Re: [RFC PATCH v2 1/3] x86: cpu/bugs: update SpectreRSB comments for AMD

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Thu, Nov 14, 2024 at 12:01:16AM -0800, Pawan Gupta wrote:
> > For PBRSB, I guess we don't need to worry about that since there would
> > be at least one kernel CALL before context switch.
> 
> Right. So the case where we need RSB filling at context switch is
> retpoline+CDT mitigation.

According to the docs, classic IBRS also needs RSB filling at context
switch to protect against corrupt RSB entries (as opposed to RSB
underflow).


Something like so...


diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 47a01d4028f6..7b9c0a21e478 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:
@@ -1608,8 +1625,8 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
 	case SPECTRE_V2_EIBRS_LFENCE:
 	case SPECTRE_V2_EIBRS:
 		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;
 
@@ -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 context switch and VMEXIT\n");
+		setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
 		setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
-		pr_info("Spectre v2 / SpectreRSB : Filling RSB on VMEXIT\n");
 		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