diff mbox series

[RFC,05/19] mm: add early FAULT_FLAG_WRITE consistency checks

Message ID 20221107161740.144456-6-david@redhat.com
State New
Headers
Series mm/gup: remove FOLL_FORCE usage from drivers (reliable R/O long-term pinning) |

Commit Message

David Hildenbrand Nov. 7, 2022, 4:17 p.m. UTC 
  Let's catch abuse of FAULT_FLAG_WRITE early, such that we don't have to
care in all other handlers and might get "surprises" if we forget to do
so.

Write faults without VM_MAYWRITE don't make any sense, and our
maybe_mkwrite() logic could have hidden such abuse for now.

Write faults without VM_WRITE on something that is not a COW mapping is
similarly broken, and e.g., do_wp_page() could end up placing an
anonymous page into a shared mapping, which would be bad.

This is a preparation for reliable R/O long-term pinning of pages in
private mappings, whereby we want to make sure that we will never break
COW in a read-only private mapping.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/memory.c | 8 ++++++++
 1 file changed, 8 insertions(+)

Comments

Nadav Amit Nov. 7, 2022, 7:03 p.m. UTC | #1 
On Nov 7, 2022, at 8:17 AM, David Hildenbrand <david@redhat.com> wrote:

> !! External Email
> 
> Let's catch abuse of FAULT_FLAG_WRITE early, such that we don't have to
> care in all other handlers and might get "surprises" if we forget to do
> so.
> 
> Write faults without VM_MAYWRITE don't make any sense, and our
> maybe_mkwrite() logic could have hidden such abuse for now.
> 
> Write faults without VM_WRITE on something that is not a COW mapping is
> similarly broken, and e.g., do_wp_page() could end up placing an
> anonymous page into a shared mapping, which would be bad.
> 
> This is a preparation for reliable R/O long-term pinning of pages in
> private mappings, whereby we want to make sure that we will never break
> COW in a read-only private mapping.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> mm/memory.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index fe131273217a..826353da7b23 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5159,6 +5159,14 @@ static vm_fault_t sanitize_fault_flags(struct vm_area_struct *vma,
>                 */
>                if (!is_cow_mapping(vma->vm_flags))
>                        *flags &= ~FAULT_FLAG_UNSHARE;
> +       } else if (*flags & FAULT_FLAG_WRITE) {
> +               /* Write faults on read-only mappings are impossible ... */
> +               if (WARN_ON_ONCE(!(vma->vm_flags & VM_MAYWRITE)))
> +                       return VM_FAULT_SIGSEGV;
> +               /* ... and FOLL_FORCE only applies to COW mappings. */
> +               if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE) &&
> +                                !is_cow_mapping(vma->vm_flags)))
> +                       return VM_FAULT_SIGSEGV;

Not sure about the WARN_*(). Seems as if it might trigger in benign even if
rare scenarios, e.g., mprotect() racing with page-fault.
David Hildenbrand Nov. 7, 2022, 7:27 p.m. UTC | #2 
On 07.11.22 20:03, Nadav Amit wrote:
> On Nov 7, 2022, at 8:17 AM, David Hildenbrand <david@redhat.com> wrote:
> 
>> !! External Email
>>
>> Let's catch abuse of FAULT_FLAG_WRITE early, such that we don't have to
>> care in all other handlers and might get "surprises" if we forget to do
>> so.
>>
>> Write faults without VM_MAYWRITE don't make any sense, and our
>> maybe_mkwrite() logic could have hidden such abuse for now.
>>
>> Write faults without VM_WRITE on something that is not a COW mapping is
>> similarly broken, and e.g., do_wp_page() could end up placing an
>> anonymous page into a shared mapping, which would be bad.
>>
>> This is a preparation for reliable R/O long-term pinning of pages in
>> private mappings, whereby we want to make sure that we will never break
>> COW in a read-only private mapping.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> mm/memory.c | 8 ++++++++
>> 1 file changed, 8 insertions(+)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index fe131273217a..826353da7b23 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -5159,6 +5159,14 @@ static vm_fault_t sanitize_fault_flags(struct vm_area_struct *vma,
>>                  */
>>                 if (!is_cow_mapping(vma->vm_flags))
>>                         *flags &= ~FAULT_FLAG_UNSHARE;
>> +       } else if (*flags & FAULT_FLAG_WRITE) {
>> +               /* Write faults on read-only mappings are impossible ... */
>> +               if (WARN_ON_ONCE(!(vma->vm_flags & VM_MAYWRITE)))
>> +                       return VM_FAULT_SIGSEGV;
>> +               /* ... and FOLL_FORCE only applies to COW mappings. */
>> +               if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE) &&
>> +                                !is_cow_mapping(vma->vm_flags)))
>> +                       return VM_FAULT_SIGSEGV;
> 
> Not sure about the WARN_*(). Seems as if it might trigger in benign even if
> rare scenarios, e.g., mprotect() racing with page-fault.
> 

We most certainly would want to catch any such broken/racy cases. There 
are no benign cases I could possibly think of.

Page faults need the mmap lock in read. mprotect() / VMA changes need 
the mmap lock in write. Whoever calls handle_mm_fault() is supposed to 
properly check VMA permissions.
Nadav Amit Nov. 7, 2022, 7:50 p.m. UTC | #3 
On Nov 7, 2022, at 11:27 AM, David Hildenbrand <david@redhat.com> wrote:

> !! External Email
> 
> On 07.11.22 20:03, Nadav Amit wrote:
>> On Nov 7, 2022, at 8:17 AM, David Hildenbrand <david@redhat.com> wrote:
>> 
>>> !! External Email
>>> 
>>> Let's catch abuse of FAULT_FLAG_WRITE early, such that we don't have to
>>> care in all other handlers and might get "surprises" if we forget to do
>>> so.
>>> 
>>> Write faults without VM_MAYWRITE don't make any sense, and our
>>> maybe_mkwrite() logic could have hidden such abuse for now.
>>> 
>>> Write faults without VM_WRITE on something that is not a COW mapping is
>>> similarly broken, and e.g., do_wp_page() could end up placing an
>>> anonymous page into a shared mapping, which would be bad.
>>> 
>>> This is a preparation for reliable R/O long-term pinning of pages in
>>> private mappings, whereby we want to make sure that we will never break
>>> COW in a read-only private mapping.
>>> 
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>> ---
>>> mm/memory.c | 8 ++++++++
>>> 1 file changed, 8 insertions(+)
>>> 
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index fe131273217a..826353da7b23 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -5159,6 +5159,14 @@ static vm_fault_t sanitize_fault_flags(struct vm_area_struct *vma,
>>>                 */
>>>                if (!is_cow_mapping(vma->vm_flags))
>>>                        *flags &= ~FAULT_FLAG_UNSHARE;
>>> +       } else if (*flags & FAULT_FLAG_WRITE) {
>>> +               /* Write faults on read-only mappings are impossible ... */
>>> +               if (WARN_ON_ONCE(!(vma->vm_flags & VM_MAYWRITE)))
>>> +                       return VM_FAULT_SIGSEGV;
>>> +               /* ... and FOLL_FORCE only applies to COW mappings. */
>>> +               if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE) &&
>>> +                                !is_cow_mapping(vma->vm_flags)))
>>> +                       return VM_FAULT_SIGSEGV;
>> 
>> Not sure about the WARN_*(). Seems as if it might trigger in benign even if
>> rare scenarios, e.g., mprotect() racing with page-fault.
> 
> We most certainly would want to catch any such broken/racy cases. There
> are no benign cases I could possibly think of.
> 
> Page faults need the mmap lock in read. mprotect() / VMA changes need
> the mmap lock in write. Whoever calls handle_mm_fault() is supposed to
> properly check VMA permissions.

My bad. I now see it. Thanks for explaining.
diff mbox series

Patch

diff --git a/mm/memory.c b/mm/memory.c
index fe131273217a..826353da7b23 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5159,6 +5159,14 @@  static vm_fault_t sanitize_fault_flags(struct vm_area_struct *vma,
 		 */
 		if (!is_cow_mapping(vma->vm_flags))
 			*flags &= ~FAULT_FLAG_UNSHARE;
+	} else if (*flags & FAULT_FLAG_WRITE) {
+		/* Write faults on read-only mappings are impossible ... */
+		if (WARN_ON_ONCE(!(vma->vm_flags & VM_MAYWRITE)))
+			return VM_FAULT_SIGSEGV;
+		/* ... and FOLL_FORCE only applies to COW mappings. */
+		if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE) &&
+				 !is_cow_mapping(vma->vm_flags)))
+			return VM_FAULT_SIGSEGV;
 	}
 	return 0;
 }