From 5cc2db37124bb33914996d6fdbb2ddb3811f2945 Mon Sep 17 00:00:00 2001 From: Andrew Cooper Date: Sat, 21 Dec 2024 21:10:46 +0000 Subject: [PATCH 1/2] x86/static-call: Remove early_boot_irqs_disabled check to fix Xen PVH dom0 __static_call_update_early() has a check for early_boot_irqs_disabled, but is used before early_boot_irqs_disabled is set up in start_kernel(). Xen PV has always special cased early_boot_irqs_disabled, but Xen PVH does not and falls over the BUG when booting as dom0. It is very suspect that early_boot_irqs_disabled starts as 0, becomes 1 for a time, then becomes 0 again, but as this needs backporting to fix a breakage in a security fix, dropping the BUG_ON() is the far safer option. Fixes: 0ef8047b737d ("x86/static-call: provide a way to do very early static-call updates") Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219620 Reported-by: Alex Zenla Suggested-by: Peter Zijlstra Signed-off-by: Andrew Cooper Signed-off-by: Borislav Petkov (AMD) Reviewed-by: Juergen Gross Acked-by: Peter Zijlstra (Intel) Tested-by: Alex Zenla Link: https://lore.kernel.org/r/20241221211046.6475-1-andrew.cooper3@citrix.com --- arch/x86/kernel/static_call.c | 1 - 1 file changed, 1 deletion(-) diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c index 9eed0c144dad5..9e51242ed125e 100644 --- a/arch/x86/kernel/static_call.c +++ b/arch/x86/kernel/static_call.c @@ -175,7 +175,6 @@ EXPORT_SYMBOL_GPL(arch_static_call_transform); noinstr void __static_call_update_early(void *tramp, void *func) { BUG_ON(system_state != SYSTEM_BOOTING); - BUG_ON(!early_boot_irqs_disabled); BUG_ON(static_call_initialized); __text_gen_insn(tramp, JMP32_INSN_OPCODE, tramp, func, JMP32_INSN_SIZE); sync_core(); From a9d9c33132d49329ada647e4514d210d15e31d81 Mon Sep 17 00:00:00 2001 From: Rick Edgecombe Date: Tue, 7 Jan 2025 15:30:56 -0800 Subject: [PATCH 2/2] x86/fpu: Ensure shadow stack is active before "getting" registers The x86 shadow stack support has its own set of registers. Those registers are XSAVE-managed, but they are "supervisor state components" which means that userspace can not touch them with XSAVE/XRSTOR. It also means that they are not accessible from the existing ptrace ABI for XSAVE state. Thus, there is a new ptrace get/set interface for it. The regset code that ptrace uses provides an ->active() handler in addition to the get/set ones. For shadow stack this ->active() handler verifies that shadow stack is enabled via the ARCH_SHSTK_SHSTK bit in the thread struct. The ->active() handler is checked from some call sites of the regset get/set handlers, but not the ptrace ones. This was not understood when shadow stack support was put in place. As a result, both the set/get handlers can be called with XFEATURE_CET_USER in its init state, which would cause get_xsave_addr() to return NULL and trigger a WARN_ON(). The ssp_set() handler luckily has an ssp_active() check to avoid surprising the kernel with shadow stack behavior when the kernel is not ready for it (ARCH_SHSTK_SHSTK==0). That check just happened to avoid the warning. But the ->get() side wasn't so lucky. It can be called with shadow stacks disabled, triggering the warning in practice, as reported by Christina Schimpe: WARNING: CPU: 5 PID: 1773 at arch/x86/kernel/fpu/regset.c:198 ssp_get+0x89/0xa0 [...] Call Trace: ? show_regs+0x6e/0x80 ? ssp_get+0x89/0xa0 ? __warn+0x91/0x150 ? ssp_get+0x89/0xa0 ? report_bug+0x19d/0x1b0 ? handle_bug+0x46/0x80 ? exc_invalid_op+0x1d/0x80 ? asm_exc_invalid_op+0x1f/0x30 ? __pfx_ssp_get+0x10/0x10 ? ssp_get+0x89/0xa0 ? ssp_get+0x52/0xa0 __regset_get+0xad/0xf0 copy_regset_to_user+0x52/0xc0 ptrace_regset+0x119/0x140 ptrace_request+0x13c/0x850 ? wait_task_inactive+0x142/0x1d0 ? do_syscall_64+0x6d/0x90 arch_ptrace+0x102/0x300 [...] Ensure that shadow stacks are active in a thread before looking them up in the XSAVE buffer. Since ARCH_SHSTK_SHSTK and user_ssp[SHSTK_EN] are set at the same time, the active check ensures that there will be something to find in the XSAVE buffer. [ dhansen: changelog/subject tweaks ] Fixes: 2fab02b25ae7 ("x86: Add PTRACE interface for shadow stack") Reported-by: Christina Schimpe Signed-off-by: Rick Edgecombe Signed-off-by: Dave Hansen Tested-by: Christina Schimpe Cc:stable@vger.kernel.org Link: https://lore.kernel.org/all/20250107233056.235536-1-rick.p.edgecombe%40intel.com --- arch/x86/kernel/fpu/regset.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c index 6bc1eb2a21bd9..887b0b8e21e36 100644 --- a/arch/x86/kernel/fpu/regset.c +++ b/arch/x86/kernel/fpu/regset.c @@ -190,7 +190,8 @@ int ssp_get(struct task_struct *target, const struct user_regset *regset, struct fpu *fpu = &target->thread.fpu; struct cet_user_state *cetregs; - if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK)) + if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) || + !ssp_active(target, regset)) return -ENODEV; sync_fpstate(fpu);