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)? -Kees -- Kees Cook Pixel Security -- 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
