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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id k15-20020a5d66cf000000b00228d52b935asm21570694wrw.71.2022.12.08.03.45.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 08 Dec 2022 03:45:26 -0800 (PST) Message-ID: <83259ac7-1aa4-e186-43d9-2b280795e510@redhat.com> Date: Thu, 8 Dec 2022 12:45:26 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.0 Subject: Re: [PATCH v1] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA Content-Language: en-US To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, Ives van Hoorne , Peter Xu , stable@vger.kernel.org, Andrew Morton , Hugh Dickins , Alistair Popple , Mike Rapoport , Nadav Amit , Andrea Arcangeli References: <20221208114137.35035-1-david@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20221208114137.35035-1-david@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On 08.12.22 12:41, David Hildenbrand wrote: > Currently, we don't enable writenotify when enabling userfaultfd-wp on > a shared writable mapping (for now only shmem and hugetlb). The consequence > is that vma->vm_page_prot will still include write permissions, to be set > as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting, > page migration, ...). > > So far, vma->vm_page_prot is assumed to be a safe default, meaning that > we only add permissions (e.g., mkwrite) but not remove permissions (e.g., > wrprotect). For example, when enabling softdirty tracking, we enable > writenotify. With uffd-wp on shared mappings, that changed. More details > on vma->vm_page_prot semantics were summarized in [1]. > > This is problematic for uffd-wp: we'd have to manually check for > a uffd-wp PTEs/PMDs and manually write-protect PTEs/PMDs, which is error > prone. Prone to such issues is any code that uses vma->vm_page_prot to set > PTE permissions: primarily pte_modify() and mk_pte(). > > Instead, let's enable writenotify such that PTEs/PMDs/... will be mapped > write-protected as default and we will only allow selected PTEs that are > definitely safe to be mapped without write-protection (see > can_change_pte_writable()) to be writable. In the future, we might want > to enable write-bit recovery -- e.g., can_change_pte_writable() -- at > more locations, for example, also when removing uffd-wp protection. > > This fixes two known cases: > > (a) remove_migration_pte() mapping uffd-wp'ed PTEs writable, resulting > in uffd-wp not triggering on write access. > (b) do_numa_page() / do_huge_pmd_numa_page() mapping uffd-wp'ed PTEs/PMDs > writable, resulting in uffd-wp not triggering on write access. > > Note that do_numa_page() / do_huge_pmd_numa_page() can be reached even > without NUMA hinting (which currently doesn't seem to be applicable to > shmem), for example, by using uffd-wp with a PROT_WRITE shmem VMA. > On such a VMA, userfaultfd-wp is currently non-functional. > > Note that when enabling userfaultfd-wp, there is no need to walk page > tables to enforce the new default protection for the PTEs: we know that > they cannot be uffd-wp'ed yet, because that can only happen after > enabling uffd-wp for the VMA in general. > > Also note that this makes mprotect() on ranges with uffd-wp'ed PTEs not > accidentally set the write bit -- which would result in uffd-wp not > triggering on later write access. This commit makes uffd-wp on shmem behave > just like uffd-wp on anonymous memory (iow, less special) in that regard, > even though, mixing mprotect with uffd-wp is controversial. > > [1] https://lkml.kernel.org/r/92173bad-caa3-6b43-9d1e-9a471fdbc184@redhat.com > > Reported-by: Ives van Hoorne > Debugged-by: Peter Xu > Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs") > Cc: stable@vger.kernel.org > Cc: Andrew Morton > Cc: Hugh Dickins No idea how a wrong mail address from Hugh sneaked in 2 (I assume, copy-paste issue from de1ccfb64824). Let's properly cc him and keep the full patch. > Cc: Alistair Popple > Cc: Mike Rapoport > Cc: Nadav Amit > Cc: Andrea Arcangeli > Signed-off-by: David Hildenbrand > --- > > As discussed in [2], this is supposed to replace the fix by Peter: > [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover > pte > > This survives vm/selftests and my reproducers: > * migrating pages that are uffd-wp'ed using mbind() on a machine with 2 > NUMA nodes > * Using a PROT_WRITE mapping with uffd-wp > * Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and > mprotect()'ing it PROT_WRITE > * Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and > temporarily mprotect()'ing it PROT_READ > > uffd-wp properly triggers in all cases. On v8.1-rc8, all mre reproducers > fail. > > It would be good to get some more testing feedback and review. > > [2] https://lkml.kernel.org/r/20221202122748.113774-1-david@redhat.com > > --- > fs/userfaultfd.c | 28 ++++++++++++++++++++++------ > mm/mmap.c | 4 ++++ > 2 files changed, 26 insertions(+), 6 deletions(-) > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > index 98ac37e34e3d..fb0733f2e623 100644 > --- a/fs/userfaultfd.c > +++ b/fs/userfaultfd.c > @@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx) > return ctx->features & UFFD_FEATURE_INITIALIZED; > } > > +static void userfaultfd_set_vm_flags(struct vm_area_struct *vma, > + vm_flags_t flags) > +{ > + const bool uffd_wp = !!((vma->vm_flags | flags) & VM_UFFD_WP); > + > + vma->vm_flags = flags; > + /* > + * For shared mappings, we want to enable writenotify while > + * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply > + * recalculate vma->vm_page_prot whenever userfaultfd-wp is involved. > + */ > + if ((vma->vm_flags & VM_SHARED) && uffd_wp) > + vma_set_page_prot(vma); > +} > + > static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode, > int wake_flags, void *key) > { > @@ -618,7 +633,8 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx, > for_each_vma(vmi, vma) { > if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) { > vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; > - vma->vm_flags &= ~__VM_UFFD_FLAGS; > + userfaultfd_set_vm_flags(vma, > + vma->vm_flags & ~__VM_UFFD_FLAGS); > } > } > mmap_write_unlock(mm); > @@ -652,7 +668,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs) > octx = vma->vm_userfaultfd_ctx.ctx; > if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) { > vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; > - vma->vm_flags &= ~__VM_UFFD_FLAGS; > + userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS); > return 0; > } > > @@ -733,7 +749,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma, > } else { > /* Drop uffd context if remap feature not enabled */ > vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; > - vma->vm_flags &= ~__VM_UFFD_FLAGS; > + userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS); > } > } > > @@ -895,7 +911,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file) > prev = vma; > } > > - vma->vm_flags = new_flags; > + userfaultfd_set_vm_flags(vma, new_flags); > vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; > } > mmap_write_unlock(mm); > @@ -1463,7 +1479,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx, > * the next vma was merged into the current one and > * the current one has not been updated yet. > */ > - vma->vm_flags = new_flags; > + userfaultfd_set_vm_flags(vma, new_flags); > vma->vm_userfaultfd_ctx.ctx = ctx; > > if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma)) > @@ -1651,7 +1667,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx, > * the next vma was merged into the current one and > * the current one has not been updated yet. > */ > - vma->vm_flags = new_flags; > + userfaultfd_set_vm_flags(vma, new_flags); > vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; > > skip: > diff --git a/mm/mmap.c b/mm/mmap.c > index a5eb2f175da0..6033d20198b0 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -1525,6 +1525,10 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot) > if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma)) > return 1; > > + /* Do we need write faults for uffd-wp tracking? */ > + if (userfaultfd_wp(vma)) > + return 1; > + > /* Specialty mapping? */ > if (vm_flags & VM_PFNMAP) > return 0; > > base-commit: 8ed710da2873c2aeb3bb805864a699affaf1d03b -- Thanks, David / dhildenb