[RFC PATCH] arm64: fault: Don't populate ESR context for user fault on kernel VA

[will.deacon@arm.com (Will Deacon)]

User faults on kernel addresses are a good sign that the faulting task
is either up to no good or is in deep trouble. In such situations,
exposing the optional ESR context on the sigframe as part of the
delivered signal is only useful to attackers who are using information
about underlying hardware fault (e.g. translation vs permission) as a
mechanism to defeat KASLR.

Remove the ESR context from the sigframe for user faults on kernel
addresses.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---

Here's another one that doesn't make a huge amount of difference when
kpti is enabled, but I think is a change worth making all the same.

 arch/arm64/mm/fault.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 49dfb08a6c4d..b9800395788e 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -292,8 +292,10 @@ static void __do_kernel_fault(unsigned long addr, unsigned int esr,
 
 static void __do_user_fault(struct siginfo *info, unsigned int esr)
 {
-	current->thread.fault_address = (unsigned long)info->si_addr;
-	current->thread.fault_code = esr;
+	unsigned long addr = (unsigned long)info->si_addr;
+
+	current->thread.fault_address = addr;
+	current->thread.fault_code = addr < TASK_SIZE ? esr : 0;
 	arm64_force_sig_info(info, esr_to_fault_info(esr)->name, current);
 }
 
-- 
2.1.4