[RFC PATCH 4/4] arm64: Check user stack pointer on syscall entry

[Will Deacon]

Allow the user stack pointer value to be checked on system call entry
and deliver a SIGSEGV if the check does not pass.

Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
 arch/arm64/Kconfig          | 1 +
 arch/arm64/kernel/syscall.c | 4 ++++
 2 files changed, 5 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index a4168d366127..e87304a06a85 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -29,6 +29,7 @@ config ARM64
 	select ARCH_HAS_SYNC_DMA_FOR_CPU
 	select ARCH_HAS_SYSCALL_WRAPPER
 	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
+	select ARCH_HAS_USP_CHECK_SYSCALL
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
 	select ARCH_INLINE_READ_LOCK if !PREEMPT
 	select ARCH_INLINE_READ_LOCK_BH if !PREEMPT
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index 5610ac01c1ec..06566010f6d1 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -11,6 +11,7 @@
 #include <asm/fpsimd.h>
 #include <asm/syscall.h>
 #include <asm/thread_info.h>
+#include <asm/traps.h>
 #include <asm/unistd.h>
 
 long compat_arm_syscall(struct pt_regs *regs, int scno);
@@ -71,6 +72,9 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 	local_daif_restore(DAIF_PROCCTX);
 	user_exit();
 
+	if (!usp_check_syscall())
+		force_signal_inject(SIGSEGV, SEGV_MAPERR, GET_USP(regs));
+
 	if (has_syscall_work(flags)) {
 		/* set default errno for user-issued syscall(-1) */
 		if (scno == NO_SYSCALL)
-- 
2.11.0