From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f199.google.com (mail-pf1-f199.google.com [209.85.210.199]) by kanga.kvack.org (Postfix) with ESMTP id 8A5926B030A for ; Fri, 26 Oct 2018 08:29:04 -0400 (EDT) Received: by mail-pf1-f199.google.com with SMTP id 14-v6so545386pfk.22 for ; Fri, 26 Oct 2018 05:29:04 -0700 (PDT) Received: from mga14.intel.com (mga14.intel.com. [192.55.52.115]) by mx.google.com with ESMTPS id 89-v6si11401786plc.383.2018.10.26.05.29.02 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 26 Oct 2018 05:29:02 -0700 (PDT) From: "Kirill A. Shutemov" Subject: [PATCHv3 2/3] x86/ldt: Unmap PTEs for the slot before freeing LDT pages Date: Fri, 26 Oct 2018 15:28:55 +0300 Message-Id: <20181026122856.66224-3-kirill.shutemov@linux.intel.com> In-Reply-To: <20181026122856.66224-1-kirill.shutemov@linux.intel.com> References: <20181026122856.66224-1-kirill.shutemov@linux.intel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org Cc: boris.ostrovsky@oracle.com, jgross@suse.com, bhe@redhat.com, willy@infradead.org, x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" modify_ldt(2) leaves old LDT mapped after we switch over to the new one. Memory for the old LDT gets freed and the pages can be re-used. Leaving the mapping in place can have security implications. The mapping is present in userspace copy of page tables and Meltdown-like attack can read these freed and possibly reused pages. It's relatively simple to fix: just unmap the old LDT and flush TLB before freeing LDT memory. We can now avoid flushing TLB on map_ldt_struct() as the slot is unmapped and flushed by unmap_ldt_struct() (or never mapped in the first place). The overhead of the change should be negligible. It shouldn't be a particularly hot path anyway. Signed-off-by: Kirill A. Shutemov Fixes: f55f0501cbf6 ("x86/pti: Put the LDT in its own PGD if PTI is on") --- arch/x86/kernel/ldt.c | 51 ++++++++++++++++++++++++++++++++----------- 1 file changed, 38 insertions(+), 13 deletions(-) diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c index ab18e0884dc6..e32f3427438a 100644 --- a/arch/x86/kernel/ldt.c +++ b/arch/x86/kernel/ldt.c @@ -199,14 +199,6 @@ static void sanity_check_ldt_mapping(struct mm_struct *mm) /* * If PTI is enabled, this maps the LDT into the kernelmode and * usermode tables for the given mm. - * - * There is no corresponding unmap function. Even if the LDT is freed, we - * leave the PTEs around until the slot is reused or the mm is destroyed. - * This is harmless: the LDT is always in ordinary memory, and no one will - * access the freed slot. - * - * If we wanted to unmap freed LDTs, we'd also need to do a flush to make - * it useful, and the flush would slow down modify_ldt(). */ static int map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot) @@ -215,7 +207,7 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot) bool is_vmalloc; spinlock_t *ptl; pgd_t *pgd; - int i; + int i, nr_pages; if (!static_cpu_has(X86_FEATURE_PTI)) return 0; @@ -238,7 +230,8 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot) is_vmalloc = is_vmalloc_addr(ldt->entries); - for (i = 0; i * PAGE_SIZE < ldt->nr_entries * LDT_ENTRY_SIZE; i++) { + nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE); + for (i = 0; i < nr_pages; i++) { unsigned long offset = i << PAGE_SHIFT; const void *src = (char *)ldt->entries + offset; unsigned long pfn; @@ -272,13 +265,39 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot) /* Propagate LDT mapping to the user page-table */ map_ldt_struct_to_user(mm); - va = (unsigned long)ldt_slot_va(slot); - flush_tlb_mm_range(mm, va, va + LDT_SLOT_STRIDE, PAGE_SHIFT, false); - ldt->slot = slot; return 0; } +static void +unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt) +{ + unsigned long va; + int i, nr_pages; + + if (!ldt) + return; + + /* LDT map/unmap is only required for PTI */ + if (!static_cpu_has(X86_FEATURE_PTI)) + return; + + nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE); + for (i = 0; i < nr_pages; i++) { + unsigned long offset = i << PAGE_SHIFT; + pte_t *ptep; + spinlock_t *ptl; + + va = (unsigned long)ldt_slot_va(ldt->slot) + offset; + ptep = get_locked_pte(mm, va, &ptl); + pte_clear(mm, va, ptep); + pte_unmap_unlock(ptep, ptl); + } + + va = (unsigned long)ldt_slot_va(ldt->slot); + flush_tlb_mm_range(mm, va, va + nr_pages * PAGE_SIZE, 0, false); +} + #else /* !CONFIG_PAGE_TABLE_ISOLATION */ static int @@ -286,6 +305,11 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot) { return 0; } + +static void +unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt) +{ +} #endif /* CONFIG_PAGE_TABLE_ISOLATION */ static void free_ldt_pgtables(struct mm_struct *mm) @@ -524,6 +548,7 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode) } install_ldt(mm, new_ldt); + unmap_ldt_struct(mm, old_ldt); free_ldt_struct(old_ldt); error = 0; -- 2.19.1