x86: Do not leak kernel page mapping locations

Without this patch, it is trivial to determine kernel page
mappings by examining the error code reported to dmesg[1].
Instead, declare the entire kernel memory space as a violation
of a present page.

Additionally, since show_unhandled_signals is enabled by
default, switch branch hinting to the more realistic
expectation, and unobfuscate the setting of the PF_PROT bit to
improve readability.

[1] http://vulnfactory.org/blog/2013/02/06/a-linux-memory-trick/

Reported-by: Dan Rosenberg <dan.j.rosenberg@gmail.com>
Suggested-by: Brad Spengler <spender@grsecurity.net>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
Acked-by: H. Peter Anvin <hpa@zytor.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20130207174413.GA12485@www.outflux.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 027088f2f7dd..fb674fd3fc22 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -748,13 +748,15 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
                                 return;
                 }
 #endif
+                /* Kernel addresses are always protection faults: */
+                if (address >= TASK_SIZE)
+                        error_code |= PF_PROT;
 
-                if (unlikely(show_unhandled_signals))
+                if (likely(show_unhandled_signals))
                         show_signal_msg(regs, error_code, address, tsk);
 
-                /* Kernel addresses are always protection faults: */
                 tsk->thread.cr2         = address;
-                tsk->thread.error_code  = error_code | (address >= TASK_SIZE);
+                tsk->thread.error_code  = error_code;
                 tsk->thread.trap_nr     = X86_TRAP_PF;
 
                 force_sig_info_fault(SIGSEGV, si_code, address, tsk, 0);