x86/entry/64: Convert SYSRET validation tests to C

No change in functionality expected.

Signed-off-by: Brian Gerst <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Uros Bizjak <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

1 files changed, 42 insertions, 1 deletions

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 0551bcb197fb..90214652ff06 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -71,7 +71,8 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
 	return false;
 }
 
-__visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
+/* Returns true to return using SYSRET, or false to use IRET */
+__visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
 {
 	add_random_kstack_offset();
 	nr = syscall_enter_from_user_mode(regs, nr);
@@ -85,6 +86,46 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
 
 	instrumentation_end();
 	syscall_exit_to_user_mode(regs);
+
+	/*
+	 * 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 the 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.
+	 *
+	 * Change top bits to match the most significant bit (47th or 56th bit
+	 * depending on paging mode) in the address.
+	 */
+	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