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 <thgarnie@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: kernel-hardening@lists.openwall.com
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Miroslav Benes <mbenes@suse.cz>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Pratyush Anand <panand@redhat.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Will Drewry <wad@chromium.org>
Cc: linux-api@vger.kernel.org
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Link: http://lkml.kernel.org/r/20170615011203.144108-2-thgarnie@google.com

diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
index 776757d1604ab3901996bb24bb02748e54c2aee7..1d468b527b7b6928d2bb21c2152a3a282a7a366d 100644 (file)
--- a/arch/arm/include/asm/thread_info.h
+++ b/arch/arm/include/asm/thread_info.h
@@ -139,10 +139,11 @@ extern int vfp_restore_user_hwstate(struct user_vfp __user *,
 #define TIF_NEED_RESCHED       1       /* rescheduling necessary */
 #define TIF_NOTIFY_RESUME      2       /* callback before returning to user */
 #define TIF_UPROBE             3       /* breakpointed or singlestepping */
-#define TIF_SYSCALL_TRACE      4       /* syscall trace active */
-#define TIF_SYSCALL_AUDIT      5       /* syscall auditing active */
-#define TIF_SYSCALL_TRACEPOINT 6       /* syscall tracepoint instrumentation */
-#define TIF_SECCOMP            7       /* seccomp syscall filtering active */
+#define TIF_FSCHECK            4       /* Check FS is USER_DS on return */
+#define TIF_SYSCALL_TRACE      5       /* syscall trace active */
+#define TIF_SYSCALL_AUDIT      6       /* syscall auditing active */
+#define TIF_SYSCALL_TRACEPOINT 7       /* syscall tracepoint instrumentation */
+#define TIF_SECCOMP            8       /* seccomp syscall filtering active */
 
 #define TIF_NOHZ               12      /* in adaptive nohz mode */
 #define TIF_USING_IWMMXT       17
@@ -153,6 +154,7 @@ extern int vfp_restore_user_hwstate(struct user_vfp __user *,
 #define _TIF_NEED_RESCHED      (1 << TIF_NEED_RESCHED)
 #define _TIF_NOTIFY_RESUME     (1 << TIF_NOTIFY_RESUME)
 #define _TIF_UPROBE            (1 << TIF_UPROBE)
+#define _TIF_FSCHECK           (1 << TIF_FSCHECK)
 #define _TIF_SYSCALL_TRACE     (1 << TIF_SYSCALL_TRACE)
 #define _TIF_SYSCALL_AUDIT     (1 << TIF_SYSCALL_AUDIT)
 #define _TIF_SYSCALL_TRACEPOINT        (1 << TIF_SYSCALL_TRACEPOINT)
@@ -166,8 +168,9 @@ extern int vfp_restore_user_hwstate(struct user_vfp __user *,
 /*
  * Change these and you break ASM code in entry-common.S
  */
-#define _TIF_WORK_MASK         (_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
-                                _TIF_NOTIFY_RESUME | _TIF_UPROBE)
+#define _TIF_WORK_MASK         (_TIF_NEED_RESCHED | _TIF_SIGPENDING |  \
+                                _TIF_NOTIFY_RESUME | _TIF_UPROBE |     \
+                                _TIF_FSCHECK)
 
 #endif /* __KERNEL__ */
 #endif /* __ASM_ARM_THREAD_INFO_H */

diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h
index 2577405d082dab01004f13adee7a91398942d399..6cc882223e341ed2401370c864e6bd9b492726ec 100644 (file)
--- a/arch/arm/include/asm/uaccess.h
+++ b/arch/arm/include/asm/uaccess.h
@@ -77,6 +77,8 @@ static inline void set_fs(mm_segment_t fs)
 {
        current_thread_info()->addr_limit = fs;
        modify_domain(DOMAIN_KERNEL, fs ? DOMAIN_CLIENT : DOMAIN_MANAGER);
+       /* On user-mode return, check fs is correct */
+       set_thread_flag(TIF_FSCHECK);
 }
 
 #define segment_eq(a, b)       ((a) == (b))

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index eb5cd77bf1d8326aa69fe05ff7dc223aa5eb707a..e33c32d561934c36b84d294d85ff27fa05aa0ec2 100644 (file)
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -41,7 +41,9 @@ ret_fast_syscall:
  UNWIND(.cantunwind    )
        disable_irq_notrace                     @ disable interrupts
        ldr     r1, [tsk, #TI_FLAGS]            @ re-check for syscall tracing
-       tst     r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
+       tst     r1, #_TIF_SYSCALL_WORK
+       bne     fast_work_pending
+       tst     r1, #_TIF_WORK_MASK
        bne     fast_work_pending
 
        /* perform architecture specific actions before user return */
@@ -67,12 +69,15 @@ ret_fast_syscall:
        str     r0, [sp, #S_R0 + S_OFF]!        @ save returned r0
        disable_irq_notrace                     @ disable interrupts
        ldr     r1, [tsk, #TI_FLAGS]            @ re-check for syscall tracing
-       tst     r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
+       tst     r1, #_TIF_SYSCALL_WORK
+       bne     fast_work_pending
+       tst     r1, #_TIF_WORK_MASK
        beq     no_work_pending
  UNWIND(.fnend         )
 ENDPROC(ret_fast_syscall)
 
        /* Slower path - fall through to work_pending */
+fast_work_pending:
 #endif
 
        tst     r1, #_TIF_SYSCALL_WORK

diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index 7b8f2141427bda172899bfe8ae5113367163af47..3a48b54c6405acf67042574856de4e3a655c5cd4 100644 (file)
--- 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();