arm/syscalls: Check address limit on user-mode return

Ensure the address limit is a user-mode segment before returning to
user-mode. Otherwise a process can corrupt kernel-mode memory and
elevate privileges [1].

The set_fs function sets the TIF_SETFS flag to force a slow path on
return. In the slow path, the address limit is checked to be USER_DS if
needed.

The TIF_SETFS flag is added to _TIF_WORK_MASK shifting _TIF_SYSCALL_WORK
for arm instruction immediate support. The global work mask is too big
to used on a single instruction so adapt ret_fast_syscall.

[1] https://bugs.chromium.org/p/project-zero/issues/detail?id=990

Signed-off-by: Thomas Garnier <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: [email protected]
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: David Howells <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Miroslav Benes <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Pratyush Anand <[email protected]>
Cc: Russell King <[email protected]>
Cc: Petr Mladek <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: [email protected]
Cc: Will Drewry <[email protected]>
Cc: [email protected]
Cc: Oleg Nesterov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]

1 files changed, 5 insertions, 0 deletions

diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index 7b8f2141427b..3a48b54c6405 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -14,6 +14,7 @@
 #include <linux/uaccess.h>
 #include <linux/tracehook.h>
 #include <linux/uprobes.h>
+#include <linux/syscalls.h>
 
 #include <asm/elf.h>
 #include <asm/cacheflush.h>
@@ -571,6 +572,10 @@ do_work_pending(struct pt_regs *regs, unsigned int thread_flags, int syscall)
 	 * Update the trace code with the current status.
 	 */
 	trace_hardirqs_off();
+
+	/* Check valid user FS if needed */
+	addr_limit_user_check();
+
 	do {
 		if (likely(thread_flags & _TIF_NEED_RESCHED)) {
 			schedule();