Re: TLB flushes on fixmap changes

[Andy Lutomirski <luto@kernel.org>]

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On Sat, Aug 25, 2018 at 7:23 PM, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> On Fri, 24 Aug 2018 21:23:26 -0700
> Andy Lutomirski <luto@kernel.org> wrote:
>> Couldn't text_poke() use kmap_atomic()?  Or, even better, just change CR3?
>
> No, since kmap_atomic() is only for x86_32 and highmem support kernel.
> In x86-64, it seems that returns just a page address. That is not
> good for text_poke, since it needs to make a writable alias for RO
> code page. Hmm, maybe, can we mimic copy_oldmem_page(), it uses ioremap_cache?
>

I just re-read text_poke().  It's, um, horrible.  Not only is the
implementation overcomplicated and probably buggy, but it's SLOOOOOW.
It's totally the wrong API -- poking one instruction at a time
basically can't be efficient on x86.  The API should either poke lots
of instructions at once or should be text_poke_begin(); ...;
text_poke_end();.

Anyway, the attached patch seems to boot.  Linus, Kees, etc: is this
too scary of an approach?  With the patch applied, text_poke() is a
fantastic exploit target.  On the other hand, even without the patch
applied, text_poke() is every bit as juicy.

--Andy

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diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 014f214da581..811c8735b129 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -690,40 +690,15 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
 void *text_poke(void *addr, const void *opcode, size_t len)
 {
 	unsigned long flags;
-	char *vaddr;
-	struct page *pages[2];
-	int i;
-
-	/*
-	 * While boot memory allocator is runnig we cannot use struct
-	 * pages as they are not yet initialized.
-	 */
-	BUG_ON(!after_bootmem);
+	unsigned long old_cr0;
 
-	if (!core_kernel_text((unsigned long)addr)) {
-		pages[0] = vmalloc_to_page(addr);
-		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
-	} else {
-		pages[0] = virt_to_page(addr);
-		WARN_ON(!PageReserved(pages[0]));
-		pages[1] = virt_to_page(addr + PAGE_SIZE);
-	}
-	BUG_ON(!pages[0]);
 	local_irq_save(flags);
-	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
-	if (pages[1])
-		set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
-	vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
-	memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
-	clear_fixmap(FIX_TEXT_POKE0);
-	if (pages[1])
-		clear_fixmap(FIX_TEXT_POKE1);
-	local_flush_tlb();
-	sync_core();
-	/* Could also do a CLFLUSH here to speed up CPU recovery; but
-	   that causes hangs on some VIA CPUs. */
-	for (i = 0; i < len; i++)
-		BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
+	old_cr0 = read_cr0();
+	write_cr0(old_cr0 & ~X86_CR0_WP);
+
+	memcpy(addr, opcode, len);
+
+	write_cr0(old_cr0);	/* also serializes */
 	local_irq_restore(flags);
 	return addr;
 }