[Xen-devel] [PATCH v2 2/2] x86/xpti: Don't leak TSS-adjacent percpu data via Meltdown

[Andrew Cooper <andrew.cooper3@citrix.com>]

The XPTI work restricted the visibility of most of memory, but missed a few
aspects when it came to the TSS.

Given that the TSS is just an object in percpu data, the 4k mapping for it
created in setup_cpu_root_pgt() maps adjacent percpu data, making it all
leakable via Meltdown, even when XPTI is in use.

Furthermore, no care is taken to check that the TSS doesn't cross a page
boundary.  As it turns out, struct tss_struct is aligned on its size which
does prevent it straddling a page boundary, but this will cease to be true
once CET and Shadow Stack support is added to Xen.

Move the TSS into the page aligned percpu area, so no adjacent data can be
leaked.  Move the definition from setup.c to traps.c, which is a more
appropriate place for it to live.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wl@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>

v2:
 * Rebase over changes to include __aligned() within
   DEFINE_PER_CPU_PAGE_ALIGNED()
 * Drop now-unused xen/percpu.h from setup.c
---
 xen/arch/x86/setup.c            | 3 ---
 xen/arch/x86/traps.c            | 6 ++++++
 xen/arch/x86/xen.lds.S          | 2 ++
 xen/include/asm-x86/processor.h | 4 ++--
 4 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index d2011910fa..f9d38155d3 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -16,7 +16,6 @@
 #include <xen/domain_page.h>
 #include <xen/version.h>
 #include <xen/gdbstub.h>
-#include <xen/percpu.h>
 #include <xen/hypercall.h>
 #include <xen/keyhandler.h>
 #include <xen/numa.h>
@@ -100,8 +99,6 @@ unsigned long __read_mostly xen_phys_start;
 
 unsigned long __read_mostly xen_virt_end;
 
-DEFINE_PER_CPU(struct tss_struct, init_tss);
-
 char __section(".bss.stack_aligned") __aligned(STACK_SIZE)
     cpu0_stack[STACK_SIZE];
 
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 38d12013db..de3ac135f5 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -108,6 +108,12 @@ idt_entry_t __section(".bss.page_aligned") __aligned(PAGE_SIZE)
 /* Pointer to the IDT of every CPU. */
 idt_entry_t *idt_tables[NR_CPUS] __read_mostly;
 
+/*
+ * The TSS is smaller than a page, but we give it a full page to avoid
+ * adjacent per-cpu data leaking via Meltdown when XPTI is in use.
+ */
+DEFINE_PER_CPU_PAGE_ALIGNED(struct tss_struct, init_tss);
+
 bool (*ioemul_handle_quirk)(
     u8 opcode, char *io_emul_stub, struct cpu_user_regs *regs);
 
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index b8a2ea4259..c82e1e504a 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -368,6 +368,8 @@ ASSERT(IS_ALIGNED(__2M_rwdata_end,   SECTION_ALIGN), "__2M_rwdata_end misaligned
 
 ASSERT(IS_ALIGNED(cpu0_stack, STACK_SIZE), "cpu0_stack misaligned")
 
+ASSERT(IS_ALIGNED(per_cpu__init_tss, PAGE_SIZE), "per_cpu(init_tss) misaligned")
+
 ASSERT(IS_ALIGNED(__init_begin, PAGE_SIZE), "__init_begin misaligned")
 ASSERT(IS_ALIGNED(__init_end,   PAGE_SIZE), "__init_end misaligned")
 
diff --git a/xen/include/asm-x86/processor.h b/xen/include/asm-x86/processor.h
index 2862321eee..b5bee94931 100644
--- a/xen/include/asm-x86/processor.h
+++ b/xen/include/asm-x86/processor.h
@@ -411,7 +411,7 @@ static always_inline void __mwait(unsigned long eax, unsigned long ecx)
 #define IOBMP_BYTES             8192
 #define IOBMP_INVALID_OFFSET    0x8000
 
-struct __packed __cacheline_aligned tss_struct {
+struct __packed tss_struct {
     uint32_t :32;
     uint64_t rsp0, rsp1, rsp2;
     uint64_t :64;
@@ -425,6 +425,7 @@ struct __packed __cacheline_aligned tss_struct {
     /* Pads the TSS to be cacheline-aligned (total size is 0x80). */
     uint8_t __cacheline_filler[24];
 };
+DECLARE_PER_CPU(struct tss_struct, init_tss);
 
 #define IST_NONE 0UL
 #define IST_DF   1UL
@@ -463,7 +464,6 @@ static inline void disable_each_ist(idt_entry_t *idt)
 extern idt_entry_t idt_table[];
 extern idt_entry_t *idt_tables[];
 
-DECLARE_PER_CPU(struct tss_struct, init_tss);
 DECLARE_PER_CPU(root_pgentry_t *, root_pgt);
 
 extern void write_ptbase(struct vcpu *v);
-- 
2.11.0


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