[PATCH] x86: do not leak kernel page mapping locations

[PATCH] x86: do not leak kernel page mapping locations

[Kees Cook]

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>
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/x86/mm/fault.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 027088f..fb674fd 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);
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security

Re: [PATCH] x86: do not leak kernel page mapping locations

[H. Peter Anvin]

On 02/07/2013 09:44 AM, Kees Cook wrote:
> 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>
> Cc: stable@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>

There is, of course, tons of information in dmesg which can be used for
nefarious purposes, and it is hard to avoid that without neutering dmesg
to uselessness.

So the question is to some degree how much this adds in isolation from
locking down dmesg (and if dmesg_restrict isn't enough, what more do we
need...)?

That being said, the patch doesn't seem to do any harm so I acked it and
Ingo is going to apply it.

	-hpa

[tip:x86/urgent] x86: Do not leak kernel page mapping locations

[tip-bot for Kees Cook]

Commit-ID:  e575a86fdc50d013bf3ad3aa81d9100e8e6cc60d
Gitweb:     http://git.kernel.org/tip/e575a86fdc50d013bf3ad3aa81d9100e8e6cc60d
Author:     Kees Cook <keescook@chromium.org>
AuthorDate: Thu, 7 Feb 2013 09:44:13 -0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 7 Feb 2013 19:57:44 +0100

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>
---
 arch/x86/mm/fault.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 027088f..fb674fd 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);