU: 0 UID: 0 PID: 1 Comm: systemd Not tainted 6.13.0-rc1+ #28
>>>> [    5.045560][    T1] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
>>>> [    5.046328][    T1] Call trace:
>>>> [    5.046670][    T1]  show_stack+0x20/0x38 (C)
>>>> [    5.047127][    T1]  dump_stack_lvl+0x80/0xf8
>>>> [    5.047533][    T1]  print_address_description.constprop.0+0x58/0x358
>>>> [    5.048092][   T72] hid-generic 0003:0627:0001.0001: input,hidraw0: USB HID v0.01 Mouse [QEMU 0
>>>> [    5.048126][    T1]  print_report+0xb0/0x280
>>>> [    5.049682][    T1]  kasan_report+0xb8/0x108
>>>> [    5.050170][    T1]  kasan_check_range+0xe8/0x190
>>>> [    5.050685][    T1]  memcpy+0x58/0xa0
>>>> [    5.051135][    T1]  move_module+0x2c0/0x708
>>>> [    5.051586][    T1]  layout_and_allocate.constprop.0+0x308/0x5b8
>>>> [    5.052219][    T1]  load_module+0x134/0x16c8
>>>> [    5.052671][    T1]  init_module_from_file+0xdc/0x138
>>>> [    5.053193][    T1]  idempotent_init_module+0x344/0x600
>>>> [    5.053742][    T1]  __arm64_sys_finit_module+0xbc/0x150
>>>> [    5.054289][    T1]  invoke_syscall+0xd4/0x258
>>>> [    5.054749][    T1]  el0_svc_common.constprop.0+0xb4/0x240
>>>> [    5.055319][    T1]  do_el0_svc+0x48/0x68
>>>> [    5.055743][    T1]  el0_svc+0x40/0xe0
>>>> [    5.056142][    T1]  el0t_64_sync_handler+0x10c/0x138
>>>> [    5.056658][    T1]  el0t_64_sync+0x1ac/0x1b0
>>>>
>>>> Fixes: 233e89322cbe ("alloc_tag: fix module allocation tags populated area calculation")
>>>> Signed-off-by: Hao Ge <gehao@kylinos.cn>
>>> Thanks for the fix!
>>>
>>>> ---
>>>> v2: Add comments to kasan_unpoison_vmalloc like other places.
>>>>
>>>> commit 233e89322cbe ("alloc_tag: fix module allocation
>>>> tags populated area calculation") is currently in the
>>>> mm-hotfixes-unstable branch, so this patch is
>>>> developed based on the mm-hotfixes-unstable branch.
>>>> ---
>>>>    lib/alloc_tag.c | 13 +++++++++++++
>>>>    1 file changed, 13 insertions(+)
>>>>
>>>> diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
>>>> index 4ee6caa6d2da..f885b3f3af0e 100644
>>>> --- a/lib/alloc_tag.c
>>>> +++ b/lib/alloc_tag.c
>>>> @@ -421,7 +421,20 @@ static int vm_module_tags_populate(void)
>>>>                                   __free_page(next_page[i]);
>>>>                           return -ENOMEM;
>>>>                   }
>>>> +
>>>> +               kasan_poison_vmalloc((void *)module_tags.start_addr,
>>>> +                                    vm_module_tags->nr_pages << PAGE_SHIFT);
>>>> +
>>>>                   vm_module_tags->nr_pages += nr;
>>>> +
>>>> +               /*
>>>> +                * Mark the pages as accessible, now that they are mapped.
>>>> +                * With hardware tag-based KASAN, marking is skipped for
>>>> +                * non-VM_ALLOC mappings, see __kasan_unpoison_vmalloc().
>>>> +                */
>>>> +               kasan_unpoison_vmalloc((void *)module_tags.start_addr,
>>>> +                                      vm_module_tags->nr_pages << PAGE_SHIFT,
>>>> +                                      KASAN_VMALLOC_PROT_NORMAL);
>>> Instead of poisoning [module_tags.start_addr,
>>> vm_module_tags->nr_pages], incrementing vm_module_tags->nr_pages and
>>> the unpoisoning [module_tags.start_addr, vm_module_tags->nr_pages]
>>> could we simply poisons the additional area like this:
>>>
>>>                   kasan_unpoison_vmalloc((void *)module_tags.start_addr +
>>>                                          (vm_module_tags->nr_pages << PAGE_SHIFT),
>>>                                          nr << PAGE_SHIFT,
>>>                                          KASAN_VMALLOC_PROT_NORMAL);
>>>                  vm_module_tags->nr_pages += nr;
>>> ?
>>
>> I had considered making such modifications earlier.
>>
>> But considering the following situation,
>>
>> A module tags spans across the regions of [module_tags.start_addr,
>> vm_module_tags->nr_pages] and [module_tags.start_addr +
>> vm_module_tags->nr_pages, ...].
>>
>> It may result in false positives for out-of-bounds errors.
> Sorry, maybe I'm missing something but I don't see why poisoning only
> newly mapped area would lead to false positives. Could you please
> clarify?


Because KASAN may perceive the two as distinct address spaces, despite 
their addresses being contiguous.

So, when a module tag spans across these two contiguous address spaces, 
KASAN may incorrectly consider it as an out-of-bounds access.


> Also, if you do need to unpoison and then poison, using phys_end and
> new_end would be better, like this:
>
> kasan_poison_vmalloc((void *)module_tags.start_addr,
>                                        phys_end - module_tags.start_addr)
>
> kasan_unpoison_vmalloc((void *)module_tags.start_addr,
>                                            new_end - module_tags.start_addr,
>                                            KASAN_VMALLOC_PROT_NORMAL);

OK, the next version will include.


Thanks

Best regards

Hao


>>
>>>>           }
>>>>
>>>>           return 0;
>>>> --
>>>> 2.25.1
>>>>