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Date: Tue, 15 May 2018 15:04:26 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Michael Ellerman <mpe@...erman.id.au>
Cc: linuxppc-dev@...abs.org, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 2/2] powerpc: Check address limit on user-mode return (TIF_FSCHECK)

On Mon, May 14, 2018 at 6:03 AM Michael Ellerman <mpe@...erman.id.au> wrote:

> set_fs() sets the addr_limit, which is used in access_ok() to
> determine if an address is a user or kernel address.

> Some code paths use set_fs() to temporarily elevate the addr_limit so
> that kernel code can read/write kernel memory as if it were user
> memory. That is fine as long as the code can't ever return to
> userspace with the addr_limit still elevated.

> If that did happen, then userspace can read/write kernel memory as if
> it were user memory, eg. just with write(2). In case it's not clear,
> that is very bad. It has also happened in the past due to bugs.

> Commit 5ea0727b163c ("x86/syscalls: Check address limit on user-mode
> return") added a mechanism to check the addr_limit value before
> returning to userspace. Any call to set_fs() sets a thread flag,
> TIF_FSCHECK, and if we see that on the return to userspace we go out
> of line to check that the addr_limit value is not elevated.

> For further info see the above commit, as well as:
>    https://lwn.net/Articles/722267/
>    https://bugs.chromium.org/p/project-zero/issues/detail?id=990

> Verified to work on 64-bit Book3S using a POC that objdumps the system
> call handler, and a modified lkdtm_CORRUPT_USER_DS() that doesn't kill
> the caller.

> Before:
>    $ sudo ./test-tif-fscheck
>    ...
>    0000000000000000 <.data>:
>           0:       e1 f7 8a 79     rldicl. r10,r12,30,63
>           4:       80 03 82 40     bne     0x384
>           8:       00 40 8a 71     andi.   r10,r12,16384
>           c:       78 0b 2a 7c     mr      r10,r1
>          10:       10 fd 21 38     addi    r1,r1,-752
>          14:       08 00 c2 41     beq-    0x1c
>          18:       58 09 2d e8     ld      r1,2392(r13)
>          1c:       00 00 41 f9     std     r10,0(r1)
>          20:       70 01 61 f9     std     r11,368(r1)
>          24:       78 01 81 f9     std     r12,376(r1)
>          28:       70 00 01 f8     std     r0,112(r1)
>          2c:       78 00 41 f9     std     r10,120(r1)
>          30:       20 00 82 41     beq     0x50
>          34:       a6 42 4c 7d     mftb    r10

> After:

>    $ sudo ./test-tif-fscheck
>    Killed

> And in dmesg:
>    Invalid address limit on user-mode return
>    WARNING: CPU: 1 PID: 3689 at ../include/linux/syscalls.h:260
do_notify_resume+0x140/0x170
>    ...
>    NIP [c00000000001ee50] do_notify_resume+0x140/0x170
>    LR [c00000000001ee4c] do_notify_resume+0x13c/0x170
>    Call Trace:
>      do_notify_resume+0x13c/0x170 (unreliable)
>      ret_from_except_lite+0x70/0x74

> Performance overhead is essentially zero in the usual case, because
> the bit is checked as part of the existing _TIF_USER_WORK_MASK check.

> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>

The change looks good to me.

> ---
>   arch/powerpc/include/asm/thread_info.h | 8 +++++---
>   arch/powerpc/include/asm/uaccess.h     | 8 +++++++-
>   arch/powerpc/kernel/signal.c           | 4 ++++
>   3 files changed, 16 insertions(+), 4 deletions(-)

> diff --git a/arch/powerpc/include/asm/thread_info.h
b/arch/powerpc/include/asm/thread_info.h
> index 5964145db03d..f308dfeb2746 100644
> --- a/arch/powerpc/include/asm/thread_info.h
> +++ b/arch/powerpc/include/asm/thread_info.h
> @@ -79,8 +79,7 @@ extern int arch_dup_task_struct(struct task_struct
*dst, struct task_struct *src
>   #define TIF_SYSCALL_TRACE      0       /* syscall trace active */
>   #define TIF_SIGPENDING         1       /* signal pending */
>   #define TIF_NEED_RESCHED       2       /* rescheduling necessary */
> -#define TIF_POLLING_NRFLAG     3       /* true if poll_idle() is polling
> -                                          TIF_NEED_RESCHED */
> +#define TIF_FSCHECK            3       /* Check FS is USER_DS on return
*/
>   #define TIF_32BIT              4       /* 32 bit binary */
>   #define TIF_RESTORE_TM         5       /* need to restore TM FP/VEC/VSX
*/
>   #define TIF_PATCH_PENDING      6       /* pending live patching update */
> @@ -99,6 +98,7 @@ extern int arch_dup_task_struct(struct task_struct
*dst, struct task_struct *src
>   #if defined(CONFIG_PPC64)
>   #define TIF_ELF2ABI            18      /* function descriptors must die!
*/
>   #endif
> +#define TIF_POLLING_NRFLAG     19      /* true if poll_idle() is polling
TIF_NEED_RESCHED */

>   /* as above, but as bit values */
>   #define _TIF_SYSCALL_TRACE     (1<<TIF_SYSCALL_TRACE)
> @@ -118,13 +118,15 @@ extern int arch_dup_task_struct(struct task_struct
*dst, struct task_struct *src
>   #define _TIF_SYSCALL_TRACEPOINT        (1<<TIF_SYSCALL_TRACEPOINT)
>   #define _TIF_EMULATE_STACK_STORE       (1<<TIF_EMULATE_STACK_STORE)
>   #define _TIF_NOHZ              (1<<TIF_NOHZ)
> +#define _TIF_FSCHECK           (1<<TIF_FSCHECK)
>   #define _TIF_SYSCALL_DOTRACE   (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT
| \
>                                   _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT |
\
>                                   _TIF_NOHZ)

>   #define _TIF_USER_WORK_MASK    (_TIF_SIGPENDING | _TIF_NEED_RESCHED | \
>                                   _TIF_NOTIFY_RESUME | _TIF_UPROBE | \
> -                                _TIF_RESTORE_TM | _TIF_PATCH_PENDING)
> +                                _TIF_RESTORE_TM | _TIF_PATCH_PENDING | \
> +                                _TIF_FSCHECK)
>   #define _TIF_PERSYSCALL_MASK   (_TIF_RESTOREALL|_TIF_NOERROR)

>   /* Bits in local_flags */
> diff --git a/arch/powerpc/include/asm/uaccess.h
b/arch/powerpc/include/asm/uaccess.h
> index a91cea15187b..abba80f8ff04 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -31,7 +31,13 @@

>   #define get_ds()       (KERNEL_DS)
>   #define get_fs()       (current->thread.addr_limit)
> -#define set_fs(val)    (current->thread.addr_limit = (val))
> +
> +static inline void set_fs(mm_segment_t fs)
> +{
> +       current->thread.addr_limit = fs;
> +       /* On user-mode return check addr_limit (fs) is correct */
> +       set_thread_flag(TIF_FSCHECK);
> +}

>   #define segment_eq(a, b)       ((a).seg == (b).seg)

> diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
> index 61db86ecd318..fb932f1202c7 100644
> --- a/arch/powerpc/kernel/signal.c
> +++ b/arch/powerpc/kernel/signal.c
> @@ -15,6 +15,7 @@
>   #include <linux/key.h>
>   #include <linux/context_tracking.h>
>   #include <linux/livepatch.h>
> +#include <linux/syscalls.h>
>   #include <asm/hw_breakpoint.h>
>   #include <linux/uaccess.h>
>   #include <asm/unistd.h>
> @@ -150,6 +151,9 @@ void do_notify_resume(struct pt_regs *regs, unsigned
long thread_info_flags)
>   {
>          user_exit();

> +       /* Check valid addr_limit, TIF check is done there */
> +       addr_limit_user_check();
> +
>          if (thread_info_flags & _TIF_UPROBE)
>                  uprobe_notify_resume(regs);

> --
> 2.14.1



-- 
Thomas

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