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Date: Wed, 26 Apr 2023 06:18:38 +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,
Oleg Nesterov <oleg@...hat.com>,
Jason Gunthorpe <jgg@...dia.com>,
John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
"Kirill A . Shutemov" <kirill@...temov.name>,
Pavel Begunkov <asml.silence@...il.com>
Subject: Re: [PATCH v4] mm/gup: disallow GUP writing to file-backed mappings
by default
Hi,
On 26.4.2023 2.15, Lorenzo Stoakes wrote:
> GUP does not correctly implement write-notify semantics, nor does it
> guarantee that the underlying pages are correctly dirtied, which could lead
> to a kernel oops or data corruption when writing to file-backed mappings.
>
> This is only relevant when the mappings are file-backed and the underlying
> file system requires folio dirty tracking. File systems which do not, such
> as shmem or hugetlb, are not at risk and therefore can be written to
> without issue.
>
> Unfortunately this limitation of GUP has been present for some time and
> requires future rework of the GUP API in order to provide correct write
> access to such mappings.
>
> In the meantime, we add a check for the most broken GUP case -
> FOLL_LONGTERM - which really under no circumstances can safely access
> dirty-tracked file mappings.
>
> As part of this change we separate out vma_needs_dirty_tracking() as a
> helper function to determine this, which is distinct from
> vma_wants_writenotify() which is specific to determining which PTE flags to
> set.
>
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
> v4:
> - Split out vma_needs_dirty_tracking() from vma_wants_writenotify() to reduce
> duplication and update to use this in the GUP check. Note that both separately
> check vm_ops_needs_writenotify() as the latter needs to test this before the
> vm_pgprot_modify() test, resulting in vma_wants_writenotify() checking this
> twice, however it is such a small check this should not be egregious.
>
> v3:
> - Rebased on latest mm-unstable as of 24th April 2023.
> - Explicitly check whether file system requires folio dirtying. Note that
> vma_wants_writenotify() could not be used directly as it is very much focused
> on determining if the PTE r/w should be set (e.g. assuming private mapping
> does not require it as already set, soft dirty considerations).
> - Tested code against shmem and hugetlb mappings - confirmed that these are not
> disallowed by the check.
> - Eliminate FOLL_ALLOW_BROKEN_FILE_MAPPING flag and instead perform check only
> for FOLL_LONGTERM pins.
> - As a result, limit check to internal GUP code.
> https://lore.kernel.org/all/23c19e27ef0745f6d3125976e047ee0da62569d4.1682406295.git.lstoakes@gmail.com/
>
> v2:
> - Add accidentally excluded ptrace_access_vm() use of
> FOLL_ALLOW_BROKEN_FILE_MAPPING.
> - Tweak commit message.
> https://lore.kernel.org/all/c8ee7e02d3d4f50bb3e40855c53bda39eec85b7d.1682321768.git.lstoakes@gmail.com/
>
> v1:
> https://lore.kernel.org/all/f86dc089b460c80805e321747b0898fd1efe93d7.1682168199.git.lstoakes@gmail.com/
>
> include/linux/mm.h | 1 +
> mm/gup.c | 26 +++++++++++++++++++++++++-
> mm/mmap.c | 37 ++++++++++++++++++++++++++++---------
> 3 files changed, 54 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..53652453037c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -959,16 +959,37 @@ 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
> + * may not adhere to the semantics expected by a file system.
> + */
> +static inline bool can_write_file_mapping(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;
> +
> + /* 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 +999,9 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
> return -EFAULT;
>
> if (write) {
> + if (!vma_anon && !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/mmap.c b/mm/mmap.c
> index 536bbb8fa0ae..aac638dd22cf 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1475,6 +1475,32 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
> }
> #endif /* __ARCH_WANT_SYS_OLD_MMAP */
>
> +/* Do VMA operations imply write notify is required? */
> +static inline bool vm_ops_needs_writenotify(
> + const struct vm_operations_struct *vm_ops)
> +{
> + return vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite);
> +}
> +
> +/*
> + * Does this VMA require the underlying folios to have their dirty state
> + * tracked?
> + */
> +bool vma_needs_dirty_tracking(struct vm_area_struct *vma)
> +{
> + /* Does the filesystem need to be notified? */
> + if (vm_ops_needs_writenotify(vma->vm_ops))
> + return true;
> +
> + /* Specialty mapping? */
> + if (vma->vm_flags & VM_PFNMAP)
> + return false;
> +
> + /* Can the mapping track the dirty pages? */
> + return vma->vm_file && vma->vm_file->f_mapping &&
> + mapping_can_writeback(vma->vm_file->f_mapping);
> +}
> +
What would be the exact reproducer of the problem? AFAIK writenotify is
handled (by handle_mm_fault()) for non cow mappings (shared), where it
only matters.
GUP will only allow FOLL_FORCE without faulting for PageAnonExclusive
pages. So if you want something beyond normal cow semantics you have
custom vm_ops (and mmap() and fault())
Also for longterm pinning gups vs fork vs swap there has been fixes by
david recently.
> /*
> * Some shared mappings will want the pages marked read-only
> * to track write events. If so, we'll downgrade vm_page_prot
> @@ -1484,14 +1510,13 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
> int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
> {
> vm_flags_t vm_flags = vma->vm_flags;
> - const struct vm_operations_struct *vm_ops = vma->vm_ops;
>
> /* If it was private or non-writable, the write bit is already clear */
> if ((vm_flags & (VM_WRITE|VM_SHARED)) != ((VM_WRITE|VM_SHARED)))
> return 0;
>
> /* The backer wishes to know when pages are first written to? */
> - if (vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite))
> + if (vm_ops_needs_writenotify(vma->vm_ops))
> return 1;
>
> /* The open routine did something to the protections that pgprot_modify
> @@ -1511,13 +1536,7 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
> if (userfaultfd_wp(vma))
> return 1;
>
> - /* Specialty mapping? */
> - if (vm_flags & VM_PFNMAP)
> - return 0;
> -
> - /* Can the mapping track the dirty pages? */
> - return vma->vm_file && vma->vm_file->f_mapping &&
> - mapping_can_writeback(vma->vm_file->f_mapping);
> + return vma_needs_dirty_tracking(vma);
> }
>
> /*
--Mika
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