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 <[email protected]>
---
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;
}
return 0;
}
--
2.38.1
On Nov 7, 2022, at 8:17 AM, David Hildenbrand <[email protected]> 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 <[email protected]>
> ---
> 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.
On 07.11.22 20:03, Nadav Amit wrote:
> On Nov 7, 2022, at 8:17 AM, David Hildenbrand <[email protected]> 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 <[email protected]>
>> ---
>> 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.
--
Thanks,
David / dhildenb
On Nov 7, 2022, at 11:27 AM, David Hildenbrand <[email protected]> wrote:
> !! External Email
>
> On 07.11.22 20:03, Nadav Amit wrote:
>> On Nov 7, 2022, at 8:17 AM, David Hildenbrand <[email protected]> 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 <[email protected]>
>>> ---
>>> 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.