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SubjectRe: [PATCH v12 18/31] mm: protect against PTE changes done by dup_mmap()
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Le 22/04/2019 à 22:32, Jerome Glisse a écrit :
> On Tue, Apr 16, 2019 at 03:45:09PM +0200, Laurent Dufour wrote:
>> Vinayak Menon and Ganesh Mahendran reported that the following scenario may
>> lead to thread being blocked due to data corruption:
>>
>> CPU 1 CPU 2 CPU 3
>> Process 1, Process 1, Process 1,
>> Thread A Thread B Thread C
>>
>> while (1) { while (1) { while(1) {
>> pthread_mutex_lock(l) pthread_mutex_lock(l) fork
>> pthread_mutex_unlock(l) pthread_mutex_unlock(l) }
>> } }
>>
>> In the details this happens because :
>>
>> CPU 1 CPU 2 CPU 3
>> fork()
>> copy_pte_range()
>> set PTE rdonly
>> got to next VMA...
>> . PTE is seen rdonly PTE still writable
>> . thread is writing to page
>> . -> page fault
>> . copy the page Thread writes to page
>> . . -> no page fault
>> . update the PTE
>> . flush TLB for that PTE
>> flush TLB PTE are now rdonly
>
> Should the fork be on CPU3 to be consistant with the top thing (just to
> make it easier to read and go from one to the other as thread can move
> from one CPU to another).

Sure, this is quite confusing this way ;)

>>
>> So the write done by the CPU 3 is interfering with the page copy operation
>> done by CPU 2, leading to the data corruption.
>>
>> To avoid this we mark all the VMA involved in the COW mechanism as changing
>> by calling vm_write_begin(). This ensures that the speculative page fault
>> handler will not try to handle a fault on these pages.
>> The marker is set until the TLB is flushed, ensuring that all the CPUs will
>> now see the PTE as not writable.
>> Once the TLB is flush, the marker is removed by calling vm_write_end().
>>
>> The variable last is used to keep tracked of the latest VMA marked to
>> handle the error path where part of the VMA may have been marked.
>>
>> Since multiple VMA from the same mm may have the sequence count increased
>> during this process, the use of the vm_raw_write_begin/end() is required to
>> avoid lockdep false warning messages.
>>
>> Reported-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
>> Reported-by: Vinayak Menon <vinmenon@codeaurora.org>
>> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
>
> A minor comment (see below)
>
> Reviewed-by: Jérome Glisse <jglisse@redhat.com>

Thanks for the review Jérôme.

>> ---
>> kernel/fork.c | 30 ++++++++++++++++++++++++++++--
>> 1 file changed, 28 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index f8dae021c2e5..2992d2c95256 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -462,7 +462,7 @@ EXPORT_SYMBOL(free_task);
>> static __latent_entropy int dup_mmap(struct mm_struct *mm,
>> struct mm_struct *oldmm)
>> {
>> - struct vm_area_struct *mpnt, *tmp, *prev, **pprev;
>> + struct vm_area_struct *mpnt, *tmp, *prev, **pprev, *last = NULL;
>> struct rb_node **rb_link, *rb_parent;
>> int retval;
>> unsigned long charge;
>> @@ -581,8 +581,18 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>> rb_parent = &tmp->vm_rb;
>>
>> mm->map_count++;
>> - if (!(tmp->vm_flags & VM_WIPEONFORK))
>> + if (!(tmp->vm_flags & VM_WIPEONFORK)) {
>> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT)) {
>> + /*
>> + * Mark this VMA as changing to prevent the
>> + * speculative page fault hanlder to process
>> + * it until the TLB are flushed below.
>> + */
>> + last = mpnt;
>> + vm_raw_write_begin(mpnt);
>> + }
>> retval = copy_page_range(mm, oldmm, mpnt);
>> + }
>>
>> if (tmp->vm_ops && tmp->vm_ops->open)
>> tmp->vm_ops->open(tmp);
>> @@ -595,6 +605,22 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>> out:
>> up_write(&mm->mmap_sem);
>> flush_tlb_mm(oldmm);
>> +
>> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT)) {
>
> You do not need to check for CONFIG_SPECULATIVE_PAGE_FAULT as last
> will always be NULL if it is not enabled but maybe the compiler will
> miss the optimization opportunity if you only have the for() loop
> below.

I didn't check for the generated code, perhaps the compiler will be
optimize that correctly.
This being said, I think the if block is better for the code
readability, highlighting that this block is only needed in the case of SPF.

>> + /*
>> + * Since the TLB has been flush, we can safely unmark the
>> + * copied VMAs and allows the speculative page fault handler to
>> + * process them again.
>> + * Walk back the VMA list from the last marked VMA.
>> + */
>> + for (; last; last = last->vm_prev) {
>> + if (last->vm_flags & VM_DONTCOPY)
>> + continue;
>> + if (!(last->vm_flags & VM_WIPEONFORK))
>> + vm_raw_write_end(last);
>> + }
>> + }
>> +
>> up_write(&oldmm->mmap_sem);
>> dup_userfaultfd_complete(&uf);
>> fail_uprobe_end:
>> --
>> 2.21.0
>>
>

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