Subject: Re: [PATCH v2 1/4] mm: Add optional close() to struct vm_special_mapping

From: Sven Schnelle <[email-masked]>
Date: Mon, 02 Sep 2024 21:06:48 +0200

Nathan Chancellor <[email-masked]> writes:

> Hi Michael,
>
> On Mon, Aug 12, 2024 at 06:26:02PM +1000, Michael Ellerman wrote:
>> Add an optional close() callback to struct vm_special_mapping. It will
>> be used, by powerpc at least, to handle unmapping of the VDSO.
>>
>> Although support for unmapping the VDSO was initially added
>> for CRIU[1], it is not desirable to guard that support behind
>> CONFIG_CHECKPOINT_RESTORE.
>>
>> There are other known users of unmapping the VDSO which are not related
>> to CRIU, eg. Valgrind [2] and void-ship [3].
>>
>> The powerpc arch_unmap() hook has been in place for ~9 years, with no
>> ifdef, so there may be other unknown users that have come to rely on
>> unmapping the VDSO. Even if the code was behind an ifdef, major distros
>> enable CHECKPOINT_RESTORE so users may not realise unmapping the VDSO
>> depends on that configuration option.
>>
>> It's also undesirable to have such core mm behaviour behind a relatively
>> obscure CONFIG option.
>>
>> Longer term the unmap behaviour should be standardised across
>> architectures, however that is complicated by the fact the VDSO pointer
>> is stored differently across architectures. There was a previous attempt
>> to unify that handling [4], which could be revived.
>>
>> See [5] for further discussion.
>>
>> [1]: commit 83d3f0e90c6c ("powerpc/mm: tracking vDSO remap")
>> [2]: https://sourceware.org/git/?p=valgrind.git;a=commit;h=3a004915a2cbdcdebafc1612427576bf3321eef5
>> [3]: https://github.com/insanitybit/void-ship
>> [4]: https://lore.kernel.org/lkml/[email-masked]/
>> [5]: https://lore.kernel.org/linuxppc-dev/shiq5v3jrmyi6ncwke7wgl76ojysgbhrchsk32q4lbx2hadqqc@kzyy2igem256
>>
>> Suggested-by: Linus Torvalds <[email-masked]>
>> Signed-off-by: Michael Ellerman <[email-masked]>
>> Reviewed-by: David Hildenbrand <[email-masked]>
>> ---
>>  include/linux/mm_types.h | 3 +++
>>  mm/mmap.c                | 6 ++++++
>>  2 files changed, 9 insertions(+)
>>
>> v2:
>> - Add some blank lines as requested.
>> - Expand special_mapping_close() comment.
>> - Add David's reviewed-by.
>> - Expand change log to capture review discussion.
>>
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 485424979254..78bdfc59abe5 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -1313,6 +1313,9 @@ struct vm_special_mapping {
>>
>>  	int (*mremap)(const struct vm_special_mapping *sm,
>>  		     struct vm_area_struct *new_vma);
>> +
>> +	void (*close)(const struct vm_special_mapping *sm,
>> +		      struct vm_area_struct *vma);
>>  };
>>
>>  enum tlb_flush_reason {
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index d0dfc85b209b..af4dbf0d3bd4 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -3620,10 +3620,16 @@ void vm_stat_account(struct mm_struct *mm, vm_flags_t flags, long npages)
>>  static vm_fault_t special_mapping_fault(struct vm_fault *vmf);
>>
>>  /*
>> + * Close hook, called for unmap() and on the old vma for mremap().
>> + *
>>   * Having a close hook prevents vma merging regardless of flags.
>>   */
>>  static void special_mapping_close(struct vm_area_struct *vma)
>>  {
>> +	const struct vm_special_mapping *sm = vma->vm_private_data;
>> +
>> +	if (sm->close)
>> +		sm->close(sm, vma);
>>  }
>>
>>  static const char *special_mapping_name(struct vm_area_struct *vma)
>> --
>> 2.45.2
>>
>
> This change is now in -next and I bisected a crash that our CI sees with
> ARCH=um to it:

I see another crash on s390, which seems related, but rather an issue in
uprobe. This can be reproduced by

# cd linux-next/tools/testing/selftests/ftrace
# ./ftracetest ./test.d/dynevent/add_remove_uprobe.tc

The 'mm: Add optional close() to struct vm_special_mapping' patch just
makes it visible. I enabled KASAN, and that shows me:

[   44.505448] ==================================================================                                                                      20:37:27 [3421/145075]
[   44.505455] BUG: KASAN: slab-use-after-free in special_mapping_close+0x9c/0xc8
[   44.505471] Read of size 8 at addr 00000000868dac48 by task sh/1384
[   44.505479]
[   44.505486] CPU: 51 UID: 0 PID: 1384 Comm: sh Not tainted 6.11.0-rc6-next-20240902-dirty #1496
[   44.505503] Hardware name: IBM 3931 A01 704 (z/VM 7.3.0)
[   44.505508] Call Trace:
[   44.505511]  [<000b0324d2f78080>] dump_stack_lvl+0xd0/0x108
[   44.505521]  [<000b0324d2f5435c>] print_address_description.constprop.0+0x34/0x2e0
[   44.505529]  [<000b0324d2f5464c>] print_report+0x44/0x138
[   44.505536]  [<000b0324d1383192>] kasan_report+0xc2/0x140
[   44.505543]  [<000b0324d2f52904>] special_mapping_close+0x9c/0xc8
[   44.505550]  [<000b0324d12c7978>] remove_vma+0x78/0x120
[   44.505557]  [<000b0324d128a2c6>] exit_mmap+0x326/0x750
[   44.505563]  [<000b0324d0ba655a>] __mmput+0x9a/0x370
[   44.505570]  [<000b0324d0bbfbe0>] exit_mm+0x240/0x340
[   44.505575]  [<000b0324d0bc0228>] do_exit+0x548/0xd70
[   44.505580]  [<000b0324d0bc1102>] do_group_exit+0x132/0x390
[   44.505586]  [<000b0324d0bc13b6>] __s390x_sys_exit_group+0x56/0x60
[   44.505592]  [<000b0324d0adcbd6>] do_syscall+0x2f6/0x430
[   44.505599]  [<000b0324d2f78434>] __do_syscall+0xa4/0x170
[   44.505606]  [<000b0324d2f9454c>] system_call+0x74/0x98
[   44.505614]
[   44.505616] Allocated by task 1384:
[   44.505621]  kasan_save_stack+0x40/0x70
[   44.505630]  kasan_save_track+0x28/0x40
[   44.505636]  __kasan_kmalloc+0xa0/0xc0
[   44.505642]  __create_xol_area+0xfa/0x410
[   44.505648]  get_xol_area+0xb0/0xf0
[   44.505652]  uprobe_notify_resume+0x27a/0x470
[   44.505657]  irqentry_exit_to_user_mode+0x15e/0x1d0
[   44.505664]  pgm_check_handler+0x122/0x170
[   44.505670]
[   44.505672] Freed by task 1384:
[   44.505676]  kasan_save_stack+0x40/0x70
[   44.505682]  kasan_save_track+0x28/0x40
[   44.505687]  kasan_save_free_info+0x4a/0x70
[   44.505693]  __kasan_slab_free+0x5a/0x70
[   44.505698]  kfree+0xe8/0x3f0
[   44.505704]  __mmput+0x20/0x370
[   44.505709]  exit_mm+0x240/0x340
[   44.505713]  do_exit+0x548/0xd70
[   44.505718]  do_group_exit+0x132/0x390
[   44.505722]  __s390x_sys_exit_group+0x56/0x60
[   44.505727]  do_syscall+0x2f6/0x430
[   44.505732]  __do_syscall+0xa4/0x170
[   44.505738]  system_call+0x74/0x98

The problem is that uprobe_clear_state() kfree's struct xol_area, which
contains struct vm_special_mapping *xol_mapping. This one is passed to
_install_special_mapping() in xol_add_vma().

__mput reads:

static inline void __mmput(struct mm_struct *mm)
{
	VM_BUG_ON(atomic_read(&mm->mm_users));

	uprobe_clear_state(mm);
	exit_aio(mm);
	ksm_exit(mm);
	khugepaged_exit(mm); /* must run before exit_mmap */
	exit_mmap(mm);
	...
}

So uprobe_clear_state() in the beginning free's the memory area
containing the vm_special_mapping data, but exit_mmap() uses this
address later via vma->vm_private_data (which was set in _install_special_mapping().

The following change fixes this for me, but i'm not sure about any side
effects:

diff --git a/kernel/fork.c b/kernel/fork.c
index df8e4575ff01..cfcabba36c93 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1340,11 +1340,11 @@ static inline void __mmput(struct mm_struct *mm)
 {
        VM_BUG_ON(atomic_read(&mm->mm_users));
 
-       uprobe_clear_state(mm);
        exit_aio(mm);
        ksm_exit(mm);
        khugepaged_exit(mm); /* must run before exit_mmap */
        exit_mmap(mm);
+       uprobe_clear_state(mm);
        mm_put_huge_zero_folio(mm);
        set_mm_exe_file(mm, NULL);
        if (!list_empty(&mm->mmlist)) {

Any thoughts?