File 5a6b36cd-8-x86-boot-calculate-best-BTI-mitigati... of Package xen

Overview Repositories Revisions Requests Users Attributes Meta

File 5a6b36cd-8-x86-boot-calculate-best-BTI-mitigation.patch of Package xen (Revision f461f181be8e9ab1102295d3f581363b)

Currently displaying revision f461f181be8e9ab1102295d3f581363b , Show latest

# Commit 2713715305ca516f698d58cec5e0b322c3b2c4eb
# Date 2018-01-26 14:10:21 +0000
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86/boot: Calculate the most appropriate BTI mitigation to use

See the logic and comments in init_speculation_mitigations() for further
details.

There are two controls for RSB overwriting, because in principle there are
cases where it might be safe to forego rsb_native (Off the top of my head,
SMEP active, no 32bit PV guests at all, no use of vmevent/paging subsystems
for HVM guests, but I make no guarantees that this list of restrictions is
exhaustive).

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

# Commit 30cbd0c83ef3d0edac2d5bcc41a9a2b7a843ae58
# Date 2018-02-06 18:32:58 +0000
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86/spec_ctrl: Fix determination of when to use IBRS

The original version of this logic was:

/*
     * On Intel hardware, we'd like to use retpoline in preference to
     * IBRS, but only if it is safe on this hardware.
     */
    else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
    {
        if ( retpoline_safe() )
            thunk = THUNK_RETPOLINE;
        else
            ibrs = true;
    }

but it was changed by a request during review.  Sadly, the result is buggy as
it breaks the later fallback logic by allowing IBRS to appear as available
when in fact it isn't.

This in practice means that on repoline-unsafe hardware without IBRS, we
select THUNK_JUMP despite intending to select THUNK_RETPOLINE.

Reported-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -246,7 +246,7 @@ enough. Setting this to a high value may
 the NMI watchdog is also enabled.
 
 ### bti (x86)
-> `= List of [ thunk=retpoline|lfence|jmp ]`
+> `= List of [ thunk=retpoline|lfence|jmp, ibrs=<bool>, rsb_{vmexit,native}=<bool> ]`
 
 Branch Target Injection controls.  By default, Xen will pick the most
 appropriate BTI mitigations based on compiled in support, loaded microcode,
@@ -261,6 +261,14 @@ locations.  The default thunk is `retpol
 hardware), with the alternatives being `jmp` (a `jmp *%reg` gadget, minimal
 overhead), and `lfence` (an `lfence; jmp *%reg` gadget, preferred for AMD).
 
+On hardware supporting IBRS, the `ibrs=` option can be used to force or
+prevent Xen using the feature itself.  If Xen is not using IBRS itself,
+functionality is still set up so IBRS can be virtualised for guests.
+
+The `rsb_vmexit=` and `rsb_native=` options can be used to fine tune when the
+RSB gets overwritten.  There are individual controls for an entry from HVM
+context, and an entry from a native (PV or Xen) context.
+
 ### xenheap\_megabytes (arm32)
 > `= <size>`
 
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -20,6 +20,7 @@
 #include <xen/init.h>
 #include <xen/lib.h>
 
+#include <asm/microcode.h>
 #include <asm/msr-index.h>
 #include <asm/processor.h>
 #include <asm/spec_ctrl.h>
@@ -33,11 +34,15 @@ static enum ind_thunk {
     THUNK_LFENCE,
     THUNK_JMP,
 } opt_thunk __initdata = THUNK_DEFAULT;
+static int8_t __initdata opt_ibrs = -1;
+static bool __initdata opt_rsb_native = true;
+static bool __initdata opt_rsb_vmexit = true;
+uint8_t __read_mostly default_bti_ist_info;
 
 static int __init parse_bti(const char *s)
 {
     const char *ss;
-    int rc = 0;
+    int val, rc = 0;
 
     do {
         ss = strchr(s, ',');
@@ -57,6 +62,12 @@ static int __init parse_bti(const char *
             else
                 rc = -EINVAL;
         }
+        else if ( (val = parse_boolean("ibrs", s, ss)) >= 0 )
+            opt_ibrs = val;
+        else if ( (val = parse_boolean("rsb_native", s, ss)) >= 0 )
+            opt_rsb_native = val;
+        else if ( (val = parse_boolean("rsb_vmexit", s, ss)) >= 0 )
+            opt_rsb_vmexit = val;
         else
             rc = -EINVAL;
 
@@ -93,24 +104,84 @@ static void __init print_details(enum in
         printk(XENLOG_DEBUG "  Compiled-in support: INDIRECT_THUNK\n");
 
     printk(XENLOG_INFO
-           "BTI mitigations: Thunk %s\n",
+           "BTI mitigations: Thunk %s, Others:%s%s%s\n",
            thunk == THUNK_NONE      ? "N/A" :
            thunk == THUNK_RETPOLINE ? "RETPOLINE" :
            thunk == THUNK_LFENCE    ? "LFENCE" :
-           thunk == THUNK_JMP       ? "JMP" : "?");
+           thunk == THUNK_JMP       ? "JMP" : "?",
+           boot_cpu_has(X86_FEATURE_XEN_IBRS_SET)    ? " IBRS+" :
+           boot_cpu_has(X86_FEATURE_XEN_IBRS_CLEAR)  ? " IBRS-"      : "",
+           boot_cpu_has(X86_FEATURE_RSB_NATIVE)      ? " RSB_NATIVE" : "",
+           boot_cpu_has(X86_FEATURE_RSB_VMEXIT)      ? " RSB_VMEXIT" : "");
+}
+
+/* Calculate whether Retpoline is known-safe on this CPU. */
+static bool __init retpoline_safe(void)
+{
+    unsigned int ucode_rev = this_cpu(ucode_cpu_info).cpu_sig.rev;
+
+    if ( boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
+        return true;
+
+    if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
+         boot_cpu_data.x86 != 6 )
+        return false;
+
+    switch ( boot_cpu_data.x86_model )
+    {
+    case 0x17: /* Penryn */
+    case 0x1d: /* Dunnington */
+    case 0x1e: /* Nehalem */
+    case 0x1f: /* Auburndale / Havendale */
+    case 0x1a: /* Nehalem EP */
+    case 0x2e: /* Nehalem EX */
+    case 0x25: /* Westmere */
+    case 0x2c: /* Westmere EP */
+    case 0x2f: /* Westmere EX */
+    case 0x2a: /* SandyBridge */
+    case 0x2d: /* SandyBridge EP/EX */
+    case 0x3a: /* IvyBridge */
+    case 0x3e: /* IvyBridge EP/EX */
+    case 0x3c: /* Haswell */
+    case 0x3f: /* Haswell EX/EP */
+    case 0x45: /* Haswell D */
+    case 0x46: /* Haswell H */
+        return true;
+
+        /*
+         * Broadwell processors are retpoline-safe after specific microcode
+         * versions.
+         */
+    case 0x3d: /* Broadwell */
+        return ucode_rev >= 0x28;
+    case 0x47: /* Broadwell H */
+        return ucode_rev >= 0x1b;
+    case 0x4f: /* Broadwell EP/EX */
+        return ucode_rev >= 0xb000025;
+    case 0x56: /* Broadwell D */
+        return false; /* TBD. */
+
+        /*
+         * Skylake and later processors are not retpoline-safe.
+         */
+    default:
+        return false;
+    }
 }
 
 void __init init_speculation_mitigations(void)
 {
     enum ind_thunk thunk = THUNK_DEFAULT;
+    bool ibrs = false;
 
     /*
      * Has the user specified any custom BTI mitigations?  If so, follow their
      * instructions exactly and disable all heuristics.
      */
-    if ( opt_thunk != THUNK_DEFAULT )
+    if ( opt_thunk != THUNK_DEFAULT || opt_ibrs != -1 )
     {
         thunk = opt_thunk;
+        ibrs  = !!opt_ibrs;
     }
     else
     {
@@ -126,7 +197,18 @@ void __init init_speculation_mitigations
              */
             if ( cpu_has_lfence_dispatch )
                 thunk = THUNK_LFENCE;
+            /*
+             * On Intel hardware, we'd like to use retpoline in preference to
+             * IBRS, but only if it is safe on this hardware.
+             */
+            else if ( retpoline_safe() )
+                thunk = THUNK_RETPOLINE;
+            else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+                ibrs = true;
         }
+        /* Without compiler thunk support, use IBRS if available. */
+        else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+            ibrs = true;
     }
 
     /*
@@ -137,6 +219,13 @@ void __init init_speculation_mitigations
         thunk = THUNK_NONE;
 
     /*
+     * If IBRS is in use and thunks are compiled in, there is no point
+     * suffering extra overhead.  Switch to the least-overhead thunk.
+     */
+    if ( ibrs && thunk == THUNK_DEFAULT )
+        thunk = THUNK_JMP;
+
+    /*
      * If there are still no thunk preferences, the compiled default is
      * actually retpoline, and it is better than nothing.
      */
@@ -149,6 +238,50 @@ void __init init_speculation_mitigations
     else if ( thunk == THUNK_JMP )
         setup_force_cpu_cap(X86_FEATURE_IND_THUNK_JMP);
 
+    if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+    {
+        /*
+         * Even if we've chosen to not have IBRS set in Xen context, we still
+         * need the IBRS entry/exit logic to virtualise IBRS support for
+         * guests.
+         */
+        if ( ibrs )
+            setup_force_cpu_cap(X86_FEATURE_XEN_IBRS_SET);
+        else
+            setup_force_cpu_cap(X86_FEATURE_XEN_IBRS_CLEAR);
+
+        default_bti_ist_info |= BTI_IST_WRMSR | ibrs;
+    }
+
+    /*
+     * PV guests can poison the RSB to any virtual address from which
+     * they can execute a call instruction.  This is necessarily outside
+     * of the Xen supervisor mappings.
+     *
+     * With SMEP enabled, the processor won't speculate into user mappings.
+     * Therefore, in this case, we don't need to worry about poisoned entries
+     * from 64bit PV guests.
+     *
+     * 32bit PV guest kernels run in ring 1, so use supervisor mappings.
+     * If a processors speculates to 32bit PV guest kernel mappings, it is
+     * speculating in 64bit supervisor mode, and can leak data.
+     */
+    if ( opt_rsb_native )
+    {
+        setup_force_cpu_cap(X86_FEATURE_RSB_NATIVE);
+        default_bti_ist_info |= BTI_IST_RSB;
+    }
+
+    /*
+     * HVM guests can always poison the RSB to point at Xen supervisor
+     * mappings.
+     */
+    if ( opt_rsb_vmexit )
+        setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
+
+    /* (Re)init BSP state now that default_bti_ist_info has been calculated. */
+    init_shadow_spec_ctrl_state();
+
     print_details(thunk);
 }
 
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -24,12 +24,14 @@
 
 void init_speculation_mitigations(void);
 
+extern uint8_t default_bti_ist_info;
+
 static inline void init_shadow_spec_ctrl_state(void)
 {
     struct cpu_info *info = get_cpu_info();
 
     info->shadow_spec_ctrl = info->use_shadow_spec_ctrl = 0;
-    info->bti_ist_info = 0;
+    info->bti_ist_info = default_bti_ist_info;
 }
 
 #endif /* !__X86_SPEC_CTRL_H__ */

Places

File 5a6b36cd-8-x86-boot-calculate-best-BTI-mitigation.patch of Package xen (Revision f461f181be8e9ab1102295d3f581363b)

Places