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Subject[tip:x86/asm] x86/asm: Avoid taking an exception before cr4 restore
Commit-ID:  3c8dbef26202a83207611b3d6f4d2db5e5cbbc0f
Gitweb: https://git.kernel.org/tip/3c8dbef26202a83207611b3d6f4d2db5e5cbbc0f
Author: Kees Cook <keescook@chromium.org>
AuthorDate: Wed, 27 Feb 2019 12:01:31 -0800
Committer: Thomas Gleixner <tglx@linutronix.de>
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@openwall.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Jann Horn <jannh@google.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>
Link: https://lkml.kernel.org/r/20190227200132.24707-3-keescook@chromium.org

---
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
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