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