[PATCH 0/3] x86/bugs: Separate config for mitigations (part 2)

[PATCH 0/3] x86/bugs: Separate config for mitigations (part 2)

[Breno Leitao]

The current CONFIG_SPECULATION_MITIGATIONS namespace is only
halfway populated, where some mitigations have entries in Kconfig, and
they could be modified, while others mitigations do not have Kconfig entries,
and can not be controlled at build time.

Fine-grained control over these Kconfig entries can help in a number of ways:

1) Users can choose and pick only mitigations that are important for
 their workloads.

2) Users and developers can choose to disable mitigations that mangle
 the assembly code generation, making it hard to read.

3) Separate Kconfigs for just source code readability,
 so that we see *which* butt-ugly piece of crap code is for what
 reason...

In most cases, if a mitigation is disabled at compilation time, it
can still be enabled at runtime using kernel command line arguments.

This is the second part of the initial patchset[1] that got half landed.
The first patch did some code re-organization. This second part
contains the exact missing patches from the initial patchset, and
basically adds build-time configuration for the other mitigations that
are currently only disabled at boot time.

Here is a detailed view of each patch:

Patch 1: Create a Kconfig to disable GDS mitigation.
Patch 2: Add a Kconfig entry for each mitigation that doesn't have such
Patch 3: Make spectre v2 userspace mitigation dependent on kernel
         mitigations.

With this patch applied, setting CONFIG_SPECULATION_MITIGATIONS=n, a
simple script[2] shows that all the mitigations are disabled:

  spectre_v2_user_stibp   	 SPECTRE_V2_USER_NONE
  spectre_v2_user_ibpb    	 SPECTRE_V2_USER_NONE
  spectre_v2_cmd          	 SPECTRE_V2_CMD_NONE
  ssb_mode                	 SPEC_STORE_BYPASS_NONE
  l1tf_mitigation         	 L1TF_MITIGATION_OFF
  srso_mitigation         	 SRSO_MITIGATION_NONE
  srso_cmd                	 SRSO_CMD_SAFE_RET
  mds_mitigation          	 MDS_MITIGATION_OFF
  taa_mitigation          	 TAA_MITIGATION_OFF
  mmio_mitigation         	 MMIO_MITIGATION_OFF
  srbds_mitigation        	 SRBDS_MITIGATION_OFF
  gds_mitigation          	 GDS_MITIGATION_OFF
  spectre_v1_mitigation   	 SPECTRE_V1_MITIGATION_NONE
  spectre_v2_enabled      	 SPECTRE_V2_NONE
  retbleed_mitigation     	 RETBLEED_MITIGATION_NONE

[1] https://lore.kernel.org/all/ZZ7c9EbJ71zU5TOF@gmail.com/#t
[2] https://github.com/leitao/debug/blob/main/spec/dump_speculation.py

Breno Leitao (3):
  x86/bugs: Create a way to disable GDS mitigation
  x86/bugs: Add a separate config for missing mitigation
  x86/bugs: spectre_v2_user default mode depends on main default

 arch/x86/Kconfig           | 117 +++++++++++++++++++++++++++++++++++--
 arch/x86/kernel/cpu/bugs.c |  57 +++++++++++-------
 2 files changed, 147 insertions(+), 27 deletions(-)

-- 
2.34.1

[PATCH 1/3] x86/bugs: Create a way to disable GDS mitigation

[Breno Leitao]

Currently there is no way to disable GDS mitigation at build time.
The current config option (GDS_MITIGATION_FORCE) just enables a more
drastic mitigation.

Create a new kernel config that allows GDS to be completely disabled,
similarly to the "gather_data_sampling=off" or "mitigations=off" kernel
command-line. Move the GDS_MITIGATION_FORCE under this new mitigation.

Suggested-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 arch/x86/Kconfig           | 16 +++++++++++-----
 arch/x86/kernel/cpu/bugs.c |  7 ++++---
 2 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 0a9fea390ef3..d5e3f1a8cacd 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2587,15 +2587,21 @@ config MITIGATION_SLS
 	  against straight line speculation. The kernel image might be slightly
 	  larger.
 
+config MITIGATION_GDS
+	bool "Mitigate Gather Data Sampling"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for Gather Data Sampling (GDS). GDS is a hardware
+	  vulnerability which allows unprivileged speculative access to data
+	  which was previously stored in vector registers. The attacker uses gather
+	  instructions to infer the stale vector register data.
+
 config MITIGATION_GDS_FORCE
 	bool "Force GDS Mitigation"
-	depends on CPU_SUP_INTEL
+	depends on MITIGATION_GDS
 	default n
 	help
-	  Gather Data Sampling (GDS) is a hardware vulnerability which allows
-	  unprivileged speculative access to data which was previously stored in
-	  vector registers.
-
 	  This option is equivalent to setting gather_data_sampling=force on the
 	  command line. The microcode mitigation is used if present, otherwise
 	  AVX is disabled as a mitigation. On affected systems that are missing
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index f2775417bda2..0172bb0f61fe 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -671,10 +671,11 @@ enum gds_mitigations {
 	GDS_MITIGATION_HYPERVISOR,
 };
 
-#if IS_ENABLED(CONFIG_MITIGATION_GDS_FORCE)
-static enum gds_mitigations gds_mitigation __ro_after_init = GDS_MITIGATION_FORCE;
+#if IS_ENABLED(CONFIG_MITIGATION_GDS)
+static enum gds_mitigations gds_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_GDS_FORCE) ? GDS_MITIGATION_FORCE : GDS_MITIGATION_FULL;
 #else
-static enum gds_mitigations gds_mitigation __ro_after_init = GDS_MITIGATION_FULL;
+static enum gds_mitigations gds_mitigation __ro_after_init = GDS_MITIGATION_OFF;
 #endif
 
 static const char * const gds_strings[] = {
-- 
2.34.1

[PATCH 2/3] x86/bugs: Add a separate config for missing mitigation

[Breno Leitao]

Currently, the CONFIG_SPECULATION_MITIGATIONS is halfway populated,
where some mitigations have entries in Kconfig, and they could be
modified, while others mitigations do not have Kconfig entries, and
could not be controlled at build time.

Create an entry for each CPU mitigation under
CONFIG_SPECULATION_MITIGATIONS. This allow users to enable or disable
them at compilation time.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 arch/x86/Kconfig           | 101 +++++++++++++++++++++++++++++++++++++
 arch/x86/kernel/cpu/bugs.c |  39 ++++++++------
 2 files changed, 125 insertions(+), 15 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index d5e3f1a8cacd..e16cfaf3dd14 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2612,6 +2612,107 @@ config MITIGATION_GDS_FORCE
 
 	  If in doubt, say N.
 
+config MITIGATION_MDS
+	bool "Mitigate Microarchitectural Data Sampling (MDS) hardware bug"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for Microarchitectural Data Sampling (MDS). MDS is
+	  a hardware vulnerability which allows unprivileged speculative access
+	  to data which is available in various CPU internal buffers.
+	  See also <file:Documentation/admin-guide/hw-vuln/mds.rst>
+
+config MITIGATION_TAA
+	bool "Mitigate TSX Asynchronous Abort (TAA) hardware bug"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for TSX Asynchronous Abort (TAA). TAA is a hardware
+	  vulnerability that allows unprivileged speculative access to data
+	  which is available in various CPU internal buffers by using
+	  asynchronous aborts within an Intel TSX transactional region.
+	  See also <file:Documentation/admin-guide/hw-vuln/tsx_async_abort.rst>
+
+config MITIGATION_MMIO_STALE_DATA
+	bool "Mitigate MMIO Stale Data hardware bug"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for MMIO Stale Data hardware bugs.  Processor MMIO
+	  Stale Data Vulnerabilities are a class of memory-mapped I/O (MMIO)
+	  vulnerabilities that can expose data. The vulnerabilities require the
+	  attacker to have access to MMIO.
+	  See also
+	  <file:Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst>
+
+config MITIGATION_L1TF
+	bool "Mitigate L1 Terminal Fault (L1TF) hardware bug"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Mitigate L1 Terminal Fault (L1TF) hardware bug. L1 Terminal Fault is a
+	  hardware vulnerability which allows unprivileged speculative access to data
+	  available in the Level 1 Data Cache.
+	  See <file:Documentation/admin-guide/hw-vuln/l1tf.rst
+
+config MITIGATION_RETBLEED
+	bool "Mitigate RETBleed hardware bug"
+	depends on CPU_SUP_INTEL || (CPU_SUP_AMD && MITIGATION_UNRET_ENTRY)
+	default y
+	help
+	  Enable mitigation for RETBleed (Arbitrary Speculative Code Execution
+	  with Return Instructions) vulnerability.  RETBleed is a speculative
+	  execution attack which takes advantage of microarchitectural behavior
+	  in many modern microprocessors, similar to Spectre v2. An
+	  unprivileged attacker can use these flaws to bypass conventional
+	  memory security restrictions to gain read access to privileged memory
+	  that would otherwise be inaccessible.
+
+config MITIGATION_SPECTRE_V1
+	bool "Mitigate SPECTRE V1 hardware bug"
+	default y
+	help
+	  Enable mitigation for Spectre V1 (Bounds Check Bypass). Spectre V1 is a
+	  class of side channel attacks that takes advantage of speculative
+	  execution that bypasses conditional branch instructions used for
+	  memory access bounds check.
+	  See also <file:Documentation/admin-guide/hw-vuln/spectre.rst>
+
+config MITIGATION_SPECTRE_V2
+	bool "Mitigate SPECTRE V2 hardware bug"
+	default y
+	help
+	  Enable mitigation for Spectre V2 (Branch Target Injection). Spectre
+	  V2 is a class of side channel attacks that takes advantage of
+	  indirect branch predictors inside the processor. In Spectre variant 2
+	  attacks, the attacker can steer speculative indirect branches in the
+	  victim to gadget code by poisoning the branch target buffer of a CPU
+	  used for predicting indirect branch addresses.
+	  See also <file:Documentation/admin-guide/hw-vuln/spectre.rst>
+
+config MITIGATION_SRBDS
+	bool "Mitigate Special Register Buffer Data Sampling (SRBDS) hardware bug"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for Special Register Buffer Data Sampling (SRBDS).
+	  SRBDS is a hardware vulnerability that allows Microarchitectural Data
+	  Sampling (MDS) techniques to infer values returned from special
+	  register accesses. An unprivileged user can extract values returned
+	  from RDRAND and RDSEED executed on another core or sibling thread
+	  using MDS techniques.
+	  See also
+	  <file:Documentation/admin-guide/hw-vuln/special-register-buffer-data-sampling.rst>
+
+config MITIGATION_SSB
+	bool "Mitigate Speculative Store Bypass (SSB) hardware bug"
+	default y
+	help
+	  Enable mitigation for Speculative Store Bypass (SSB). SSB is a
+	  hardware security vulnerability and its exploitation takes advantage
+	  of speculative execution in a similar way to the Meltdown and Spectre
+	  security vulnerabilities.
+
 endif
 
 config ARCH_HAS_ADD_PAGES
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 0172bb0f61fe..11ccbadd8800 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -232,7 +232,8 @@ static void x86_amd_ssb_disable(void)
 #define pr_fmt(fmt)	"MDS: " fmt
 
 /* Default mitigation for MDS-affected CPUs */
-static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
+static enum mds_mitigations mds_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_MDS) ? MDS_MITIGATION_FULL : MDS_MITIGATION_OFF;
 static bool mds_nosmt __ro_after_init = false;
 
 static const char * const mds_strings[] = {
@@ -292,7 +293,8 @@ enum taa_mitigations {
 };
 
 /* Default mitigation for TAA-affected CPUs */
-static enum taa_mitigations taa_mitigation __ro_after_init = TAA_MITIGATION_VERW;
+static enum taa_mitigations taa_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_TAA) ? TAA_MITIGATION_VERW : TAA_MITIGATION_OFF;
 static bool taa_nosmt __ro_after_init;
 
 static const char * const taa_strings[] = {
@@ -393,7 +395,8 @@ enum mmio_mitigations {
 };
 
 /* Default mitigation for Processor MMIO Stale Data vulnerabilities */
-static enum mmio_mitigations mmio_mitigation __ro_after_init = MMIO_MITIGATION_VERW;
+static enum mmio_mitigations mmio_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_MMIO_STALE_DATA) ? MMIO_MITIGATION_VERW : MMIO_MITIGATION_OFF;
 static bool mmio_nosmt __ro_after_init = false;
 
 static const char * const mmio_strings[] = {
@@ -542,7 +545,8 @@ enum srbds_mitigations {
 	SRBDS_MITIGATION_HYPERVISOR,
 };
 
-static enum srbds_mitigations srbds_mitigation __ro_after_init = SRBDS_MITIGATION_FULL;
+static enum srbds_mitigations srbds_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_SRBDS) ? SRBDS_MITIGATION_FULL : SRBDS_MITIGATION_OFF;
 
 static const char * const srbds_strings[] = {
 	[SRBDS_MITIGATION_OFF]		= "Vulnerable",
@@ -812,7 +816,8 @@ enum spectre_v1_mitigation {
 };
 
 static enum spectre_v1_mitigation spectre_v1_mitigation __ro_after_init =
-	SPECTRE_V1_MITIGATION_AUTO;
+	IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V1) ?
+		SPECTRE_V1_MITIGATION_AUTO : SPECTRE_V1_MITIGATION_NONE;
 
 static const char * const spectre_v1_strings[] = {
 	[SPECTRE_V1_MITIGATION_NONE] = "Vulnerable: __user pointer sanitization and usercopy barriers only; no swapgs barriers",
@@ -927,7 +932,7 @@ static const char * const retbleed_strings[] = {
 static enum retbleed_mitigation retbleed_mitigation __ro_after_init =
 	RETBLEED_MITIGATION_NONE;
 static enum retbleed_mitigation_cmd retbleed_cmd __ro_after_init =
-	RETBLEED_CMD_AUTO;
+	IS_ENABLED(CONFIG_MITIGATION_RETBLEED) ? RETBLEED_CMD_AUTO : RETBLEED_CMD_OFF;
 
 static int __ro_after_init retbleed_nosmt = false;
 
@@ -1388,17 +1393,18 @@ static void __init spec_v2_print_cond(const char *reason, bool secure)
 
 static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 {
-	enum spectre_v2_mitigation_cmd cmd = SPECTRE_V2_CMD_AUTO;
+	enum spectre_v2_mitigation_cmd cmd;
 	char arg[20];
 	int ret, i;
 
+	cmd = IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V2) ?  SPECTRE_V2_CMD_AUTO : SPECTRE_V2_CMD_NONE;
 	if (cmdline_find_option_bool(boot_command_line, "nospectre_v2") ||
 	    cpu_mitigations_off())
 		return SPECTRE_V2_CMD_NONE;
 
 	ret = cmdline_find_option(boot_command_line, "spectre_v2", arg, sizeof(arg));
 	if (ret < 0)
-		return SPECTRE_V2_CMD_AUTO;
+		return cmd;
 
 	for (i = 0; i < ARRAY_SIZE(mitigation_options); i++) {
 		if (!match_option(arg, ret, mitigation_options[i].option))
@@ -1408,8 +1414,8 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 	}
 
 	if (i >= ARRAY_SIZE(mitigation_options)) {
-		pr_err("unknown option (%s). Switching to AUTO select\n", arg);
-		return SPECTRE_V2_CMD_AUTO;
+		pr_err("unknown option (%s). Switching to default mode\n", arg);
+		return cmd;
 	}
 
 	if ((cmd == SPECTRE_V2_CMD_RETPOLINE ||
@@ -1882,10 +1888,12 @@ static const struct {
 
 static enum ssb_mitigation_cmd __init ssb_parse_cmdline(void)
 {
-	enum ssb_mitigation_cmd cmd = SPEC_STORE_BYPASS_CMD_AUTO;
+	enum ssb_mitigation_cmd cmd;
 	char arg[20];
 	int ret, i;
 
+	cmd = IS_ENABLED(CONFIG_MITIGATION_SSB) ?
+		SPEC_STORE_BYPASS_CMD_AUTO : SPEC_STORE_BYPASS_CMD_NONE;
 	if (cmdline_find_option_bool(boot_command_line, "nospec_store_bypass_disable") ||
 	    cpu_mitigations_off()) {
 		return SPEC_STORE_BYPASS_CMD_NONE;
@@ -1893,7 +1901,7 @@ static enum ssb_mitigation_cmd __init ssb_parse_cmdline(void)
 		ret = cmdline_find_option(boot_command_line, "spec_store_bypass_disable",
 					  arg, sizeof(arg));
 		if (ret < 0)
-			return SPEC_STORE_BYPASS_CMD_AUTO;
+			return cmd;
 
 		for (i = 0; i < ARRAY_SIZE(ssb_mitigation_options); i++) {
 			if (!match_option(arg, ret, ssb_mitigation_options[i].option))
@@ -1904,8 +1912,8 @@ static enum ssb_mitigation_cmd __init ssb_parse_cmdline(void)
 		}
 
 		if (i >= ARRAY_SIZE(ssb_mitigation_options)) {
-			pr_err("unknown option (%s). Switching to AUTO select\n", arg);
-			return SPEC_STORE_BYPASS_CMD_AUTO;
+			pr_err("unknown option (%s). Switching to default mode\n", arg);
+			return cmd;
 		}
 	}
 
@@ -2232,7 +2240,8 @@ EXPORT_SYMBOL_GPL(itlb_multihit_kvm_mitigation);
 #define pr_fmt(fmt)	"L1TF: " fmt
 
 /* Default mitigation for L1TF-affected CPUs */
-enum l1tf_mitigations l1tf_mitigation __ro_after_init = L1TF_MITIGATION_FLUSH;
+enum l1tf_mitigations l1tf_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_L1TF) ? L1TF_MITIGATION_FLUSH : L1TF_MITIGATION_OFF;
 #if IS_ENABLED(CONFIG_KVM_INTEL)
 EXPORT_SYMBOL_GPL(l1tf_mitigation);
 #endif
-- 
2.34.1

[PATCH 3/3] x86/bugs: spectre_v2_user default mode depends on main default

[Breno Leitao]

Change the default value of spectre v2 in user mode to respect the
CONFIG_MITIGATION_SPECTRE_V2 config option.

Currently, user mode spectre v2 is set to auto
(SPECTRE_V2_USER_CMD_AUTO) by default, even if
CONFIG_MITIGATION_SPECTRE_V2 is disabled.

Set the Spectre_v2 value to auto (SPECTRE_V2_USER_CMD_AUTO) if the
Spectre v2 config (CONFIG_MITIGATION_SPECTRE_V2) is enabled, otherwise
set the value to none (SPECTRE_V2_USER_CMD_NONE).

Important to say the command line argument "spectre_v2_user" overwrites
the default value in both cases.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 arch/x86/kernel/cpu/bugs.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 11ccbadd8800..4f1da92784c6 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1221,8 +1221,11 @@ static __ro_after_init enum spectre_v2_mitigation_cmd spectre_v2_cmd;
 static enum spectre_v2_user_cmd __init
 spectre_v2_parse_user_cmdline(void)
 {
+	int ret, i, mode;
 	char arg[20];
-	int ret, i;
+
+	mode = IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V2) ?
+		SPECTRE_V2_USER_CMD_AUTO : SPECTRE_V2_USER_CMD_NONE;
 
 	switch (spectre_v2_cmd) {
 	case SPECTRE_V2_CMD_NONE:
@@ -1236,7 +1239,7 @@ spectre_v2_parse_user_cmdline(void)
 	ret = cmdline_find_option(boot_command_line, "spectre_v2_user",
 				  arg, sizeof(arg));
 	if (ret < 0)
-		return SPECTRE_V2_USER_CMD_AUTO;
+		return mode;
 
 	for (i = 0; i < ARRAY_SIZE(v2_user_options); i++) {
 		if (match_option(arg, ret, v2_user_options[i].option)) {
@@ -1246,8 +1249,8 @@ spectre_v2_parse_user_cmdline(void)
 		}
 	}
 
-	pr_err("Unknown user space protection option (%s). Switching to AUTO select\n", arg);
-	return SPECTRE_V2_USER_CMD_AUTO;
+	pr_err("Unknown user space protection option (%s). Switching to default\n", arg);
+	return mode;
 }
 
 static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
-- 
2.34.1

Re: [PATCH 2/3] x86/bugs: Add a separate config for missing mitigation

[Pawan Gupta]

On Thu, Jan 18, 2024 at 09:32:12AM -0800, Breno Leitao wrote:
> +config MITIGATION_RETBLEED
> +	bool "Mitigate RETBleed hardware bug"
> +	depends on CPU_SUP_INTEL || (CPU_SUP_AMD && MITIGATION_UNRET_ENTRY)

Atleast on Intel CPUs, Retbleed mitigation is meaningless without
spectre-v2 being mitigated, shouldn't this depend on MITIGATION_SPECTRE_V2?

Re: [PATCH 3/3] x86/bugs: spectre_v2_user default mode depends on main default

[Pawan Gupta]

On Thu, Jan 18, 2024 at 09:32:13AM -0800, Breno Leitao wrote:
>  static enum spectre_v2_user_cmd __init
>  spectre_v2_parse_user_cmdline(void)
>  {
> +	int ret, i, mode;

mode is being returned, it is better to match its type with function's
return type "enum spectre_v2_user_cmd".

>  	char arg[20];
> -	int ret, i;
> +
> +	mode = IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V2) ?
> +		SPECTRE_V2_USER_CMD_AUTO : SPECTRE_V2_USER_CMD_NONE;
>  
>  	switch (spectre_v2_cmd) {
>  	case SPECTRE_V2_CMD_NONE:
> @@ -1236,7 +1239,7 @@ spectre_v2_parse_user_cmdline(void)
>  	ret = cmdline_find_option(boot_command_line, "spectre_v2_user",
>  				  arg, sizeof(arg));
>  	if (ret < 0)
> -		return SPECTRE_V2_USER_CMD_AUTO;
> +		return mode;
>  
>  	for (i = 0; i < ARRAY_SIZE(v2_user_options); i++) {
>  		if (match_option(arg, ret, v2_user_options[i].option)) {
> @@ -1246,8 +1249,8 @@ spectre_v2_parse_user_cmdline(void)
>  		}
>  	}
>  
> -	pr_err("Unknown user space protection option (%s). Switching to AUTO select\n", arg);
> -	return SPECTRE_V2_USER_CMD_AUTO;
> +	pr_err("Unknown user space protection option (%s). Switching to default\n", arg);
> +	return mode;
>  }
>  
>  static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
> -- 
> 2.34.1
> 

Re: [PATCH 3/3] x86/bugs: spectre_v2_user default mode depends on main default

[Breno Leitao]

On Mon, Jan 22, 2024 at 04:04:31PM -0800, Pawan Gupta wrote:
> On Thu, Jan 18, 2024 at 09:32:13AM -0800, Breno Leitao wrote:
> >  static enum spectre_v2_user_cmd __init
> >  spectre_v2_parse_user_cmdline(void)
> >  {
> > +	int ret, i, mode;
> 
> mode is being returned, it is better to match its type with function's
> return type "enum spectre_v2_user_cmd".

Agree. I will update. Thanks!

Re: [PATCH 2/3] x86/bugs: Add a separate config for missing mitigation

[Breno Leitao]

On Mon, Jan 22, 2024 at 12:58:21PM -0800, Pawan Gupta wrote:
> On Thu, Jan 18, 2024 at 09:32:12AM -0800, Breno Leitao wrote:
> > +config MITIGATION_RETBLEED
> > +	bool "Mitigate RETBleed hardware bug"
> > +	depends on CPU_SUP_INTEL || (CPU_SUP_AMD && MITIGATION_UNRET_ENTRY)
> 
> Atleast on Intel CPUs, Retbleed mitigation is meaningless without
> spectre-v2 being mitigated, shouldn't this depend on MITIGATION_SPECTRE_V2?

I suppose it is the same for AMD, right?

So, I suppose it should be something as:

	depends on (MITIGATION_SPECTRE_V2 && (CPU_SUP_INTEL || (CPU_SUP_AMD && MITIGATION_UNRET_ENTRY))

Is this better?

Thanks!

Re: [PATCH 2/3] x86/bugs: Add a separate config for missing mitigation

[Pawan Gupta]

On Tue, Jan 23, 2024 at 07:05:47AM -0800, Breno Leitao wrote:
> On Mon, Jan 22, 2024 at 12:58:21PM -0800, Pawan Gupta wrote:
> > On Thu, Jan 18, 2024 at 09:32:12AM -0800, Breno Leitao wrote:
> > > +config MITIGATION_RETBLEED
> > > +	bool "Mitigate RETBleed hardware bug"
> > > +	depends on CPU_SUP_INTEL || (CPU_SUP_AMD && MITIGATION_UNRET_ENTRY)
> > 
> > Atleast on Intel CPUs, Retbleed mitigation is meaningless without
> > spectre-v2 being mitigated, shouldn't this depend on MITIGATION_SPECTRE_V2?
> 
> I suppose it is the same for AMD, right?

AMD's retbleed problem and mitigation is a bit different, it has to do
with Branch Type Confusion(BTC). I am not sure if it is possible to
mitigate it without spectre-v2 mitigation. Someone from AMD should be
able to clarify. Looking at the code it appears the dependency is on
CONFIG_CPU_UNRET_ENTRY or CONFIG_CPU_IBPB_ENTRY being set:

retbleed_select_mitigation()
{
...
         case RETBLEED_CMD_AUTO:
                 if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
                     boot_cpu_data.x86_vendor == X86_VENDOR_HYGON) {
                         if (IS_ENABLED(CONFIG_CPU_UNRET_ENTRY))
                                 retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
                         else if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY) && boot_cpu_has(X86_FEATURE_IBPB))
                                 retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
                 }

> So, I suppose it should be something as:
> 
> 	depends on (MITIGATION_SPECTRE_V2 && (CPU_SUP_INTEL || (CPU_SUP_AMD && MITIGATION_UNRET_ENTRY))
> 
> Is this better?
> 
> Thanks!