[tip:x86/asm] x86/asm: Avoid taking an exception before cr4 restore

[tip-bot for Kees Cook]

Commit-ID:  3c8dbef26202a83207611b3d6f4d2db5e5cbbc0f
Gitweb:     https://git.kernel.org/tip/3c8dbef26202a83207611b3d6f4d2db5e5cbbc0f
Author:     Kees Cook <keescook@xxxxxxxxxxxx>
AuthorDate: Wed, 27 Feb 2019 12:01:31 -0800
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Wed, 6 Mar 2019 13:25:55 +0100

x86/asm: Avoid taking an exception before cr4 restore

Instead of taking a full WARN() exception before restoring a potentially
missed CR4 bit, retain the missing bit for later reporting.

Additionally update the comments to note the required use of "volatile".

Suggested-by: Solar Designer <solar@xxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Cc: Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190227200132.24707-3-keescook@xxxxxxxxxxxx

---
 arch/x86/include/asm/special_insns.h | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index fabda1400137..99607f142cad 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -76,18 +76,24 @@ extern volatile unsigned long cr4_pin;
 
 static inline void native_write_cr4(unsigned long val)
 {
+	unsigned long warn = 0;
+
 again:
 	val |= cr4_pin;
 	asm volatile("mov %0,%%cr4": : "r" (val), "m" (__force_order));
 	/*
 	 * If the MOV above was used directly as a ROP gadget we can
 	 * notice the lack of pinned bits in "val" and start the function
-	 * from the beginning to gain the cr4_pin bits for sure.
+	 * from the beginning to gain the cr4_pin bits for sure. Note
+	 * that "val" must be volatile to keep the compiler from
+	 * optimizing away this check.
 	 */
-	if (WARN_ONCE((val & cr4_pin) != cr4_pin,
-		      "Attempt to unpin cr4 bits: %lx, cr4 bypass attack?!",
-		      ~val & cr4_pin))
+	if ((val & cr4_pin) != cr4_pin) {
+		warn = ~val & cr4_pin;
 		goto again;
+	}
+	WARN_ONCE(warn, "Attempt to unpin cr4 bits: %lx; bypass attack?!\n",
+		  warn);
 }
 
 #ifdef CONFIG_X86_64