ARM: software-based priviledged-no-access support

Provide a software-based implementation of the priviledged no access
support found in ARMv8.1.

Userspace pages are mapped using a different domain number from the
kernel and IO mappings.  If we switch the user domain to "no access"
when we enter the kernel, we can prevent the kernel from touching
userspace.

However, the kernel needs to be able to access userspace via the
various user accessor functions.  With the wrapping in the previous
patch, we can temporarily enable access when the kernel needs user
access, and re-disable it afterwards.

This allows us to trap non-intended accesses to userspace, eg, caused
by an inadvertent dereference of the LIST_POISON* values, which, with
appropriate user mappings setup, can be made to succeed.  This in turn
can allow use-after-free bugs to be further exploited than would
otherwise be possible.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 14 insertions, 0 deletions

diff --git a/arch/arm/lib/csumpartialcopyuser.S b/arch/arm/lib/csumpartialcopyuser.S
index 1d0957e61f89..1712f132b80d 100644
--- a/arch/arm/lib/csumpartialcopyuser.S
+++ b/arch/arm/lib/csumpartialcopyuser.S
@@ -17,6 +17,19 @@
 
                 .text
 
+#ifdef CONFIG_CPU_SW_DOMAIN_PAN
+                .macro  save_regs
+                mrc     p15, 0, ip, c3, c0, 0
+                stmfd   sp!, {r1, r2, r4 - r8, ip, lr}
+                uaccess_enable ip
+                .endm
+
+                .macro  load_regs
+                ldmfd   sp!, {r1, r2, r4 - r8, ip, lr}
+                mcr     p15, 0, ip, c3, c0, 0
+                ret     lr
+                .endm
+#else
                 .macro  save_regs
                 stmfd   sp!, {r1, r2, r4 - r8, lr}
                 .endm
@@ -24,6 +37,7 @@
                 .macro  load_regs
                 ldmfd   sp!, {r1, r2, r4 - r8, pc}
                 .endm
+#endif
 
                 .macro  load1b, reg1
                 ldrusr  \reg1, r0, 1