From: Xin Li (Intel) Date: Fri, 16 Aug 2024 10:43:16 +0000 (-0700) Subject: x86/fred: Set SS to __KERNEL_DS when enabling FRED X-Git-Tag: v6.12-rc1~179^2~3 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=723edbd2ca5fb4c78ac4a5644511c63895fd1c57;p=thirdparty%2Fkernel%2Flinux.git x86/fred: Set SS to __KERNEL_DS when enabling FRED SS is initialized to NULL during boot time and not explicitly set to __KERNEL_DS. With FRED enabled, if a kernel event is delivered before a CPU goes to user level for the first time, its SS is NULL thus NULL is pushed into the SS field of the FRED stack frame. But before ERETS is executed, the CPU may context switch to another task and go to user level. Then when the CPU comes back to kernel mode, SS is changed to __KERNEL_DS. Later when ERETS is executed to return from the kernel event handler, a #GP fault is generated because SS doesn't match the SS saved in the FRED stack frame. Initialize SS to __KERNEL_DS when enabling FRED to prevent that. Note, IRET doesn't check if SS matches the SS saved in its stack frame, thus IDT doesn't have this problem. For IDT it doesn't matter whether SS is set to __KERNEL_DS or not, because it's set to NULL upon interrupt or exception delivery and __KERNEL_DS upon SYSCALL. Thus it's pointless to initialize SS for IDT. Signed-off-by: Xin Li (Intel) Signed-off-by: Thomas Gleixner Link: https://lore.kernel.org/all/20240816104316.2276968-1-xin@zytor.com --- diff --git a/arch/x86/kernel/fred.c b/arch/x86/kernel/fred.c index 99a134fcd5bf4..266c69e332a43 100644 --- a/arch/x86/kernel/fred.c +++ b/arch/x86/kernel/fred.c @@ -26,6 +26,20 @@ void cpu_init_fred_exceptions(void) /* When FRED is enabled by default, remove this log message */ pr_info("Initialize FRED on CPU%d\n", smp_processor_id()); + /* + * If a kernel event is delivered before a CPU goes to user level for + * the first time, its SS is NULL thus NULL is pushed into the SS field + * of the FRED stack frame. But before ERETS is executed, the CPU may + * context switch to another task and go to user level. Then when the + * CPU comes back to kernel mode, SS is changed to __KERNEL_DS. Later + * when ERETS is executed to return from the kernel event handler, a #GP + * fault is generated because SS doesn't match the SS saved in the FRED + * stack frame. + * + * Initialize SS to __KERNEL_DS when enabling FRED to avoid such #GPs. + */ + loadsegment(ss, __KERNEL_DS); + wrmsrl(MSR_IA32_FRED_CONFIG, /* Reserve for CALL emulation */ FRED_CONFIG_REDZONE |