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Subject[PATCH] 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>
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


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