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SubjectRe: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12)
----- On Apr 3, 2018, at 12:36 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> ----- On Apr 2, 2018, at 11:33 AM, Mathieu Desnoyers
> mathieu.desnoyers@efficios.com wrote:
>
>> ----- On Apr 1, 2018, at 12:13 PM, One Thousand Gnomes
>> gnomes@lxorguk.ukuu.org.uk wrote:
>>
[...]
>>> I still like the idea it's just the latencies concern me.
>>
[...]
>
> Looking into this a bit more, I notice the following: The pgprot_noncached
> (_PAGE_NOCACHE on x86) pgprot is part of the vma->vm_page_prot. Therefore,
> in order to have userspace provide pointers to noncached pages as input
> to cpu_opv, they need to be part of a userspace vma which has a
> pgprot_noncached vm_page_prot.
>
> The cpu_opv system call uses get_user_pages_fast() to grab the struct page
> from the userspace addresses, and then passes those pages to vm_map_ram(),
> with a PAGE_KERNEL pgprot. This creates a temporary kernel mapping to those
> pages, which is then used to read/write from/to those pages with preemption
> disabled.
>
> Therefore, with the proposed cpu_opv implementation, the kernel is not
> touching noncached mappings with preemption disabled, which should take
> care of your latency concern.

[...]

The following extra check should let userspace know it's trying to
provide a pointer to noncached memory by returning -1, errno=EFAULT.

Is the approach acceptable ?

Thanks,

Mathieu

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ad06d42..0245481 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2425,6 +2425,18 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
return follow_page_mask(vma, address, foll_flags, &unused_page_mask);
}

+static inline bool is_vma_noncached(struct vm_area_struct *vma)
+{
+ pgprot_t pgprot = vma->vm_page_prot;
+
+ /* Check whether architecture implements noncached pages. */
+ if (pgprot_val(pgprot_noncached(PAGE_KERNEL)) == pgprot_val(PAGE_KERNEL))
+ return false;
+ if (pgprot_val(pgprot) != pgprot_val(pgprot_noncached(pgprot)))
+ return false;
+ return true;
+}
+
#define FOLL_WRITE 0x01 /* check pte is writable */
#define FOLL_TOUCH 0x02 /* mark page accessed */
#define FOLL_GET 0x04 /* do get_page on page */
diff --git a/kernel/cpu_opv.c b/kernel/cpu_opv.c
index 197339e..e4395b4 100644
--- a/kernel/cpu_opv.c
+++ b/kernel/cpu_opv.c
@@ -362,7 +362,19 @@ static int cpu_op_pin_pages(unsigned long addr, unsigned long len,
int ret, nr_pages, nr_put_pages, n;
unsigned long _vaddr;
struct vaddr *va;
+ struct vm_area_struct *vma;

+ vma = find_vma_intersection(current->mm, addr, addr + len);
+ if (!vma)
+ return -EFAULT;
+ /*
+ * cpu_opv() accesses its own cached mapping of the userspace pages.
+ * Considering that concurrent noncached and cached accesses may yield
+ * to unexpected results in terms of memory consistency, explicitly
+ * disallow cpu_opv on noncached memory.
+ */
+ if (is_vma_noncached(vma))
+ return -EFAULT;
nr_pages = cpu_op_count_pages(addr, len);
if (!nr_pages)
return 0;
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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