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@kernel.org Cc: stable@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@k... --- 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(¤t->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(¤t->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,