[PATCH 0/2] x86/bugs: RSB tweaks

[PATCH 0/2] x86/bugs: RSB tweaks

[Josh Poimboeuf]

Some RSB filling tweaks as discussed in the following thread:

  [RFC PATCH v2 0/3] Add support for the ERAPS feature
  https://lore.kernel.org/20241111163913.36139-1-amit@kernel.org

Josh Poimboeuf (2):
  x86/bugs: Don't fill RSB on VMEXIT with eIBRS+retpoline
  x86/bugs: Don't fill RSB on context switch with eIBRS

 arch/x86/kernel/cpu/bugs.c | 97 +++++++++++++++-----------------------
 arch/x86/mm/tlb.c          |  2 +-
 2 files changed, 38 insertions(+), 61 deletions(-)

-- 
2.47.0

[PATCH 1/2] x86/bugs: Don't fill RSB on VMEXIT with eIBRS+retpoline

[Josh Poimboeuf]

eIBRS protects against RSB underflow/poisoning attacks.  Adding
retpoline to the mix doesn't change that.  Retpoline has a balanced
CALL/RET anyway.

So the current full RSB filling on VMEXIT with eIBRS+retpoline is
overkill.  Disable it (or do the VMEXIT_LITE mitigation if needed).

Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 arch/x86/kernel/cpu/bugs.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 47a01d4028f6..68bed17f0980 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1605,20 +1605,20 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
 	case SPECTRE_V2_NONE:
 		return;
 
-	case SPECTRE_V2_EIBRS_LFENCE:
 	case SPECTRE_V2_EIBRS:
+	case SPECTRE_V2_EIBRS_LFENCE:
+	case SPECTRE_V2_EIBRS_RETPOLINE:
 		if (boot_cpu_has_bug(X86_BUG_EIBRS_PBRSB)) {
-			setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT_LITE);
 			pr_info("Spectre v2 / PBRSB-eIBRS: Retire a single CALL on VMEXIT\n");
+			setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT_LITE);
 		}
 		return;
 
-	case SPECTRE_V2_EIBRS_RETPOLINE:
 	case SPECTRE_V2_RETPOLINE:
 	case SPECTRE_V2_LFENCE:
 	case SPECTRE_V2_IBRS:
-		setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
 		pr_info("Spectre v2 / SpectreRSB : Filling RSB on VMEXIT\n");
+		setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
 		return;
 	}
 
-- 
2.47.0

[PATCH 2/2] x86/bugs: Don't fill RSB on context switch with eIBRS

[Josh Poimboeuf]

User->user Spectre v2 attacks (including RSB) across context switches
are already mitigated by IBPB in cond_mitigation(), if enabled globally
or if at least one of the tasks has opted in to protection.  RSB filling
without IBPB serves no purpose for protecting user space, as indirect
branches are still vulnerable.

User->kernel RSB attacks are mitigated by eIBRS.  In which case the RSB
filling on context switch isn't needed.  Fix that.

While at it, update and coalesce the comments describing the various RSB
mitigations.

Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 arch/x86/kernel/cpu/bugs.c | 91 ++++++++++++++------------------------
 arch/x86/mm/tlb.c          |  2 +-
 2 files changed, 35 insertions(+), 58 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 68bed17f0980..e261f41749b0 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1579,27 +1579,44 @@ static void __init spec_ctrl_disable_kernel_rrsba(void)
 	rrsba_disabled = true;
 }
 
-static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_mitigation mode)
+static void __init spectre_v2_mitigate_rsb(enum spectre_v2_mitigation mode)
 {
 	/*
-	 * Similar to context switches, there are two types of RSB attacks
-	 * after VM exit:
+	 * In general there are two types of RSB attacks:
 	 *
-	 * 1) RSB underflow
+	 * 1) RSB underflow ("Intel Retbleed")
+	 *
+	 *    Some Intel parts have "bottomless RSB".  When the RSB is empty,
+	 *    speculated return targets may come from the branch predictor,
+	 *    which could have a user-poisoned BTB or BHB entry.
+	 *
+	 *    user->user attacks are mitigated by IBPB on context switch.
+	 *
+	 *    user->kernel attacks via context switch are mitigated by IBRS,
+	 *    eIBRS, or RSB filling.
+	 *
+	 *    user->kernel attacks via kernel entry are mitigated by IBRS,
+	 *    eIBRS, or call depth tracking.
+	 *
+	 *    On VMEXIT, guest->host attacks are mitigated by IBRS, eIBRS, or
+	 *    RSB filling.
 	 *
 	 * 2) Poisoned RSB entry
 	 *
-	 * When retpoline is enabled, both are mitigated by filling/clearing
-	 * the RSB.
+	 *    On a context switch, the previous task can poison RSB entries
+	 *    used by the next task, controlling its speculative return
+	 *    targets.  Poisoned RSB entries can also be created by "AMD
+	 *    Retbleed" or SRSO.
 	 *
-	 * When IBRS is enabled, while #1 would be mitigated by the IBRS branch
-	 * prediction isolation protections, RSB still needs to be cleared
-	 * because of #2.  Note that SMEP provides no protection here, unlike
-	 * user-space-poisoned RSB entries.
+	 *    user->user attacks are mitigated by IBPB on context switch.
 	 *
-	 * eIBRS should protect against RSB poisoning, but if the EIBRS_PBRSB
-	 * bug is present then a LITE version of RSB protection is required,
-	 * just a single call needs to retire before a RET is executed.
+	 *    user->kernel attacks via context switch are prevented by
+	 *    SMEP+eIBRS+SRSO mitigations, or RSB clearing.
+	 *
+	 *    guest->host attacks are mitigated by eIBRS or RSB clearing on
+	 *    VMEXIT.  eIBRS implementations with X86_BUG_EIBRS_PBRSB still
+	 *    need "lite" RSB filling which retires a CALL before the first
+	 *    RET.
 	 */
 	switch (mode) {
 	case SPECTRE_V2_NONE:
@@ -1617,12 +1634,13 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
 	case SPECTRE_V2_RETPOLINE:
 	case SPECTRE_V2_LFENCE:
 	case SPECTRE_V2_IBRS:
-		pr_info("Spectre v2 / SpectreRSB : Filling RSB on VMEXIT\n");
+		pr_info("Spectre v2 / SpectreRSB : Filling RSB on context switch and VMEXIT\n");
+		setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
 		setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
 		return;
 	}
 
-	pr_warn_once("Unknown Spectre v2 mode, disabling RSB mitigation at VM exit");
+	pr_warn_once("Unknown Spectre v2 mode, disabling RSB mitigation\n");
 	dump_stack();
 }
 
@@ -1817,48 +1835,7 @@ static void __init spectre_v2_select_mitigation(void)
 	spectre_v2_enabled = mode;
 	pr_info("%s\n", spectre_v2_strings[mode]);
 
-	/*
-	 * If Spectre v2 protection has been enabled, fill the RSB during a
-	 * context switch.  In general there are two types of RSB attacks
-	 * across context switches, for which the CALLs/RETs may be unbalanced.
-	 *
-	 * 1) RSB underflow
-	 *
-	 *    Some Intel parts have "bottomless RSB".  When the RSB is empty,
-	 *    speculated return targets may come from the branch predictor,
-	 *    which could have a user-poisoned BTB or BHB entry.
-	 *
-	 *    AMD has it even worse: *all* returns are speculated from the BTB,
-	 *    regardless of the state of the RSB.
-	 *
-	 *    When IBRS or eIBRS is enabled, the "user -> kernel" attack
-	 *    scenario is mitigated by the IBRS branch prediction isolation
-	 *    properties, so the RSB buffer filling wouldn't be necessary to
-	 *    protect against this type of attack.
-	 *
-	 *    The "user -> user" attack scenario is mitigated by RSB filling.
-	 *
-	 * 2) Poisoned RSB entry
-	 *
-	 *    If the 'next' in-kernel return stack is shorter than 'prev',
-	 *    'next' could be tricked into speculating with a user-poisoned RSB
-	 *    entry.
-	 *
-	 *    The "user -> kernel" attack scenario is mitigated by SMEP and
-	 *    eIBRS.
-	 *
-	 *    The "user -> user" scenario, also known as SpectreBHB, requires
-	 *    RSB clearing.
-	 *
-	 * So to mitigate all cases, unconditionally fill RSB on context
-	 * switches.
-	 *
-	 * FIXME: Is this pointless for retbleed-affected AMD?
-	 */
-	setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
-	pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch\n");
-
-	spectre_v2_determine_rsb_fill_type_at_vmexit(mode);
+	spectre_v2_mitigate_rsb(mode);
 
 	/*
 	 * Retpoline protects the kernel, but doesn't protect firmware.  IBRS
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 86593d1b787d..c693b877d4df 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -388,7 +388,7 @@ static void cond_mitigation(struct task_struct *next)
 	prev_mm = this_cpu_read(cpu_tlbstate.last_user_mm_spec);
 
 	/*
-	 * Avoid user/user BTB poisoning by flushing the branch predictor
+	 * Avoid user/user BTB/RSB poisoning by flushing the branch predictor
 	 * when switching between processes. This stops one process from
 	 * doing Spectre-v2 attacks on another.
 	 *
-- 
2.47.0

Re: [PATCH 2/2] x86/bugs: Don't fill RSB on context switch with eIBRS

[Shah, Amit]

On Tue, 2024-11-19 at 23:27 -0800, Josh Poimboeuf wrote:
> User->user Spectre v2 attacks (including RSB) across context switches
> are already mitigated by IBPB in cond_mitigation(), if enabled
> globally
> or if at least one of the tasks has opted in to protection.  RSB
> filling
> without IBPB serves no purpose for protecting user space, as indirect
> branches are still vulnerable.
> 
> User->kernel RSB attacks are mitigated by eIBRS.  In which case the
> RSB
> filling on context switch isn't needed.  Fix that.
> 
> While at it, update and coalesce the comments describing the various
> RSB
> mitigations.

Looks good from first impressions - but there's something that needs
some deeper analysis: AMD's Automatic IBRS piggybacks on eIBRS, and has
some special cases.  Adding Kim to CC to check and confirm if
everything's still as expected.

(cf commits
e7862eda309 x86/cpu: Support AMD Automatic IBRS
fd470a8beed x86/cpu: Enable STIBP on AMD if Automatic IBRS is enabled
acaa4b5c4c8 x86/speculation: Do not enable Automatic IBRS if SEV-SNP is
enabled
)

		Amit
> 
> Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
>  arch/x86/kernel/cpu/bugs.c | 91 ++++++++++++++----------------------
> --
>  arch/x86/mm/tlb.c          |  2 +-
>  2 files changed, 35 insertions(+), 58 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 68bed17f0980..e261f41749b0 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1579,27 +1579,44 @@ static void __init
> spec_ctrl_disable_kernel_rrsba(void)
>  	rrsba_disabled = true;
>  }
>  
> -static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum
> spectre_v2_mitigation mode)
> +static void __init spectre_v2_mitigate_rsb(enum
> spectre_v2_mitigation mode)
>  {
>  	/*
> -	 * Similar to context switches, there are two types of RSB
> attacks
> -	 * after VM exit:
> +	 * In general there are two types of RSB attacks:
>  	 *
> -	 * 1) RSB underflow
> +	 * 1) RSB underflow ("Intel Retbleed")
> +	 *
> +	 *    Some Intel parts have "bottomless RSB".  When the RSB
> is empty,
> +	 *    speculated return targets may come from the branch
> predictor,
> +	 *    which could have a user-poisoned BTB or BHB entry.
> +	 *
> +	 *    user->user attacks are mitigated by IBPB on context
> switch.
> +	 *
> +	 *    user->kernel attacks via context switch are mitigated
> by IBRS,
> +	 *    eIBRS, or RSB filling.
> +	 *
> +	 *    user->kernel attacks via kernel entry are mitigated by
> IBRS,
> +	 *    eIBRS, or call depth tracking.
> +	 *
> +	 *    On VMEXIT, guest->host attacks are mitigated by IBRS,
> eIBRS, or
> +	 *    RSB filling.
>  	 *
>  	 * 2) Poisoned RSB entry
>  	 *
> -	 * When retpoline is enabled, both are mitigated by
> filling/clearing
> -	 * the RSB.
> +	 *    On a context switch, the previous task can poison RSB
> entries
> +	 *    used by the next task, controlling its speculative
> return
> +	 *    targets.  Poisoned RSB entries can also be created by
> "AMD
> +	 *    Retbleed" or SRSO.
>  	 *
> -	 * When IBRS is enabled, while #1 would be mitigated by the
> IBRS branch
> -	 * prediction isolation protections, RSB still needs to be
> cleared
> -	 * because of #2.  Note that SMEP provides no protection
> here, unlike
> -	 * user-space-poisoned RSB entries.
> +	 *    user->user attacks are mitigated by IBPB on context
> switch.
>  	 *
> -	 * eIBRS should protect against RSB poisoning, but if the
> EIBRS_PBRSB
> -	 * bug is present then a LITE version of RSB protection is
> required,
> -	 * just a single call needs to retire before a RET is
> executed.
> +	 *    user->kernel attacks via context switch are prevented
> by
> +	 *    SMEP+eIBRS+SRSO mitigations, or RSB clearing.
> +	 *
> +	 *    guest->host attacks are mitigated by eIBRS or RSB
> clearing on
> +	 *    VMEXIT.  eIBRS implementations with
> X86_BUG_EIBRS_PBRSB still
> +	 *    need "lite" RSB filling which retires a CALL before
> the first
> +	 *    RET.
>  	 */
>  	switch (mode) {
>  	case SPECTRE_V2_NONE:
> @@ -1617,12 +1634,13 @@ static void __init
> spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
>  	case SPECTRE_V2_RETPOLINE:
>  	case SPECTRE_V2_LFENCE:
>  	case SPECTRE_V2_IBRS:
> -		pr_info("Spectre v2 / SpectreRSB : Filling RSB on
> VMEXIT\n");
> +		pr_info("Spectre v2 / SpectreRSB : Filling RSB on
> context switch and VMEXIT\n");
> +		setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
>  		setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
>  		return;
>  	}
>  
> -	pr_warn_once("Unknown Spectre v2 mode, disabling RSB
> mitigation at VM exit");
> +	pr_warn_once("Unknown Spectre v2 mode, disabling RSB
> mitigation\n");
>  	dump_stack();
>  }
>  
> @@ -1817,48 +1835,7 @@ static void __init
> spectre_v2_select_mitigation(void)
>  	spectre_v2_enabled = mode;
>  	pr_info("%s\n", spectre_v2_strings[mode]);
>  
> -	/*
> -	 * If Spectre v2 protection has been enabled, fill the RSB
> during a
> -	 * context switch.  In general there are two types of RSB
> attacks
> -	 * across context switches, for which the CALLs/RETs may be
> unbalanced.
> -	 *
> -	 * 1) RSB underflow
> -	 *
> -	 *    Some Intel parts have "bottomless RSB".  When the RSB
> is empty,
> -	 *    speculated return targets may come from the branch
> predictor,
> -	 *    which could have a user-poisoned BTB or BHB entry.
> -	 *
> -	 *    AMD has it even worse: *all* returns are speculated
> from the BTB,
> -	 *    regardless of the state of the RSB.
> -	 *
> -	 *    When IBRS or eIBRS is enabled, the "user -> kernel"
> attack
> -	 *    scenario is mitigated by the IBRS branch prediction
> isolation
> -	 *    properties, so the RSB buffer filling wouldn't be
> necessary to
> -	 *    protect against this type of attack.
> -	 *
> -	 *    The "user -> user" attack scenario is mitigated by RSB
> filling.
> -	 *
> -	 * 2) Poisoned RSB entry
> -	 *
> -	 *    If the 'next' in-kernel return stack is shorter than
> 'prev',
> -	 *    'next' could be tricked into speculating with a user-
> poisoned RSB
> -	 *    entry.
> -	 *
> -	 *    The "user -> kernel" attack scenario is mitigated by
> SMEP and
> -	 *    eIBRS.
> -	 *
> -	 *    The "user -> user" scenario, also known as SpectreBHB,
> requires
> -	 *    RSB clearing.
> -	 *
> -	 * So to mitigate all cases, unconditionally fill RSB on
> context
> -	 * switches.
> -	 *
> -	 * FIXME: Is this pointless for retbleed-affected AMD?
> -	 */
> -	setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
> -	pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on
> context switch\n");
> -
> -	spectre_v2_determine_rsb_fill_type_at_vmexit(mode);
> +	spectre_v2_mitigate_rsb(mode);
>  
>  	/*
>  	 * Retpoline protects the kernel, but doesn't protect
> firmware.  IBRS
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 86593d1b787d..c693b877d4df 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -388,7 +388,7 @@ static void cond_mitigation(struct task_struct
> *next)
>  	prev_mm = this_cpu_read(cpu_tlbstate.last_user_mm_spec);
>  
>  	/*
> -	 * Avoid user/user BTB poisoning by flushing the branch
> predictor
> +	 * Avoid user/user BTB/RSB poisoning by flushing the branch
> predictor
>  	 * when switching between processes. This stops one process
> from
>  	 * doing Spectre-v2 attacks on another.
>  	 *