Messages in this thread | | | From | "Li, Xin3" <> | Subject | RE: [PATCH v2 2/6] x86/entry/64: Convert SYSRET validation tests to C | Date | Sun, 23 Jul 2023 09:53:43 +0000 |
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> @@ -84,6 +85,43 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, int > nr) > > instrumentation_end(); > syscall_exit_to_user_mode(regs);
Would it be better to make the following code a new function?
And then the similar changes in patch 6 could be merged into the new function with #ifdef CONFIG_X86_64.
> + > + /* > + * Check that the register state is valid for using SYSRET to exit > + * to userspace. Otherwise use the slower but fully capable IRET > + * exit path. > + */ > + > + /* XEN PV guests always use IRET path */ > + if (cpu_feature_enabled(X86_FEATURE_XENPV)) > + return false; > + > + /* SYSRET requires RCX == RIP and R11 == EFLAGS */ > + if (unlikely(regs->cx != regs->ip || regs->r11 != regs->flags)) > + return false; > + > + /* CS and SS must match the values set in MSR_STAR */ > + if (unlikely(regs->cs != __USER_CS || regs->ss != __USER_DS)) > + return false; > + > + /* > + * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP > + * in kernel space. This essentially lets the user take over > + * the kernel, since userspace controls RSP. > + */ > + if (unlikely(!__is_canonical_address(regs->ip, __VIRTUAL_MASK_SHIFT + > 1))) > + return false; > + > + /* > + * SYSRET cannot restore RF. It can restore TF, but unlike IRET, > + * restoring TF results in a trap from userspace immediately after > + * SYSRET. > + */ > + if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF))) > + return false; > + > + /* Use SYSRET to exit to userspace */ > + return true; > } > #endif >
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