From eeea00c427376c269ffa34587f16fa1182d8c29d Mon Sep 17 00:00:00 2001 From: George Dunlap Date: Fri, 22 Sep 2017 11:46:55 +0100 Subject: [PATCH 2/2] x86/mm: Disable PV linear pagetables by default Allowing pagetables to point to other pagetables of the same level (often called 'linear pagetables') has been included in Xen since its inception. But it is not used by the most common PV guests (Linux, NetBSD, minios), and has been the source of a number of subtle reference-counting bugs. Add a command-line option to control whether PV linear pagetables are allowed (disabled by default). Reported-by: Jann Horn Signed-off-by: George Dunlap Reviewed-by: Andrew Cooper --- Changes since v2: - s/_/-/; in command-line option - Added __read_mostly --- docs/misc/xen-command-line.markdown | 15 +++++++++++++++ xen/arch/x86/mm.c | 9 +++++++++ 2 files changed, 24 insertions(+) diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown index d99a20a44b..c91acaa464 100644 --- a/docs/misc/xen-command-line.markdown +++ b/docs/misc/xen-command-line.markdown @@ -1231,6 +1231,21 @@ The following resources are available: the cache allocation. * `cat` instructs Xen to enable/disable Cache Allocation Technology. * `cos_max` indicates the max value for COS ID. + +### pv-linear-pt +> `= ` + +> Default: `false` + +Allow PV guests to have pagetable entries pointing to other pagetables +of the same level (i.e., allowing L2 PTEs to point to other L2 pages). +This technique is often called "linear pagetables", and is sometimes +used to allow operating systems a simple way to consistently map the +current process's pagetables into its own virtual address space. + +None of the most common PV operating systems (Linux, NetBSD, MiniOS) +use this technique, but there may be custom operating systems which +do. ### reboot > `= t[riple] | k[bd] | a[cpi] | p[ci] | P[ower] | e[fi] | n[o] [, [w]arm | [c]old]` diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c index 81074aa473..75dd077046 100644 --- a/xen/arch/x86/mm.c +++ b/xen/arch/x86/mm.c @@ -727,6 +727,9 @@ static void dec_linear_uses(struct page_info *pg) * frame if it is mapped by a different root table. This is sufficient and * also necessary to allow validation of a root table mapping itself. */ +static bool_t __read_mostly pv_linear_pt_enable = 0; +boolean_param("pv-linear-pt", pv_linear_pt_enable); + #define define_get_linear_pagetable(level) \ static int \ get_##level##_linear_pagetable( \ @@ -736,6 +739,12 @@ get_##level##_linear_pagetable( \ struct page_info *page; \ unsigned long pfn; \ \ + if ( !pv_linear_pt_enable ) \ + { \ + MEM_LOG("Attempt to create linear p.t. (feature disabled)"); \ + return 0; \ + } \ + \ if ( (level##e_get_flags(pde) & _PAGE_RW) ) \ { \ MEM_LOG("Attempt to create linear p.t. with write perms"); \ -- 2.14.1