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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [xen master] x86/shstk: Activate Supervisor Shadow Stacks
commit b60ab42db2f04dcf56ebbfedfb9b0c65a75e4bac
Author: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed Apr 22 13:44:37 2020 +0100
Commit: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri May 29 23:09:46 2020 +0100
x86/shstk: Activate Supervisor Shadow Stacks
With all other plumbing in place, activate shadow stacks when possible.
Note that CET shares the similar problems to SMEP/SMAP with Ring1 being
supervisor to the processor, and that the layout of the shadow stack differs
between an IRET to Ring 1 and Ring 3. Therefore, we disable PV32 when CET
is
enabled. Compatibility can be maintained if necessary via PV-Shim.
The BSP needs to wait until alternatives have run (to avoid interaction with
CR0.WP), and after the first reset_stack_and_jump() to avoid having a
pristine
shadow stack interact in problematic ways with an in-use regular stack.
Activate shadow stack in reinit_bsp_stack().
APs have all infrastructure set up by the booting CPU, so enable shadow
stacks
before entering C. Adjust the logic to call start_secondary rather than
jump
to it, so stack traces make more sense.
The crash path needs to turn CET off to avoid interfering with the crash
kernel's environment.
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
docs/misc/xen-command-line.pandoc | 8 ++++++++
xen/arch/x86/boot/x86_64.S | 35 +++++++++++++++++++++++++++++++++--
xen/arch/x86/cpu/common.c | 5 +++++
xen/arch/x86/crash.c | 7 +++++++
xen/arch/x86/setup.c | 27 +++++++++++++++++++++++++++
xen/arch/x86/spec_ctrl.c | 8 ++++++++
6 files changed, 88 insertions(+), 2 deletions(-)
diff --git a/docs/misc/xen-command-line.pandoc
b/docs/misc/xen-command-line.pandoc
index ec1c12cbe0..2d4d9639d6 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -287,6 +287,10 @@ call/jmp COP/JOP) attacks.
`cet=no-shstk` will cause Xen not to use Shadow Stacks even when support
is available in hardware.
+ Shadow Stacks are incompatible with 32bit PV guests. This option will
+ override the `pv=32` boolean to false. Backwards compatibility can be
+ maintained with the `pv-shim` mechanism.
+
### clocksource (x86)
> `= pit | hpet | acpi | tsc`
@@ -1726,6 +1730,10 @@ Controls for aspects of PV guest support.
* The `32` boolean controls whether 32bit PV guests can be created. It
defaults to `true`, and is ignored when `CONFIG_PV32` is compiled out.
+ 32bit PV guests are incompatible with CET Shadow Stacks. If Xen is using
+ shadow stacks, this option will be overridden to `false`. Backwards
+ compatibility can be maintained with the `pv-shim` mechanism.
+
### pv-linear-pt (x86)
> `= <boolean>`
diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index 314a32a19f..551acd9e94 100644
--- a/xen/arch/x86/boot/x86_64.S
+++ b/xen/arch/x86/boot/x86_64.S
@@ -28,8 +28,39 @@ ENTRY(__high_start)
lretq
1:
test %ebx,%ebx
- jnz start_secondary
-
+ jz .L_bsp
+
+ /* APs. Set up shadow stacks before entering C. */
+
+ testl $cpufeat_mask(X86_FEATURE_XEN_SHSTK), \
+ CPUINFO_FEATURE_OFFSET(X86_FEATURE_XEN_SHSTK) +
boot_cpu_data(%rip)
+ je .L_ap_shstk_done
+
+ /* Set up MSR_S_CET. */
+ mov $MSR_S_CET, %ecx
+ xor %edx, %edx
+ mov $CET_SHSTK_EN | CET_WRSS_EN, %eax
+ wrmsr
+
+ /* Derive MSR_PL0_SSP from %rsp (token written when stack is
allocated). */
+ mov $MSR_PL0_SSP, %ecx
+ mov %rsp, %rdx
+ shr $32, %rdx
+ mov %esp, %eax
+ and $~(STACK_SIZE - 1), %eax
+ or $(PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 8, %eax
+ wrmsr
+
+ /* Enable CET. MSR_INTERRUPT_SSP_TABLE is set up later in
load_system_tables(). */
+ mov $XEN_MINIMAL_CR4 | X86_CR4_CET, %ecx
+ mov %rcx, %cr4
+ setssbsy
+
+.L_ap_shstk_done:
+ call start_secondary
+ BUG /* start_secondary() shouldn't return. */
+
+.L_bsp:
/* Pass off the Multiboot info structure to C land (if applicable). */
mov multiboot_ptr(%rip),%edi
call __start_xen
diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index f483c81b7c..58f0876180 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -329,6 +329,11 @@ void __init early_cpu_init(void)
x86_cpuid_vendor_to_str(c->x86_vendor), c->x86, c->x86,
c->x86_model, c->x86_model, c->x86_mask, eax);
+ if (c->cpuid_level >= 7) {
+ cpuid_count(7, 0, &eax, &ebx, &ecx, &edx);
+ c->x86_capability[cpufeat_word(X86_FEATURE_CET_SS)] = ecx;
+ }
+
eax = cpuid_eax(0x80000000);
if ((eax >> 16) == 0x8000 && eax >= 0x80000008) {
eax = cpuid_eax(0x80000008);
diff --git a/xen/arch/x86/crash.c b/xen/arch/x86/crash.c
index 450eecd46b..0611b4fb9b 100644
--- a/xen/arch/x86/crash.c
+++ b/xen/arch/x86/crash.c
@@ -200,6 +200,13 @@ void machine_crash_shutdown(void)
/* Reset CPUID masking and faulting to the host's default. */
ctxt_switch_levelling(NULL);
+ /* Disable shadow stacks. */
+ if ( cpu_has_xen_shstk )
+ {
+ wrmsrl(MSR_S_CET, 0);
+ write_cr4(read_cr4() & ~X86_CR4_CET);
+ }
+
info = kexec_crash_save_info();
info->xen_phys_start = xen_phys_start;
info->dom0_pfn_to_mfn_frame_list_list =
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 584589baff..665274674d 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -664,6 +664,14 @@ static void __init noreturn reinit_bsp_stack(void)
stack_base[0] = stack;
memguard_guard_stack(stack);
+ if ( cpu_has_xen_shstk )
+ {
+ wrmsrl(MSR_PL0_SSP,
+ (unsigned long)stack + (PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE -
8);
+ wrmsrl(MSR_S_CET, CET_SHSTK_EN | CET_WRSS_EN);
+ asm volatile ("setssbsy" ::: "memory");
+ }
+
reset_stack_and_jump_nolp(init_done);
}
@@ -1065,6 +1073,21 @@ void __init noreturn __start_xen(unsigned long mbi_p)
/* This must come before e820 code because it sets paddr_bits. */
early_cpu_init();
+ /* Choose shadow stack early, to set infrastructure up appropriately. */
+ if ( opt_xen_shstk && boot_cpu_has(X86_FEATURE_CET_SS) )
+ {
+ printk("Enabling Supervisor Shadow Stacks\n");
+
+ setup_force_cpu_cap(X86_FEATURE_XEN_SHSTK);
+#ifdef CONFIG_PV32
+ if ( opt_pv32 )
+ {
+ opt_pv32 = 0;
+ printk(" - Disabling PV32 due to Shadow Stacks\n");
+ }
+#endif
+ }
+
/* Sanitise the raw E820 map to produce a final clean version. */
max_page = raw_max_page = init_e820(memmap_type, &e820_raw);
@@ -1801,6 +1824,10 @@ void __init noreturn __start_xen(unsigned long mbi_p)
alternative_branches();
+ /* Defer CR4.CET until alternatives have finished playing with CR0.WP */
+ if ( cpu_has_xen_shstk )
+ set_in_cr4(X86_CR4_CET);
+
/*
* NB: when running as a PV shim VCPUOP_up/down is wired to the shim
* physical cpu_add/remove functions, so launch the guest with only
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index c5d8e587a8..a94be2d594 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -881,6 +881,14 @@ void __init init_speculation_mitigations(void)
hw_smt_enabled = check_smt_enabled();
+ /*
+ * First, disable the use of retpolines if Xen is using shadow stacks, as
+ * they are incompatible.
+ */
+ if ( cpu_has_xen_shstk &&
+ (opt_thunk == THUNK_DEFAULT || opt_thunk == THUNK_RETPOLINE) )
+ thunk = THUNK_JMP;
+
/*
* Has the user specified any custom BTI mitigations? If so, follow their
* instructions exactly and disable all heuristics.
--
generated by git-patchbot for /home/xen/git/xen.git#master
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