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Date: Mon, 24 Apr 2023 06:41:38 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
Matthew Wilcox <willy@...radead.org>,
Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
Leon Romanovsky <leon@...nel.org>,
Christian Benvenuti <benve@...co.com>,
Nelson Escobar <neescoba@...co.com>,
Bernard Metzler <bmt@...ich.ibm.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Ian Rogers <irogers@...gle.com>,
Adrian Hunter <adrian.hunter@...el.com>,
Bjorn Topel <bjorn@...nel.org>,
Magnus Karlsson <magnus.karlsson@...el.com>,
Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
Jonathan Lemon <jonathan.lemon@...il.com>,
"David S . Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Christian Brauner <brauner@...nel.org>,
Richard Cochran <richardcochran@...il.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Jesper Dangaard Brouer <hawk@...nel.org>,
John Fastabend <john.fastabend@...il.com>,
linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by
default
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@...pe.ca>
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.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..
--Mika
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