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, 30 insertions, 0 deletions

diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
index a91177043467..3ae0eda5e64f 100644
--- a/arch/arm/include/asm/assembler.h
+++ b/arch/arm/include/asm/assembler.h
@@ -446,15 +446,45 @@ THUMB(	orr	\reg , \reg , #PSR_T_BIT	)
         .endm
 
         .macro  uaccess_disable, tmp, isb=1
+#ifdef CONFIG_CPU_SW_DOMAIN_PAN
+        /*
+         * Whenever we re-enter userspace, the domains should always be
+         * set appropriately.
+         */
+        mov     \tmp, #DACR_UACCESS_DISABLE
+        mcr     p15, 0, \tmp, c3, c0, 0         @ Set domain register
+        .if     \isb
+        instr_sync
+        .endif
+#endif
         .endm
 
         .macro  uaccess_enable, tmp, isb=1
+#ifdef CONFIG_CPU_SW_DOMAIN_PAN
+        /*
+         * Whenever we re-enter userspace, the domains should always be
+         * set appropriately.
+         */
+        mov     \tmp, #DACR_UACCESS_ENABLE
+        mcr     p15, 0, \tmp, c3, c0, 0
+        .if     \isb
+        instr_sync
+        .endif
+#endif
         .endm
 
         .macro  uaccess_save, tmp
+#ifdef CONFIG_CPU_SW_DOMAIN_PAN
+        mrc     p15, 0, \tmp, c3, c0, 0
+        str     \tmp, [sp, #S_FRAME_SIZE]
+#endif
         .endm
 
         .macro  uaccess_restore
+#ifdef CONFIG_CPU_SW_DOMAIN_PAN
+        ldr     r0, [sp, #S_FRAME_SIZE]
+        mcr     p15, 0, r0, c3, c0, 0
+#endif
         .endm
 
         .macro  uaccess_save_and_disable, tmp