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Howlett" , Lu Baolu , "H. Peter Anvin" , Peter Zijlstra , Shakeel Butt , Suren Baghdasaryan , Thomas Gleixner , Toshi Kani , Vishal Moola , Vlastimil Babka , Will Deacon , iommu@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH 3/5] x86/mm/pat: acquire init_mm read lock on attribute change to avoid UAF Message-ID: References: <20260728-cpa-fixes-v1-0-2ed2352300b3@kernel.org> <20260728-cpa-fixes-v1-3-2ed2352300b3@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260728-cpa-fixes-v1-3-2ed2352300b3@kernel.org> X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 8DECC100005 X-Stat-Signature: wpago3a89gk1zq3xcwie3eynkhd3rgie X-Rspam-User: X-HE-Tag: 1785244474-325435 X-HE-Meta: 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 IKZqKjNA 2xssVIaUJWy5+Dq3/2QgX74PSRGbbVRbZ5lzdZi83+943VStX9OqPZRg/5iKywEhJfnB4d/1/hiWHEmC9g8SlcDhwujvNaNM3XZBW7I6HbMPcCzwCxwdmJcMfePKP6vw1BYiNvnQaIAfNwm5xVCxiGwYqB/ZM9YmH4U1djMMR6bKTMo7cpb1TXLr24I6WkUwEi2KHI4rzv33VZU0/kmx1kbpwSuOLSeQxJoQRvShskP7qzhRNFrq9yIi0PMJYAHKmeUmMP2J/tcUEYNnfqVqO6LVM1SQPRiTqeY/uMKtGNiGW9Io+ytDd2ruxR4ps2dkxTVask5YPI6Lr/OO7OpFOB0JjZPk/XcPLRB+c6TUEpe2Bn7yMReY5d/cyBTCC2v1nAbd1Ix/bIxSMrAob9ia/RToubZE/MF64s1CIYTOv+1V5PzI/85qndZLd+Ure0xePt/fCFIbApsZuZMk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jul 28, 2026 at 04:07:46PM +0300, Mike Rapoport wrote: > From: "Lorenzo Stoakes (ARM)" > > A previous commit protected us against races between ptdump and CPA > collapse, however one still exists between attribute changes and collapse > as reported by Denis V. Lunev (linked). > > When an attribute change arises, a lockless page table walker obtains a PTE > entry, which is later written to via set_pte_atomic(): > > ... > -> change_page_attr_set_clr() > -> __change_page_attr_set_clr() > -> __change_page_attr() > -> _lookup_address_cpa() > -> lookup_address_in_pgd_attr() > -> [ lockless page table walker ] > -> set_pte_atomic() > > There is nothing preventing a concurrent CPA collapse which can free the > PTE that was retrieved here, resulting in a use-after-free. > > With the mmap write lock taken on init_mm over CPA collapse, we can now > resolve this race by acquiring an mmap read lock on init_mm over > __change_page_attr_set_clr(). > > This locks across the whole operation over which the walk and the PTE entry > write occurs, solving the race. > > It is safe to do this here, as no spinlocks are held upon entry to > __change_page_attr_set_clr(). > > However, the lock must not be held over an allocation, as allocation can > trigger reclaim and shrinkers may call into CPA recursively, making > deadlocks possible (init_mm -> ... -> fs_reclaim -> init_mm). > > A page table is allocated when a huge page needs to be split: > > -> change_page_attr_set_clr() > -> __change_page_attr_set_clr() > -> __change_page_attr() > -> split_large_page() > [ pagetable_alloc() ] > -> __split_large_page() > > Avoid deadlocks by dropping the mmap lock across pagetable_alloc() in > split_large_page() and track whether this is needed by adding a new > 'init_mm_read_locked' flag to struct cpa_data. > > This is safe as __split_large_page() (called with locks re-established) > revalidates that the page table entry is the same as it was prior to the > locks being dropped and __change_page_attr() repeats the entire page table > walk whenever a split occurs, so concurrent split and collapse are > accounted for. > > Concurrent ptdump is also safe as the lock is only dropped over page table > allocation during which time the page table has not yet been modified. > > The CPA_COLLAPSE flag is only set by set_memory_rox(), which exclusively > operates upon vmalloc ranges, and on x86 only within the module mapping > space. > > This is important, because some callers directly invoke > __change_page_attr_set_clr(), bypassing this lock. However, none of these > operate within the module mapping space. > > * cpa_process_alias() - a recursive helper called by > __change_page_attr_set_clr(). > * __set_memory_enc_pgtable() - operates on the direct mapping and (via > __vmbus_establish_gpadl()) the vmalloc mapping space. > * __set_pages_[n]p() - called by set_direct_map_[invalid, default, > valid]_noflush(), __kernel_map_pages() - operates on the direct map. > * kernel_[un]map_pages_in_pgd() - operates on EFI ranges. > > This work is based upon Denis V. Lunev's excellent analysis of the bug with > gratitude. > > Link: https://lore.kernel.org/all/20260626163213.2284080-1-den@openvz.org/ > Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation") > Cc: stable@vger.kernel.org > Signed-off-by: Lorenzo Stoakes (ARM) > Signed-off-by: Mike Rapoport (Microsoft) > --- > arch/x86/mm/pat/set_memory.c | 22 +++++++++++++--------- > 1 file changed, 13 insertions(+), 9 deletions(-) > > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c > index 4ba16d72a535..26131ecc0e1c 100644 > --- a/arch/x86/mm/pat/set_memory.c > +++ b/arch/x86/mm/pat/set_memory.c > @@ -50,7 +50,8 @@ struct cpa_data { > unsigned int flags; > unsigned int force_split : 1, > force_static_prot : 1, > - force_flush_all : 1; > + force_flush_all : 1, > + init_mm_read_locked : 1; > struct page **pages; > }; > > @@ -435,8 +436,6 @@ static void __cpa_collapse_large_pages(struct cpa_data *cpa) > int collapsed = 0; > int i; > > - cpa_lock(); > - > if (cpa->flags & (CPA_PAGES_ARRAY | CPA_ARRAY)) { > for (i = 0; i < cpa->numpages; i++) > collapsed += collapse_large_pages(__cpa_addr(cpa, i), > @@ -450,10 +449,8 @@ static void __cpa_collapse_large_pages(struct cpa_data *cpa) > collapsed += collapse_large_pages(addr, &pgtables); > } > > - if (!collapsed) { > - cpa_unlock(); > + if (!collapsed) > return; > - } > > flush_tlb_all(); > > @@ -461,8 +458,6 @@ static void __cpa_collapse_large_pages(struct cpa_data *cpa) > list_del(&ptdesc->pt_list); > pagetable_free(ptdesc); > } > - > - cpa_unlock(); > } > > static void cpa_collapse_large_pages(struct cpa_data *cpa) > @@ -1270,8 +1265,13 @@ static int split_large_page(struct cpa_data *cpa, pte_t *kpte, > struct ptdesc *ptdesc; > > cpa_unlock(); > + if (cpa->init_mm_read_locked) > + mmap_read_unlock(&init_mm); > ptdesc = pagetable_alloc(GFP_KERNEL, 0); > + if (cpa->init_mm_read_locked) > + mmap_read_lock(&init_mm); > cpa_lock(); Yeah this retains the correct ordering (sem -> spinlock) so LGTM! > + > if (!ptdesc) > return -ENOMEM; > > @@ -2139,7 +2139,11 @@ static int change_page_attr_set_clr(unsigned long *addr, int numpages, > cpa.curpage = 0; > cpa.force_split = force_split; > > - ret = __change_page_attr_set_clr(&cpa, 1); > + /* Avoid race with concurrent CPA collapse. */ > + cpa.init_mm_read_locked = true; > + scoped_guard(mmap_read_lock, &init_mm) > + ret = __change_page_attr_set_clr(&cpa, 1); > + cpa.init_mm_read_locked = false; > > /* > * Check whether we really changed something: > > -- > 2.53.0 > Cheers, Lorenzo