Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by default

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Mon, Apr 24, 2023 at 06:41:38AM +0300, Mika Penttilä wrote:
>
> Hi,
>
>
> On 22.4.2023 16.37, Lorenzo Stoakes wrote:
> > It isn't safe to write to file-backed mappings as GUP does not ensure that
> > the semantics associated with such a write are performed correctly, for
> > instance filesystems which rely upon write-notify will not be correctly
> > notified.
> >
> > There are exceptions to this - shmem and hugetlb mappings are (in effect)
> > anonymous mappings by other names so we do permit this operation in these
> > cases.
> >
> > In addition, if no pinning takes place (neither FOLL_GET nor FOLL_PIN is
> > specified and neither flags gets implicitly set) then no writing can occur
> > so we do not perform the check in this instance.
> >
> > This is an important exception, as populate_vma_page_range() invokes
> > __get_user_pages() in this way (and thus so does __mm_populate(), used by
> > MAP_POPULATE mmap() and mlock() invocations).
> >
> > There are GUP users within the kernel that do nevertheless rely upon this
> > behaviour, so we introduce the FOLL_ALLOW_BROKEN_FILE_MAPPING flag to
> > explicitly permit this kind of GUP access.
> >
> > This is required in order to not break userspace in instances where the
> > uAPI might permit file-mapped addresses - a number of RDMA users require
> > this for instance, as do the process_vm_[read/write]v() system calls,
> > /proc/$pid/mem, ptrace and SDT uprobes. Each of these callers have been
> > updated to use this flag.
> >
> > Making this change is an important step towards a more reliable GUP, and
> > explicitly indicates which callers might encouter issues moving forward.
> >
> > Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
> > Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> > ---
> >   drivers/infiniband/hw/qib/qib_user_pages.c |  3 +-
> >   drivers/infiniband/hw/usnic/usnic_uiom.c   |  2 +-
> >   drivers/infiniband/sw/siw/siw_mem.c        |  3 +-
> >   fs/proc/base.c                             |  3 +-
> >   include/linux/mm_types.h                   |  8 +++++
> >   kernel/events/uprobes.c                    |  3 +-
> >   mm/gup.c                                   | 36 +++++++++++++++++++++-
> >   mm/memory.c                                |  3 +-
> >   mm/process_vm_access.c                     |  2 +-
> >   net/xdp/xdp_umem.c                         |  2 +-
> >   10 files changed, 56 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/infiniband/hw/qib/qib_user_pages.c b/drivers/infiniband/hw/qib/qib_user_pages.c
> > index f693bc753b6b..b9019dad8008 100644
> > --- a/drivers/infiniband/hw/qib/qib_user_pages.c
> > +++ b/drivers/infiniband/hw/qib/qib_user_pages.c
> > @@ -110,7 +110,8 @@ int qib_get_user_pages(unsigned long start_page, size_t num_pages,
> >   	for (got = 0; got < num_pages; got += ret) {
> >   		ret = pin_user_pages(start_page + got * PAGE_SIZE,
> >   				     num_pages - got,
> > -				     FOLL_LONGTERM | FOLL_WRITE,
> > +				     FOLL_LONGTERM | FOLL_WRITE |
> > +				     FOLL_ALLOW_BROKEN_FILE_MAPPING,
> >   				     p + got, NULL);
> >   		if (ret < 0) {
> >   			mmap_read_unlock(current->mm);
> > diff --git a/drivers/infiniband/hw/usnic/usnic_uiom.c b/drivers/infiniband/hw/usnic/usnic_uiom.c
> > index 2a5cac2658ec..33cf79b248a9 100644
> > --- a/drivers/infiniband/hw/usnic/usnic_uiom.c
> > +++ b/drivers/infiniband/hw/usnic/usnic_uiom.c
> > @@ -85,7 +85,7 @@ static int usnic_uiom_get_pages(unsigned long addr, size_t size, int writable,
> >   				int dmasync, struct usnic_uiom_reg *uiomr)
> >   {
> >   	struct list_head *chunk_list = &uiomr->chunk_list;
> > -	unsigned int gup_flags = FOLL_LONGTERM;
> > +	unsigned int gup_flags = FOLL_LONGTERM | FOLL_ALLOW_BROKEN_FILE_MAPPING;
> >   	struct page **page_list;
> >   	struct scatterlist *sg;
> >   	struct usnic_uiom_chunk *chunk;
> > diff --git a/drivers/infiniband/sw/siw/siw_mem.c b/drivers/infiniband/sw/siw/siw_mem.c
> > index f51ab2ccf151..bc3e8c0898e5 100644
> > --- a/drivers/infiniband/sw/siw/siw_mem.c
> > +++ b/drivers/infiniband/sw/siw/siw_mem.c
> > @@ -368,7 +368,8 @@ struct siw_umem *siw_umem_get(u64 start, u64 len, bool writable)
> >   	struct mm_struct *mm_s;
> >   	u64 first_page_va;
> >   	unsigned long mlock_limit;
> > -	unsigned int foll_flags = FOLL_LONGTERM;
> > +	unsigned int foll_flags =
> > +		FOLL_LONGTERM | FOLL_ALLOW_BROKEN_FILE_MAPPING;
> >   	int num_pages, num_chunks, i, rv = 0;
> >   	if (!can_do_mlock())
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 96a6a08c8235..3e3f5ea9849f 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -855,7 +855,8 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
> >   	if (!mmget_not_zero(mm))
> >   		goto free;
> > -	flags = FOLL_FORCE | (write ? FOLL_WRITE : 0);
> > +	flags = FOLL_FORCE | FOLL_ALLOW_BROKEN_FILE_MAPPING |
> > +		(write ? FOLL_WRITE : 0);
> >   	while (count > 0) {
> >   		size_t this_len = min_t(size_t, count, PAGE_SIZE);
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 3fc9e680f174..e76637b4c78f 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -1185,6 +1185,14 @@ enum {
> >   	FOLL_PCI_P2PDMA = 1 << 10,
> >   	/* allow interrupts from generic signals */
> >   	FOLL_INTERRUPTIBLE = 1 << 11,
> > +	/*
> > +	 * By default we disallow write access to known broken file-backed
> > +	 * memory mappings (i.e. anything other than hugetlb/shmem
> > +	 * mappings). Some code may rely upon being able to access this
> > +	 * regardless for legacy reasons, thus we provide a flag to indicate
> > +	 * this.
> > +	 */
> > +	FOLL_ALLOW_BROKEN_FILE_MAPPING = 1 << 12,
> >   	/* See also internal only FOLL flags in mm/internal.h */
> >   };
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index 59887c69d54c..ec330d3b0218 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -373,7 +373,8 @@ __update_ref_ctr(struct mm_struct *mm, unsigned long vaddr, short d)
> >   		return -EINVAL;
> >   	ret = get_user_pages_remote(mm, vaddr, 1,
> > -			FOLL_WRITE, &page, &vma, NULL);
> > +				    FOLL_WRITE | FOLL_ALLOW_BROKEN_FILE_MAPPING,
> > +				    &page, &vma, NULL);
> >   	if (unlikely(ret <= 0)) {
> >   		/*
> >   		 * We are asking for 1 page. If get_user_pages_remote() fails,
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 1f72a717232b..68d5570c0bae 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -959,16 +959,46 @@ static int faultin_page(struct vm_area_struct *vma,
> >   	return 0;
> >   }
> > +/*
> > + * Writing to file-backed mappings using GUP is a fundamentally broken operation
> > + * as kernel write access to GUP mappings may not adhere to the semantics
> > + * expected by a file system.
> > + *
> > + * In most instances we disallow this broken behaviour, however there are some
> > + * exceptions to this enforced here.
> > + */
> > +static inline bool can_write_file_mapping(struct vm_area_struct *vma,
> > +					  unsigned long gup_flags)
> > +{
> > +	struct file *file = vma->vm_file;
> > +
> > +	/* If we aren't pinning then no problematic write can occur. */
> > +	if (!(gup_flags & (FOLL_GET | FOLL_PIN)))
> > +		return true;
> > +
> > +	/* Special mappings should pose no problem. */
> > +	if (!file)
> > +		return true;
> > +
> > +	/* Has the caller explicitly indicated this case is acceptable? */
> > +	if (gup_flags & FOLL_ALLOW_BROKEN_FILE_MAPPING)
> > +		return true;
> > +
> > +	/* shmem and hugetlb mappings do not have problematic semantics. */
> > +	return vma_is_shmem(vma) || is_file_hugepages(file);
> > +}
> > +
> >   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 +1008,10 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
> >   		return -EFAULT;
> >   	if (write) {
> > +		if (!vma_anon &&
> > +		    WARN_ON_ONCE(!can_write_file_mapping(vma, gup_flags)))
> > +			return -EFAULT;
> > +
> >   		if (!(vm_flags & VM_WRITE)) {
> >   			if (!(gup_flags & FOLL_FORCE))
> >   				return -EFAULT;
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 146bb94764f8..e3d535991548 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5683,7 +5683,8 @@ int access_process_vm(struct task_struct *tsk, unsigned long addr,
> >   	if (!mm)
> >   		return 0;
> > -	ret = __access_remote_vm(mm, addr, buf, len, gup_flags);
> > +	ret = __access_remote_vm(mm, addr, buf, len,
> > +				 gup_flags | FOLL_ALLOW_BROKEN_FILE_MAPPING);
> >   	mmput(mm);
> > diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c
> > index 78dfaf9e8990..ef126c08e89c 100644
> > --- a/mm/process_vm_access.c
> > +++ b/mm/process_vm_access.c
> > @@ -81,7 +81,7 @@ static int process_vm_rw_single_vec(unsigned long addr,
> >   	ssize_t rc = 0;
> >   	unsigned long max_pages_per_loop = PVM_MAX_KMALLOC_PAGES
> >   		/ sizeof(struct pages *);
> > -	unsigned int flags = 0;
> > +	unsigned int flags = FOLL_ALLOW_BROKEN_FILE_MAPPING;
> >   	/* Work out address and page range required */
> >   	if (len == 0)
> > diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
> > index 02207e852d79..b93cfcaccb0d 100644
> > --- a/net/xdp/xdp_umem.c
> > +++ b/net/xdp/xdp_umem.c
> > @@ -93,7 +93,7 @@ void xdp_put_umem(struct xdp_umem *umem, bool defer_cleanup)
> >   static int xdp_umem_pin_pages(struct xdp_umem *umem, unsigned long address)
> >   {
> > -	unsigned int gup_flags = FOLL_WRITE;
> > +	unsigned int gup_flags = FOLL_WRITE | FOLL_ALLOW_BROKEN_FILE_MAPPING;
> >   	long npgs;
> >   	int err;
>
> Not sure about this in general, but seemss at least ptrace
> (ptrace_access_vm()) seems to be broken here..

Ah thanks, that was an oversight as it uses __access_remote_vm() rather
than access_process_vm(). I had carefully examined both (and all other GUP
callers) but in supplying the flag for the latter I in typically squeezy
brained human fashion forgot to also do so for the former, will respin
accordingly.

>
>
> --Mika
>
>