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Wed, 29 Aug 2018 11:28:39 +0200 (CEST) Date: Wed, 29 Aug 2018 11:28:39 +0200 From: Peter Zijlstra To: Nadav Amit Cc: Thomas Gleixner , linux-kernel@vger.kernel.org, Ingo Molnar , x86@kernel.org, Arnd Bergmann , linux-arch@vger.kernel.org, Andy Lutomirski , Masami Hiramatsu , Kees Cook Subject: Re: [RFC PATCH 5/6] x86/alternatives: use temporary mm for text poking Message-ID: <20180829092839.GP24124@hirez.programming.kicks-ass.net> References: <20180829081147.184610-1-namit@vmware.com> <20180829081147.184610-6-namit@vmware.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180829081147.184610-6-namit@vmware.com> User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 29, 2018 at 01:11:46AM -0700, Nadav Amit wrote: > +static void text_poke_fixmap(void *addr, const void *opcode, size_t len, > + struct page *pages[2]) > +{ > + u8 *vaddr; > + > + set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0])); > + if (pages[1]) > + set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1])); > + vaddr = (u8 *)fix_to_virt(FIX_TEXT_POKE0); > + memcpy(vaddr + offset_in_page(addr), opcode, len); > + > + /* > + * clear_fixmap() performs a TLB flush, so no additional TLB > + * flush is needed. > + */ > + clear_fixmap(FIX_TEXT_POKE0); > + if (pages[1]) > + clear_fixmap(FIX_TEXT_POKE1); > + sync_core(); > + /* Could also do a CLFLUSH here to speed up CPU recovery; but > + that causes hangs on some VIA CPUs. */ Please take this opportunity to fix that comment style. > +} > + > +__ro_after_init struct mm_struct *poking_mm; > +__ro_after_init unsigned long poking_addr; > + > +/** > + * text_poke_safe() - Pokes the text using a separate address space. > + * > + * This is the preferable way for patching the kernel after boot, as it does not > + * allow other cores to accidentally or maliciously modify the code using the > + * temporary PTEs. > + */ > +static void text_poke_safe(void *addr, const void *opcode, size_t len, > + struct page *pages[2]) > +{ > + temporary_mm_state_t prev; > + pte_t pte, *ptep; > + spinlock_t *ptl; > + > + /* > + * The lock is not really needed, but this allows to avoid open-coding. > + */ > + ptep = get_locked_pte(poking_mm, poking_addr, &ptl); > + > + pte = mk_pte(pages[0], PAGE_KERNEL); > + set_pte_at(poking_mm, poking_addr, ptep, pte); > + > + if (pages[1]) { > + pte = mk_pte(pages[1], PAGE_KERNEL); > + set_pte_at(poking_mm, poking_addr + PAGE_SIZE, ptep + 1, pte); > + } > + > + /* > + * Loading the temporary mm behaves as a compiler barrier, which > + * guarantees that the PTE will be set at the time memcpy() is done. > + */ > + prev = use_temporary_mm(poking_mm); > + > + memcpy((u8 *)poking_addr + offset_in_page(addr), opcode, len); > + > + /* > + * Ensure that the PTE is only cleared after copying is done by using a > + * compiler barrier. > + */ > + barrier(); I tripped over the use of 'done', because even with TSO the store isn't done once the instruction retires. All we want to ensure is that the pte_clear() store is issued after the copy, and that is indeed guaranteed by this. > + pte_clear(poking_mm, poking_addr, ptep); > + > + /* > + * __flush_tlb_one_user() performs a redundant TLB flush when PTI is on, > + * as it also flushes the corresponding "user" address spaces, which > + * does not exist. > + * > + * Poking, however, is already very inefficient since it does not try to > + * batch updates, so we ignore this problem for the time being. > + * > + * Since the PTEs do not exist in other kernel address-spaces, we do > + * not use __flush_tlb_one_kernel(), which when PTI is on would cause > + * more unwarranted TLB flushes. > + */ yuck :-), but yeah. > + __flush_tlb_one_user(poking_addr); > + if (pages[1]) { > + pte_clear(poking_mm, poking_addr + PAGE_SIZE, ptep + 1); > + __flush_tlb_one_user(poking_addr + PAGE_SIZE); > + } > + /* > + * Loading the previous page-table hierarchy requires a serializing > + * instruction that already allows the core to see the updated version. > + * Xen-PV is assumed to serialize execution in a similar manner. > + */ > + unuse_temporary_mm(prev); > + > + pte_unmap_unlock(ptep, ptl); > +}