Messages in this thread | | | From | Brian Gerst <> | Date | Sun, 23 Jul 2023 07:17:53 -0400 | Subject | Re: [PATCH v2 2/6] x86/entry/64: Convert SYSRET validation tests to C |
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On Sun, Jul 23, 2023 at 5:53 AM Li, Xin3 <xin3.li@intel.com> wrote: > > > > @@ -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 > >
The tests are similar but not enough to combine them. If IA32_EMULATION is enabled, both versions are needed so one copy of the function with #ifdefs won't work..
Brian Gerst
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