[PATCH v2 1/1] x86/fred: Fix INT80 emulation for FRED

[Xin Li (Intel)]

Add a FRED-specific INT80 handler fred_int80_emulation().

Commit
  55617fb991df ("x86/entry: Do not allow external 0x80 interrupts")
added a bunch of checks to the int $0x80 path, however they are
unnecessary and event wrong in fact under FRED.

1) FRED distinguishes external interrupts from software interrupts,
   thus int80_emulation() should NEVER be called for handling an
   external interrupt, and then int80_is_external() is meaningless
   when FRED is enabled.

2) The FRED kernel entry handler NEVER dispatches INTx, which is
   of event type EVENT_TYPE_SWINT, so the user mode checking in
   do_int80_emulation() is redundant.

3) int80_emulation() does a CLEAR_BRANCH_HISTORY, which is likly
   an overkill for new x86 CPU implementations that support FRED.

A dedicated FRED INT80 handler duplicates most of the code in
do_int80_emulation(), but it avoids sprinkling moar tests and
seems more readable.

Suggested-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx>
---

Change since v1:
* Prefer a FRED-specific INT80 handler instead of sprinkling moar
  tests around (Borislav Petkov).
---
 arch/x86/entry/common.c     | 40 +++++++++++++++++++++++++++++++++++++
 arch/x86/entry/entry_fred.c |  2 +-
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 6de50b80702e..3cfc7c4fe1a2 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -255,6 +255,46 @@ __visible noinstr void do_int80_emulation(struct pt_regs *regs)
 	instrumentation_end();
 	syscall_exit_to_user_mode(regs);
 }
+
+#ifdef CONFIG_X86_FRED
+/*
+ * Called only if a user level application executes "int $0x80".
+ *
+ * Note, under FRED interrupt 0x80 could be used as a hardware interrupt.
+ */
+DEFINE_FREDENTRY_RAW(int80_emulation)
+{
+	int nr;
+
+	enter_from_user_mode(regs);
+
+	instrumentation_begin();
+	add_random_kstack_offset();
+
+	/*
+	 * FRED pushed 0 into regs::orig_ax and regs::ax contains the
+	 * syscall number.
+	 *
+	 * User tracing code (ptrace or signal handlers) might assume
+	 * that the regs::orig_ax contains a 32-bit number on invoking
+	 * a 32-bit syscall.
+	 *
+	 * Establish the syscall convention by saving the 32bit truncated
+	 * syscall number in regs::orig_ax and by invalidating regs::ax.
+	 */
+	regs->orig_ax = regs->ax & GENMASK(31, 0);
+	regs->ax = -ENOSYS;
+
+	nr = syscall_32_enter(regs);
+
+	local_irq_enable();
+	nr = syscall_enter_from_user_mode_work(regs, nr);
+	do_syscall_32_irqs_on(regs, nr);
+
+	instrumentation_end();
+	syscall_exit_to_user_mode(regs);
+}
+#endif
 #else /* CONFIG_IA32_EMULATION */
 
 /* Handles int $0x80 on a 32bit kernel */
diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
index ac120cbdaaf2..9fa18b8c7f26 100644
--- a/arch/x86/entry/entry_fred.c
+++ b/arch/x86/entry/entry_fred.c
@@ -66,7 +66,7 @@ static noinstr void fred_intx(struct pt_regs *regs)
 	/* INT80 */
 	case IA32_SYSCALL_VECTOR:
 		if (ia32_enabled())
-			return int80_emulation(regs);
+			return fred_int80_emulation(regs);
 		fallthrough;
 #endif
 

base-commit: 367dc2b68007e8ca00a0d8dc9afb69bff5451ae7
-- 
2.44.0