[ 034/153] x86: Do not leak kernel page mapping locations

[Ben Hutchings]

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kees Cook <keescook@xxxxxxxxxxxx>

commit e575a86fdc50d013bf3ad3aa81d9100e8e6cc60d upstream.

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@xxxxxxxxx>
Suggested-by: Brad Spengler <spender@xxxxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Link: http://lkml.kernel.org/r/20130207174413.GA12485@xxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/mm/fault.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -738,13 +738,15 @@ __bad_area_nosemaphore(struct pt_regs *r
 				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_no	= 14;
 
 		force_sig_info_fault(SIGSEGV, si_code, address, tsk, 0);


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