Commit-ID: 3c8dbef26202a83207611b3d6f4d2db5e5cbbc0f Gitweb: https://git.kernel.org/tip/3c8dbef26202a83207611b3d6f4d2db5e5cbbc0f Author: Kees Cook <keesc...@chromium.org> AuthorDate: Wed, 27 Feb 2019 12:01:31 -0800 Committer: Thomas Gleixner <t...@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 <so...@openwall.com> Signed-off-by: Kees Cook <keesc...@chromium.org> Signed-off-by: Thomas Gleixner <t...@linutronix.de> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Greg KH <gre...@linuxfoundation.org> Cc: Jann Horn <ja...@google.com> Cc: Sean Christopherson <sean.j.christopher...@intel.com> Cc: Dominik Brodowski <li...@dominikbrodowski.net> Cc: Kernel Hardening <kernel-harden...@lists.openwall.com> Link: https://lkml.kernel.org/r/20190227200132.24707-3-keesc...@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
