On Tue, Jun 20, 2017 at 1:18 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Wed, Jun 14, 2017 at 6:12 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote: >> 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@xxxxxxxxxx> >> --- >> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
