[PATCH v2] arm64/signal: Avoid corruption of SME state when entering signal handler

[Mark Brown]

We intend that signal handlers are entered with PSTATE.{SM,ZA}={0,0}.
The logic for this in setup_return() manipulates the saved state and
live CPU state in an unsafe manner, and consequently, when a task enters
a signal handler:

 * The task entering the signal handler might not have its PSTATE.{SM,ZA}
   bits cleared, and other register state that is affected by changes to
   PSTATE.{SM,ZA} might not be zeroed as expected.

 * An unrelated task might have its PSTATE.{SM,ZA} bits cleared
   unexpectedly, potentially zeroing other register state that is
   affected by changes to PSTATE.{SM,ZA}.

   Tasks which do not set PSTATE.{SM,ZA} (i.e. those only using plain
   FPSIMD or non-streaming SVE) are not affected, as there is no
   resulting change to PSTATE.{SM,ZA}.

Consider for example two tasks on one CPU:

 A: Begins signal entry in kernel mode, is preempted prior to SMSTOP.
 B: Using SM and/or ZA in userspace with register state current on the
    CPU, is preempted.
 A: Scheduled in, no register state changes made as in kernel mode.
 A: Executes SMSTOP, modifying live register state.
 A: Scheduled out.
 B: Scheduled in, fpsimd_thread_switch() sees the register state on the
    CPU is tracked as being that for task B so the state is not reloaded
    prior to returning to userspace.

Task B is now running with SM and ZA incorrectly cleared.

Fix this by:

 * Checking TIF_FOREIGN_FPSTATE, and only updating the saved or live
   state as appropriate.

 * Using {get,put}_cpu_fpsimd_context() to ensure mutual exclusion
   against other code which manipulates this state. To allow their use,
   the logic is moved into a new fpsimd_enter_sighandler() helper in
   fpsimd.c.

This race has been observed intermittently with fp-stress, especially
with preempt disabled, commonly but not exclusively reporting "Bad SVCR: 0".

Fixes: 40a8e87bb3285 ("arm64/sme: Disable ZA and streaming mode when handling signals")
Signed-off-by: Mark Brown <broonie at kernel.org>
Cc: stable at vger.kernel.org
---
Changes in v2:
- Commit message tweaks.
- Flush the task state when updating in memory to ensure we reload.
- Link to v1: https://lore.kernel.org/r/20241023-arm64-fp-sme-sigentry-v1-1-249ff7ec3ad0@kernel.org
---
 arch/arm64/include/asm/fpsimd.h |  1 +
 arch/arm64/kernel/fpsimd.c      | 33 +++++++++++++++++++++++++++++++++
 arch/arm64/kernel/signal.c      | 19 +------------------
 3 files changed, 35 insertions(+), 18 deletions(-)

diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
index f2a84efc361858d4deda99faf1967cc7cac386c1..09af7cfd9f6c2cec26332caa4c254976e117b1bf 100644
--- a/arch/arm64/include/asm/fpsimd.h
+++ b/arch/arm64/include/asm/fpsimd.h
@@ -76,6 +76,7 @@ extern void fpsimd_load_state(struct user_fpsimd_state *state);
 extern void fpsimd_thread_switch(struct task_struct *next);
 extern void fpsimd_flush_thread(void);
 
+extern void fpsimd_enter_sighandler(void);
 extern void fpsimd_signal_preserve_current_state(void);
 extern void fpsimd_preserve_current_state(void);
 extern void fpsimd_restore_current_state(void);
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 77006df20a75aee7c991cf116b6d06bfe953d1a4..c4149f474ce889af42bc2ce9402e7d032818c2e4 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -1693,6 +1693,39 @@ void fpsimd_signal_preserve_current_state(void)
 		sve_to_fpsimd(current);
 }
 
+/*
+ * Called by the signal handling code when preparing current to enter
+ * a signal handler. Currently this only needs to take care of exiting
+ * streaming mode and clearing ZA on SME systems.
+ */
+void fpsimd_enter_sighandler(void)
+{
+	if (!system_supports_sme())
+		return;
+
+	get_cpu_fpsimd_context();
+
+	if (test_thread_flag(TIF_FOREIGN_FPSTATE)) {
+		/* Exiting streaming mode zeros the FPSIMD state */
+		if (current->thread.svcr & SVCR_SM_MASK) {
+			memset(&current->thread.uw.fpsimd_state, 0,
+			       sizeof(current->thread.uw.fpsimd_state));
+			current->thread.fp_type = FP_STATE_FPSIMD;
+		}
+
+		current->thread.svcr &= ~(SVCR_ZA_MASK |
+					  SVCR_SM_MASK);
+
+		/* Ensure any copies on other CPUs aren't reused */
+		fpsimd_flush_task_state(current);
+	} else {
+		/* The register state is current, just update it. */
+		sme_smstop();
+	}
+
+	put_cpu_fpsimd_context();
+}
+
 /*
  * Called by KVM when entering the guest.
  */
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 5619869475304776fc005fe24a385bf86bfdd253..fe07d0bd9f7978d73973f07ce38b7bdd7914abb2 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -1218,24 +1218,7 @@ static void setup_return(struct pt_regs *regs, struct k_sigaction *ka,
 	/* TCO (Tag Check Override) always cleared for signal handlers */
 	regs->pstate &= ~PSR_TCO_BIT;
 
-	/* Signal handlers are invoked with ZA and streaming mode disabled */
-	if (system_supports_sme()) {
-		/*
-		 * If we were in streaming mode the saved register
-		 * state was SVE but we will exit SM and use the
-		 * FPSIMD register state - flush the saved FPSIMD
-		 * register state in case it gets loaded.
-		 */
-		if (current->thread.svcr & SVCR_SM_MASK) {
-			memset(&current->thread.uw.fpsimd_state, 0,
-			       sizeof(current->thread.uw.fpsimd_state));
-			current->thread.fp_type = FP_STATE_FPSIMD;
-		}
-
-		current->thread.svcr &= ~(SVCR_ZA_MASK |
-					  SVCR_SM_MASK);
-		sme_smstop();
-	}
+	fpsimd_enter_sighandler();
 
 	if (system_supports_poe())
 		write_sysreg_s(POR_EL0_INIT, SYS_POR_EL0);

---
base-commit: 8e929cb546ee42c9a61d24fae60605e9e3192354
change-id: 20241023-arm64-fp-sme-sigentry-a2bd7187e71b

Best regards,
-- 
Mark Brown <broonie at kernel.org>