Re: [PATCH v2 0/6] x86/alternatives: text_poke() fixes

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Sep 06, 2018 at 10:13:00AM +0200, Peter Zijlstra wrote:

> No, you got it the first time. There are in fact more fixmap abusers;
> see drivers/acpi/apei/ghes.c.  Also, as long as set_fixmap() allows
> overwriting a _PAGE_PRESENT pte and has that dodgy
> __flush_tlb_one_kernel() in it, the broken remains (and can return).
> 
> So we need to fix fixmap, to either disallow overwriting a _PAGE_PRESENT
> pte, or to issue a full TLB invalidate.
> 
> Either fix will terminally break GHES, but that's OK, they've known
> about this issue since 2015 and haven't cared, so I can't be bothered
> about them.

Worse, when I boot with the below patch in, I get hits on text_poke (we
knew about this), but I also get hits from:

  release_ds_buffer()

Because PEBS/BTS hardware write through the page-tables (I know right),
their buffers need to be in the PTI user map, and we do that by mapping
the pages in the CAE, and that uses fixmap.

---
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index dd519f372169..20dcd6fa690e 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -259,9 +259,16 @@ static pte_t *fill_pte(pmd_t *pmd, unsigned long vaddr)
 static void __set_pte_vaddr(pud_t *pud, unsigned long vaddr, pte_t new_pte)
 {
 	pmd_t *pmd = fill_pmd(pud, vaddr);
-	pte_t *pte = fill_pte(pmd, vaddr);
+	pte_t *ptep = fill_pte(pmd, vaddr);
 
-	set_pte(pte, new_pte);
+	pte_t pte = *ptep;
+
+	set_pte(ptep, new_pte);
+
+	WARN((pte_val(pte) & _PAGE_PRESENT) && (pte_val(pte) != pte_val(new_pte)),
+		"set_fixmap(%d, %lx), was: %lx",
+		(int)__virt_to_fix(vaddr),
+		pte_val(new_pte), pte_val(pte));
 
 	/*
 	 * It's enough to flush this one mapping.