[PATCH v3 3/3] x86/entry/32: Clean up syscall fast exit tests

From: Brian Gerst
Date: Wed Oct 11 2023 - 18:44:10 EST


Merge compat and native code and clarify comments.

Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>
---
arch/x86/entry/common.c | 48 +++++++++++++++++++----------------------
1 file changed, 22 insertions(+), 26 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index e3d6f255379f..0acf35d7fe55 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -255,34 +255,30 @@ __visible noinstr bool do_fast_syscall_32(struct pt_regs *regs)
if (!__do_fast_syscall_32(regs))
return false;

-#ifdef CONFIG_X86_64
/*
- * Opportunistic SYSRETL: if possible, try to return using SYSRETL.
- * SYSRETL is available on all 64-bit CPUs, so we don't need to
- * bother with SYSEXIT.
- *
- * Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
- * because the ECX fixup above will ensure that this is essentially
- * never the case.
+ * Check that the register state is valid for using SYSRETL/SYSEXIT
+ * to exit to userspace. Otherwise use the slower but fully capable
+ * IRET exit path.
*/
- return regs->cs == __USER32_CS && regs->ss == __USER_DS &&
- regs->ip == landing_pad &&
- (regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)) == 0;
-#else
- /*
- * Opportunistic SYSEXIT: if possible, try to return using SYSEXIT.
- *
- * Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
- * because the ECX fixup above will ensure that this is essentially
- * never the case.
- *
- * We don't allow syscalls at all from VM86 mode, but we still
- * need to check VM, because we might be returning from sys_vm86.
- */
- return regs->cs == __USER_CS && regs->ss == __USER_DS &&
- regs->ip == landing_pad &&
- (regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)) == 0;
-#endif
+
+ /* XEN PV guests always use IRET path */
+ if (cpu_feature_enabled(X86_FEATURE_XENPV))
+ return false;
+
+ /* EIP must point to the VDSO landing pad */
+ if (unlikely(regs->ip != landing_pad))
+ return false;
+
+ /* CS and SS must match the values set in MSR_STAR */
+ if (unlikely(regs->cs != __USER32_CS || regs->ss != __USER_DS))
+ return false;
+
+ /* If the TF, RF, or VM flags are set, use IRET */
+ if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)))
+ return false;
+
+ /* Use SYSRETL/SYSEXIT to exit to userspace */
+ return true;
}

/* Returns true to return using SYSEXIT/SYSRETL, or false to use IRET */
--
2.41.0