Re: [PATCH v10 2/3] arm/syscalls: Check address limit on user-mode return

On Tue, Jun 20, 2017 at 1:18 PM, Kees Cook [off-list ref] wrote:

On Wed, Jun 14, 2017 at 6:12 PM, Thomas Garnier [off-list ref] wrote:

quoted

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 <redacted>
---
v10 redesigns the change to use work flags on set_fs as recommended by
Linus and agreed by others.

Based on next-20170609
---
 arch/arm/include/asm/thread_info.h | 15 +++++++++------
 arch/arm/include/asm/uaccess.h     |  2 ++
 arch/arm/kernel/entry-common.S     |  9 +++++++--
 arch/arm/kernel/signal.c           |  5 +++++
 4 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
index 776757d1604a..1d468b527b7b 100644
--- 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 2577405d082d..6cc882223e34 100644
--- 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 eb5cd77bf1d8..e33c32d56193 100644
--- 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

(IIUC) MOV32 is 2 cycles (MOVW, MOVT), and each TST above is 1 cycle
and each BNE is 1 cycle (when not taken). So:

mov32 r2, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
tst r1, r2
bne fast_work_pending

is 4 cycles and tst, bne, tst, bne is also 4 cycles. Would mov32 be
more readable (since it keeps the flags together)?

I guess it would be more readable. Any opinion from the arm folks?

-Kees

--
Kees Cook
Pixel Security

-- 
Thomas