Re: [PATCH v5 07/39] x86: Add user control-protection fault handler
From: Borislav Petkov <bp@alien8.de>
Date: 2023-02-03 19:09:28
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On Thu, Jan 19, 2023 at 01:22:45PM -0800, Rick Edgecombe wrote:
Subject: Re: [PATCH v5 07/39] x86: Add user control-protection fault handler
Subject: x86/shstk: Add...
From: Yu-cheng Yu <redacted> A control-protection fault is triggered when a control-flow transfer attempt violates Shadow Stack or Indirect Branch Tracking constraints. For example, the return address for a RET instruction differs from the copy on the shadow stack.
...
quoted hunk ↗ jump to hunk
diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c new file mode 100644 index 000000000000..33d7d119be26 --- /dev/null +++ b/arch/x86/kernel/cet.c@@ -0,0 +1,152 @@ +// SPDX-License-Identifier: GPL-2.0 + +#include <linux/ptrace.h> +#include <asm/bugs.h> +#include <asm/traps.h> + +enum cp_error_code { + CP_EC = (1 << 15) - 1,
That looks like a mask, so CP_EC_MASK I guess.
+ + CP_RET = 1, + CP_IRET = 2, + CP_ENDBR = 3, + CP_RSTRORSSP = 4, + CP_SETSSBSY = 5, + + CP_ENCL = 1 << 15, +};
...
+static void do_user_cp_fault(struct pt_regs *regs, unsigned long error_code)
+{
+ struct task_struct *tsk;
+ unsigned long ssp;
+
+ /*
+ * An exception was just taken from userspace. Since interrupts are disabled
+ * here, no scheduling should have messed with the registers yet and they
+ * will be whatever is live in userspace. So read the SSP before enabling
+ * interrupts so locking the fpregs to do it later is not required.
+ */
+ rdmsrl(MSR_IA32_PL3_SSP, ssp);
+
+ cond_local_irq_enable(regs);
+
+ tsk = current;Hmm, should you read current before you enable interrupts? Not that it changes from under us...
+ tsk->thread.error_code = error_code;
+ tsk->thread.trap_nr = X86_TRAP_CP;
+
+ /* Ratelimit to prevent log spamming. */
+ if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
+ __ratelimit(&cpf_rate)) {
+ pr_emerg("%s[%d] control protection ip:%lx sp:%lx ssp:%lx error:%lx(%s)%s",
+ tsk->comm, task_pid_nr(tsk),
+ regs->ip, regs->sp, ssp, error_code,
+ cp_err_string(error_code),
+ error_code & CP_ENCL ? " in enclave" : "");
+ print_vma_addr(KERN_CONT " in ", regs->ip);
+ pr_cont("\n");
+ }
+
+ force_sig_fault(SIGSEGV, SEGV_CPERR, (void __user *)0);
+ cond_local_irq_disable(regs);
+}
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette