Messages in this thread | | | Date | Fri, 28 Apr 2023 17:13:07 +0200 | Subject | Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default | From | David Hildenbrand <> |
| |
[...]
>> This change has the potential to break existing setups. Simple example: >> libvirt domains configured for file-backed VM memory that also has a vfio >> device configured. It can easily be configured by users (evolving VM >> configuration, copy-paste etc.). And it works from a VM perspective, because >> the guest memory is essentially stale once the VM is shutdown and the pages >> were unpinned. At least we're not concerned about stale data on disk. >> >> With your changes, such VMs would no longer start, breaking existing user >> setups with a kernel update. > > Which vfio vm_ops are we talking about? vfio_pci_mmap_ops for example > doesn't specify page_mkwrite or pfn_mkwrite. Unless you mean some arbitrary > file system in the guest?
Sorry, you define a VM to have its memory backed by VM memory and, at the same time, define a vfio-pci device for your VM, which will end up long-term pinning the VM memory.
> > I may well be missing context on this so forgive me if I'm being a little > dumb here, but it'd be good to get a specific example.
I was giving to little details ;)
[...]
>> >> I know, Jason und John will disagree, but I don't think we want to be very >> careful with changing the default. >> >> Sure, we could warn, or convert individual users using a flag (io_uring). >> But maybe we should invest more energy on a fix? > > This is proactively blocking a cleanup (eliminating vmas) that I believe > will be useful in moving things forward. I am not against an opt-in option > (I have been responding to community feedback in adapting my approach), > which is the way I implemented it all the way back then :)
There are alternatives: just use a flag as Jason initially suggested and use that in io_uring code. Then, you can also bail out on the GUP-fast path as "cannot support it right now, never do GUP-fast".
IMHO, this patch is not a prereq.
> > But given we know this is both entirely broken and a potential security > issue, and FOLL_LONGTERM is about as egregious as you can get (user > explicitly saying they'll hold write access indefinitely) I feel it is an > important improvement and makes clear that this is not an acceptable usage. > > I see Jason has said more on this also :) > >> >> >> >> >>> Suggested-by: Jason Gunthorpe <jgg@nvidia.com> >>> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com> >>> --- >>> include/linux/mm.h | 1 + >>> mm/gup.c | 41 ++++++++++++++++++++++++++++++++++++++++- >>> mm/mmap.c | 36 +++++++++++++++++++++++++++--------- >>> 3 files changed, 68 insertions(+), 10 deletions(-) >>> >>> diff --git a/include/linux/mm.h b/include/linux/mm.h >>> index 37554b08bb28..f7da02fc89c6 100644 >>> --- a/include/linux/mm.h >>> +++ b/include/linux/mm.h >>> @@ -2433,6 +2433,7 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma, >>> #define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \ >>> MM_CP_UFFD_WP_RESOLVE) >>> >>> +bool vma_needs_dirty_tracking(struct vm_area_struct *vma); >>> int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot); >>> static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma) >>> { >>> diff --git a/mm/gup.c b/mm/gup.c >>> index 1f72a717232b..d36a5db9feb1 100644 >>> --- a/mm/gup.c >>> +++ b/mm/gup.c >>> @@ -959,16 +959,51 @@ static int faultin_page(struct vm_area_struct *vma, >>> return 0; >>> } >>> >>> +/* >>> + * Writing to file-backed mappings which require folio dirty tracking using GUP >>> + * is a fundamentally broken operation, as kernel write access to GUP mappings >>> + * do not adhere to the semantics expected by a file system. >>> + * >>> + * Consider the following scenario:- >>> + * >>> + * 1. A folio is written to via GUP which write-faults the memory, notifying >>> + * the file system and dirtying the folio. >>> + * 2. Later, writeback is triggered, resulting in the folio being cleaned and >>> + * the PTE being marked read-only. >>> + * 3. The GUP caller writes to the folio, as it is mapped read/write via the >>> + * direct mapping. >>> + * 4. The GUP caller, now done with the page, unpins it and sets it dirty >>> + * (though it does not have to). >>> + * >>> + * This results in both data being written to a folio without writenotify, and >>> + * the folio being dirtied unexpectedly (if the caller decides to do so). >>> + */ >>> +static bool writeable_file_mapping_allowed(struct vm_area_struct *vma, >>> + unsigned long gup_flags) >>> +{ >>> + /* If we aren't pinning then no problematic write can occur. */ >>> + if (!(gup_flags & (FOLL_GET | FOLL_PIN))) >>> + return true; >> >> FOLL_LONGTERM only applies to FOLL_PIN. This check can be dropped. > > I understand that of course (well maybe not of course, but I mean I do, I > have oodles of diagrams referencing this int he book :) This is intended to > document the fact that the check isn't relevant if we don't pin at all, > e.g. reading this you see:- > > - (implicit) if not writing or anon we're good > - if not pin we're good > - ok we are only currently checking one especially egregious case > - finally, perform the dirty tracking check. > > So this is intentional. > >> >>> + >>> + /* We limit this check to the most egregious case - a long term pin. */ >>> + if (!(gup_flags & FOLL_LONGTERM)) >>> + return true; >>> + >>> + /* If the VMA requires dirty tracking then GUP will be problematic. */ >>> + return vma_needs_dirty_tracking(vma); >>> +} >>> + >>> static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) >>> { >>> vm_flags_t vm_flags = vma->vm_flags; >>> int write = (gup_flags & FOLL_WRITE); >>> int foreign = (gup_flags & FOLL_REMOTE); >>> + bool vma_anon = vma_is_anonymous(vma); >>> >>> if (vm_flags & (VM_IO | VM_PFNMAP)) >>> return -EFAULT; >>> >>> - if (gup_flags & FOLL_ANON && !vma_is_anonymous(vma)) >>> + if ((gup_flags & FOLL_ANON) && !vma_anon) >>> return -EFAULT; >>> >>> if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma)) >>> @@ -978,6 +1013,10 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) >>> return -EFAULT; >>> >>> if (write) { >>> + if (!vma_anon && >>> + !writeable_file_mapping_allowed(vma, gup_flags)) >>> + return -EFAULT; >>> + >>> if (!(vm_flags & VM_WRITE)) { >>> if (!(gup_flags & FOLL_FORCE)) >>> return -EFAULT; >>> diff --git a/mm/mmap.c b/mm/mmap.c >>> index 536bbb8fa0ae..7b6344d1832a 100644 >>> --- a/mm/mmap.c >> >> >> I'm probably missing something, why don't we have to handle GUP-fast (having >> said that, it's hard to handle ;) )? The sequence you describe above should >> apply to GUP-fast as well, no? >> >> 1) Pin writable mapped page using GUP-fast >> 2) Trigger writeback >> 3) Write to page via pin >> 4) Unpin and set dirty > > You're right, and this is an excellent point. I worry about other GUP use > cases too, but we're a bit out of luck there because we don't get to check > the VMA _at all_ (which opens yet another Pandora's box about how safe it > is to do unlocked pinning :) > > But again, this comes down to the fact we're trying to make things > _incrementally__ better rather than throwing our hands up and saying one > day my ship will come in...
That's not how security fixes are supposed to work IMHO, sorry.
-- Thanks,
David / dhildenb
| |