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x86/retbleed: Add X86_FEATURE_CALL_DEPTH

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Intel SKL CPUs fall back to other predictors when the RSB underflows. The
only microcode mitigation is IBRS which is insanely expensive. It comes
with performance drops of up to 30% depending on the workload.

A way less expensive, but nevertheless horrible mitigation is to track the
call depth in software and overeagerly fill the RSB when returns underflow
the software counter.

Provide a configuration symbol and a CPU misfeature bit.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111147.056176424@infradead.org
This commit is contained in:
Thomas Gleixner 2022-09-15 13:11:19 +02:00 committed by Peter Zijlstra
parent bea75b3389
commit 80e4c1cd42
3 changed files with 28 additions and 1 deletions
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19
arch/x86/Kconfig
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@ -2523,6 +2523,25 @@ config CPU_UNRET_ENTRY
help
Compile the kernel with support for the retbleed=unret mitigation.
config CALL_DEPTH_TRACKING
bool "Mitigate RSB underflow with call depth tracking"
depends on CPU_SUP_INTEL && HAVE_CALL_THUNKS
select HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
select CALL_THUNKS
default y
help
Compile the kernel with call depth tracking to mitigate the Intel
SKL Return-Speculation-Buffer (RSB) underflow issue. The
mitigation is off by default and needs to be enabled on the
kernel command line via the retbleed=stuff option. For
non-affected systems the overhead of this option is marginal as
the call depth tracking is using run-time generated call thunks
in a compiler generated padding area and call patching. This
increases text size by ~5%. For non affected systems this space
is unused. On affected SKL systems this results in a significant
performance gain over the IBRS mitigation.
config CPU_IBPB_ENTRY
bool "Enable IBPB on kernel entry"
depends on CPU_SUP_AMD && X86_64

1
arch/x86/include/asm/cpufeatures.h
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@ -304,6 +304,7 @@
#define X86_FEATURE_UNRET (11*32+15) /* "" AMD BTB untrain return */
#define X86_FEATURE_USE_IBPB_FW (11*32+16) /* "" Use IBPB during runtime firmware calls */
#define X86_FEATURE_RSB_VMEXIT_LITE (11*32+17) /* "" Fill RSB on VM exit when EIBRS is enabled */
#define X86_FEATURE_CALL_DEPTH (11*32+18) /* "" Call depth tracking for RSB stuffing */
/* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
#define X86_FEATURE_AVX_VNNI (12*32+ 4) /* AVX VNNI instructions */

9
arch/x86/include/asm/disabled-features.h
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@ -69,6 +69,12 @@
# define DISABLE_UNRET (1 << (X86_FEATURE_UNRET & 31))
#endif
#ifdef CONFIG_CALL_DEPTH_TRACKING
# define DISABLE_CALL_DEPTH_TRACKING 0
#else
# define DISABLE_CALL_DEPTH_TRACKING (1 << (X86_FEATURE_CALL_DEPTH & 31))
#endif
#ifdef CONFIG_INTEL_IOMMU_SVM
# define DISABLE_ENQCMD 0
#else
@ -101,7 +107,8 @@
#define DISABLED_MASK8 (DISABLE_TDX_GUEST)
#define DISABLED_MASK9 (DISABLE_SGX)
#define DISABLED_MASK10 0
#define DISABLED_MASK11 (DISABLE_RETPOLINE|DISABLE_RETHUNK|DISABLE_UNRET)
#define DISABLED_MASK11 (DISABLE_RETPOLINE|DISABLE_RETHUNK|DISABLE_UNRET| \
DISABLE_CALL_DEPTH_TRACKING)
#define DISABLED_MASK12 0
#define DISABLED_MASK13 0
#define DISABLED_MASK14 0