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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH v8 05/17] x86: Introduce alternative indirect thunks
Depending on hardware and microcode availability, we will want to replace
IND_THUNK_REPOLINE with other implementations.
For AMD hardware, choose IND_THUNK_LFENCE in preference to retpoline if lfence
is known to be (or was successfully made) dispatch serialising.
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
v7:
* Rebase over compiler changes
* Spelling/grammar fixes
* Make opt_thunk static
---
docs/misc/xen-command-line.markdown | 16 ++++++++
xen/arch/x86/indirect-thunk.S | 17 +++++++--
xen/arch/x86/spec_ctrl.c | 75 +++++++++++++++++++++++++++++++++++--
xen/include/asm-x86/cpufeatures.h | 2 +
4 files changed, 104 insertions(+), 6 deletions(-)
diff --git a/docs/misc/xen-command-line.markdown
b/docs/misc/xen-command-line.markdown
index 781110d..96e57c2 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -245,6 +245,22 @@ and not running softirqs. Reduce this if softirqs are not
being run frequently
enough. Setting this to a high value may cause boot failure, particularly if
the NMI watchdog is also enabled.
+### bti (x86)
+> `= List of [ thunk=retpoline|lfence|jmp ]`
+
+Branch Target Injection controls. By default, Xen will pick the most
+appropriate BTI mitigations based on compiled in support, loaded microcode,
+and hardware details.
+
+**WARNING: Any use of this option may interfere with heuristics. Use with
+extreme care.**
+
+If Xen was compiled with INDIRECT_THUNK support, `thunk=` can be used to
+select which of the thunks gets patched into the `__x86_indirect_thunk_%reg`
+locations. The default thunk is `retpoline` (generally preferred for Intel
+hardware), with the alternatives being `jmp` (a `jmp *%reg` gadget, minimal
+overhead), and `lfence` (an `lfence; jmp *%reg` gadget, preferred for AMD).
+
### xenheap\_megabytes (arm32)
> `= <size>`
diff --git a/xen/arch/x86/indirect-thunk.S b/xen/arch/x86/indirect-thunk.S
index 3eaf505..7d34707 100644
--- a/xen/arch/x86/indirect-thunk.S
+++ b/xen/arch/x86/indirect-thunk.S
@@ -21,15 +21,26 @@
ret
.endm
+.macro IND_THUNK_LFENCE reg:req
+ lfence
+ jmp *%\reg
+.endm
+
+.macro IND_THUNK_JMP reg:req
+ jmp *%\reg
+.endm
+
/*
- * Build the __x86_indirect_thunk_* symbols. Currently implement the
- * retpoline thunk only.
+ * Build the __x86.indirect_thunk.* symbols. Execution lands on an
+ * alternative patch point which implements one of the above THUNK_*'s
*/
.macro GEN_INDIRECT_THUNK reg:req
.section .text.__x86_indirect_thunk_\reg, "ax", @progbits
ENTRY(__x86_indirect_thunk_\reg)
- IND_THUNK_RETPOLINE \reg
+ ALTERNATIVE_2 __stringify(IND_THUNK_RETPOLINE \reg), \
+ __stringify(IND_THUNK_LFENCE \reg), X86_FEATURE_IND_THUNK_LFENCE, \
+ __stringify(IND_THUNK_JMP \reg), X86_FEATURE_IND_THUNK_JMP
.endm
/* Instantiate GEN_INDIRECT_THUNK for each register except %rsp. */
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 256701a..d601c02 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -16,18 +16,54 @@
*
* Copyright (c) 2017-2018 Citrix Systems Ltd.
*/
+#include <xen/errno.h>
#include <xen/init.h>
#include <xen/lib.h>
#include <asm/processor.h>
#include <asm/spec_ctrl.h>
-enum ind_thunk {
+static enum ind_thunk {
THUNK_DEFAULT, /* Decide which thunk to use at boot time. */
THUNK_NONE, /* Missing compiler support for thunks. */
THUNK_RETPOLINE,
-};
+ THUNK_LFENCE,
+ THUNK_JMP,
+} opt_thunk __initdata = THUNK_DEFAULT;
+
+static int __init parse_bti(const char *s)
+{
+ const char *ss;
+ int rc = 0;
+
+ do {
+ ss = strchr(s, ',');
+ if ( !ss )
+ ss = strchr(s, '\0');
+
+ if ( !strncmp(s, "thunk=", 6) )
+ {
+ s += 6;
+
+ if ( !strncmp(s, "retpoline", ss - s) )
+ opt_thunk = THUNK_RETPOLINE;
+ else if ( !strncmp(s, "lfence", ss - s) )
+ opt_thunk = THUNK_LFENCE;
+ else if ( !strncmp(s, "jmp", ss - s) )
+ opt_thunk = THUNK_JMP;
+ else
+ rc = -EINVAL;
+ }
+ else
+ rc = -EINVAL;
+
+ s = ss + 1;
+ } while ( *ss );
+
+ return rc;
+}
+custom_param("bti", parse_bti);
static void __init print_details(enum ind_thunk thunk)
{
@@ -40,7 +76,9 @@ static void __init print_details(enum ind_thunk thunk)
printk(XENLOG_INFO
"BTI mitigations: Thunk %s\n",
thunk == THUNK_NONE ? "N/A" :
- thunk == THUNK_RETPOLINE ? "RETPOLINE" : "?");
+ thunk == THUNK_RETPOLINE ? "RETPOLINE" :
+ thunk == THUNK_LFENCE ? "LFENCE" :
+ thunk == THUNK_JMP ? "JMP" : "?");
}
void __init init_speculation_mitigations(void)
@@ -48,6 +86,31 @@ void __init init_speculation_mitigations(void)
enum ind_thunk thunk = THUNK_DEFAULT;
/*
+ * Has the user specified any custom BTI mitigations? If so, follow their
+ * instructions exactly and disable all heuristics.
+ */
+ if ( opt_thunk != THUNK_DEFAULT )
+ {
+ thunk = opt_thunk;
+ }
+ else
+ {
+ /*
+ * Evaluate the safest Branch Target Injection mitigations to use.
+ * First, begin with compiler-aided mitigations.
+ */
+ if ( IS_ENABLED(CONFIG_INDIRECT_THUNK) )
+ {
+ /*
+ * AMD's recommended mitigation is to set lfence as being dispatch
+ * serialising, and to use IND_THUNK_LFENCE.
+ */
+ if ( cpu_has_lfence_dispatch )
+ thunk = THUNK_LFENCE;
+ }
+ }
+
+ /*
* Supplimentary minor adjustments. Without compiler support, there are
* no thunks.
*/
@@ -61,6 +124,12 @@ void __init init_speculation_mitigations(void)
if ( thunk == THUNK_DEFAULT )
thunk = THUNK_RETPOLINE;
+ /* Apply the chosen settings. */
+ if ( thunk == THUNK_LFENCE )
+ setup_force_cpu_cap(X86_FEATURE_IND_THUNK_LFENCE);
+ else if ( thunk == THUNK_JMP )
+ setup_force_cpu_cap(X86_FEATURE_IND_THUNK_JMP);
+
print_details(thunk);
}
diff --git a/xen/include/asm-x86/cpufeatures.h
b/xen/include/asm-x86/cpufeatures.h
index 58b37d6..ba1771b 100644
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -23,3 +23,5 @@ XEN_CPUFEATURE(MFENCE_RDTSC, (FSCAPINTS+0)*32+ 9) /*
MFENCE synchronizes RDTS
XEN_CPUFEATURE(XEN_SMEP, (FSCAPINTS+0)*32+10) /* SMEP gets used by Xen
itself */
XEN_CPUFEATURE(XEN_SMAP, (FSCAPINTS+0)*32+11) /* SMAP gets used by Xen
itself */
XEN_CPUFEATURE(LFENCE_DISPATCH, (FSCAPINTS+0)*32+12) /* lfence set as Dispatch
Serialising */
+XEN_CPUFEATURE(IND_THUNK_LFENCE,(FSCAPINTS+0)*32+13) /* Use IND_THUNK_LFENCE */
+XEN_CPUFEATURE(IND_THUNK_JMP, (FSCAPINTS+0)*32+14) /* Use IND_THUNK_JMP */
--
2.1.4
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