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[85.23.48.202]) by smtp.gmail.com with ESMTPSA id f27-20020ac2533b000000b004db3e445f1fsm2382730lfh.97.2023.04.26.00.10.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Apr 2023 00:10:15 -0700 (PDT) Message-ID: Date: Wed, 26 Apr 2023 10:10:14 +0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH v4] mm/gup: disallow GUP writing to file-backed mappings by default Content-Language: en-US To: Lorenzo Stoakes Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , Jens Axboe , Matthew Wilcox , Dennis Dalessandro , Leon Romanovsky , Christian Benvenuti , Nelson Escobar , Bernard Metzler , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , Bjorn Topel , Magnus Karlsson , Maciej Fijalkowski , Jonathan Lemon , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Christian Brauner , Richard Cochran , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org, Oleg Nesterov , Jason Gunthorpe , John Hubbard , Jan Kara , "Kirill A . Shutemov" , Pavel Begunkov , David Hildenbrand References: <3b92d56f55671a0389252379237703df6e86ea48.1682464032.git.lstoakes@gmail.com> <5ffd7f32-d236-4da4-93f7-c2fe39a6e035@lucifer.local> From: =?UTF-8?Q?Mika_Penttil=c3=a4?= In-Reply-To: <5ffd7f32-d236-4da4-93f7-c2fe39a6e035@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 26.4.2023 10.00, Lorenzo Stoakes wrote: > On Wed, Apr 26, 2023 at 06:18:38AM +0300, Mika Penttilä wrote: >> 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 >>> Signed-off-by: Lorenzo Stoakes >>> --- >>> 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. > > The issue is reproduced simply by page_to_virt(pinned_page)[0] = 'x' :) > > The problem is that no faulting actually occurs, so no writenotify, and no Could you elaborate? GUP calls handle_mm_fault() that invokes the write notify the pte is made first writable. Of course, virt(pinned_page)[0] = 'x' is not supposed to fault while using the kernel mapping. > PG_dirty tracking does either. Unexpected page dirtying can occur even > after they are cleaned in folio_clear_dirty_for_io(), because the caller > might manually mark the page dirty at an unexpected time as with the > unpin_*dirty*() helpers. > > I think the long-term solution is to provide a different interface where > pages are passed back briefly with locks held and with a manual invocation > of writeprotect, or perhaps some kthread_use_mm() thing so we actually > trigger the faulting logic, but in the meantime this change helps restore > some sanity. > >> >> 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()) > > This has nothing to do with FOLL_FORCE. > >> >> Also for longterm pinning gups vs fork vs swap there has been fixes by david >> recently. > > I don't think these are relevant in any way to this issue. > >> >> >> >>> /* >>> * 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 >> >