x86/shskt: Disable CET-SS on parts susceptible to fractured updates

Refer to Intel SDM Rev 70 (Dec 2022), Vol3 17.2.3 "Supervisor Shadow Stack
Token".

Architecturally, an event delivery which starts in CPL<3 and switches shadow
stack will first validate the Supervisor Shadow Stack Token (setting the busy
bit), then pushes CS/LIP/SSP.  One example of this is an NMI interrupting Xen.

Some CPUs suffer from an issue called fracturing, whereby a fault/vmexit/etc
between setting the busy bit and completing the event injection renders the
action non-restartable, because when it comes time to restart, the busy bit is
found to be already set.

This is far more easily encountered under virt, yet it is not the fault of the
hypervisor, nor the fault of the guest kernel.  The fault lies somewhere
between the architectural specification, and the uarch behaviour.

Intel have allocated CPUID.7[1].ecx[18] CET_SSS to enumerate that supervisor
shadow stacks are safe to use.  Because of how Xen lays out its shadow stacks,
fracturing is not expected to be a problem on native.

Detect this case on boot and default to not using shstk if virtualised.
Specifying `cet=shstk` on the command line will override this heuristic and
enable shadow stacks irrespective.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index 923910f553c5364357ff0751d95369e2bfe8ef11..19d4d815bdeec6c07392c649c7fa791f92bae3b4 100644 (file)
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -287,10 +287,15 @@ can be maintained with the pv-shim mechanism.
     protection.
 
     The option is available when `CONFIG_XEN_SHSTK` is compiled in, and
-    defaults to `true` on hardware supporting CET-SS.  Specifying
+    generally defaults to `true` on hardware supporting CET-SS.  Specifying
     `cet=no-shstk` will cause Xen not to use Shadow Stacks even when support
     is available in hardware.
 
+    Some hardware suffers from an issue known as Supervisor Shadow Stack
+    Fracturing.  On such hardware, Xen will default to not using Shadow Stacks
+    when virtualised.  Specifying `cet=shstk` will override this heuristic and
+    enable Shadow Stacks unilaterally.
+
 *   The `ibt=` boolean controls whether Xen uses Indirect Branch Tracking for
     its own protection.
 

diff --git a/tools/libs/light/libxl_cpuid.c b/tools/libs/light/libxl_cpuid.c
index 8da78773a88618c52afaa9c29255cdaf428418f0..06b1227a2fd339a0c5261d5629fa2f22118f5b03 100644 (file)
--- a/tools/libs/light/libxl_cpuid.c
+++ b/tools/libs/light/libxl_cpuid.c
@@ -237,6 +237,8 @@ int libxl_cpuid_parse_config(libxl_cpuid_policy_list *cpuid, const char* str)
         {"fsrcs",        0x00000007,  1, CPUID_REG_EAX, 12,  1},
         {"wrmsrns",      0x00000007,  1, CPUID_REG_EAX, 19,  1},
 
+        {"cet-sss",      0x00000007,  1, CPUID_REG_EDX, 18,  1},
+
         {"intel-psfd",   0x00000007,  2, CPUID_REG_EDX,  0,  1},
         {"mcdt-no",      0x00000007,  2, CPUID_REG_EDX,  5,  1},
 

diff --git a/tools/misc/xen-cpuid.c b/tools/misc/xen-cpuid.c
index ba749fcb704a9e2791987f575212e25d6c2411be..3ab820571d8d75d6c8c7e73b6236c2d5e78b66b4 100644 (file)
--- a/tools/misc/xen-cpuid.c
+++ b/tools/misc/xen-cpuid.c
@@ -210,6 +210,7 @@ static const char *const str_7c1[32] =
 
 static const char *const str_7d1[32] =
 {
+    [18] = "cet-sss",
 };
 
 static const char *const str_7d2[32] =

diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index 860f2dea5c7dd8b598686085828f4a019d968112..5ad347534a22a2491d21ecf126697fd61ee2fe00 100644 (file)
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -349,11 +349,18 @@ 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,
+       if (c->cpuid_level >= 7) {
+               uint32_t max_subleaf;
+
+               cpuid_count(7, 0, &max_subleaf, &ebx,
                            &c->x86_capability[FEATURESET_7c0],
                            &c->x86_capability[FEATURESET_7d0]);
 
+               if (max_subleaf >= 1)
+                       cpuid_count(7, 1, &eax, &ebx, &ecx,
+                                   &c->x86_capability[FEATURESET_7d1]);
+       }
+
        eax = cpuid_eax(0x80000000);
        if ((eax >> 16) == 0x8000 && eax >= 0x80000008) {
                ebx = eax >= 0x8000001f ? cpuid_ebx(0x8000001f) : 0;

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 567a0a42ac50cd3aa753c380e2df6c934e806e5a..23b1834e94fc94a91c85044bff7506ade9ef5665 100644 (file)
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -96,11 +96,7 @@ unsigned long __initdata highmem_start;
 size_param("highmem-start", highmem_start);
 #endif
 
-#ifdef CONFIG_XEN_SHSTK
-static bool __initdata opt_xen_shstk = true;
-#else
-#define opt_xen_shstk false
-#endif
+static int8_t __initdata opt_xen_shstk = -IS_ENABLED(CONFIG_XEN_SHSTK);
 
 #ifdef CONFIG_XEN_IBT
 static bool __initdata opt_xen_ibt = true;
@@ -1100,11 +1096,45 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     early_cpu_init();
 
     /* Choose shadow stack early, to set infrastructure up appropriately. */
-    if ( opt_xen_shstk && boot_cpu_has(X86_FEATURE_CET_SS) )
+    if ( !boot_cpu_has(X86_FEATURE_CET_SS) )
+        opt_xen_shstk = 0;
+
+    if ( opt_xen_shstk )
     {
-        printk("Enabling Supervisor Shadow Stacks\n");
+        /*
+         * Some CPUs suffer from Shadow Stack Fracturing, an issue whereby a
+         * fault/VMExit/etc between setting a Supervisor Busy bit and the
+         * event delivery completing renders the operation non-restartable.
+         * On restart, event delivery will find the Busy bit already set.
+         *
+         * This is a problem on bare metal, but outside of synthetic cases or
+         * a very badly timed #MC, it's not believed to be a problem.  It is a
+         * much bigger problem under virt, because we can VMExit for a number
+         * of legitimate reasons and tickle this bug.
+         *
+         * CPUs with this addressed enumerate CET-SSS to indicate that
+         * supervisor shadow stacks are now safe to use.
+         */
+        bool cpu_has_bug_shstk_fracture =
+            boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
+            !boot_cpu_has(X86_FEATURE_CET_SSS);
 
-        setup_force_cpu_cap(X86_FEATURE_XEN_SHSTK);
+        /*
+         * On bare metal, assume that Xen won't be impacted by shstk
+         * fracturing problems.  Under virt, be more conservative and disable
+         * shstk by default.
+         */
+        if ( opt_xen_shstk == -1 )
+            opt_xen_shstk =
+                cpu_has_hypervisor ? !cpu_has_bug_shstk_fracture
+                                   : true;
+
+        if ( opt_xen_shstk )
+        {
+            printk("Enabling Supervisor Shadow Stacks\n");
+
+            setup_force_cpu_cap(X86_FEATURE_XEN_SHSTK);
+        }
     }
 
     if ( opt_xen_ibt && boot_cpu_has(X86_FEATURE_CET_IBT) )

diff --git a/xen/include/public/arch-x86/cpufeatureset.h b/xen/include/public/arch-x86/cpufeatureset.h
index 9fb19b7057b58a6359e69dc95804941027ac8fe9..336744b47119a83d619703bde5750b68fd86882a 100644 (file)
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -300,6 +300,7 @@ XEN_CPUFEATURE(MCDT_NO,            13*32+ 5) /*A  MCDT_NO */
 /* Intel-defined CPU features, CPUID level 0x00000007:1.ecx, word 14 */
 
 /* Intel-defined CPU features, CPUID level 0x00000007:1.edx, word 15 */
+XEN_CPUFEATURE(CET_SSS,            15*32+18) /*   CET Supervisor Shadow Stacks safe to use */
 
 #endif /* XEN_CPUFEATURE */