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

From: Thomas Garnier
Date: Tue Jul 18 2017 - 12:05:03 EST

On Tue, Jul 18, 2017 at 7:36 AM, Leonard Crestez
<leonard.crestez@xxxxxxx> wrote:
> On Wed, 2017-06-14 at 18:12 -0700, Thomas Garnier 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
>> 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 7b8f2141427b..3a48b54c6405 100644
>> --- a/arch/arm/kernel/signal.c
>> +++ b/arch/arm/kernel/signal.c
>> @@ -14,6 +14,7 @@
>> #include
>> #include
>> #include
>> +#include
>>
>> #include
>> #include
>> @@ -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();
>
> This patch made it's way into linux-next next-20170717 and it seems to
> cause hangs when booting some boards over NFS (found via bisection). I
> don't know exactly what determines the issue but I can reproduce hangs
> if even if I just boot with init=/bin/bash and do stuff like
>
> # sleep 1 & sleep 1 & sleep 1 & wait; wait; wait; echo done!
>
> When this happens sysrq-t shows a sleep task hung in the 'R' state
> spinning in do_work_pending, so maybe there is a potential infinite
> loop here?
>
> The addr_limit_user_check at the start of do_work_pending will check
> for TIF_FSCHECK once and clear it but the function loops while
> (thread_flags & _TIF_WORK_MASK), so it if TIF_FSCHECK is set again then
> the loop will never terminate. Does this make sense?

Yes, it does. Thanks for looking into this.

>
> I added some instrumentation to check if TIF_FSCHECK can show up during
> the do_work_pending loop and the answer seems to be yes. I also tried
> to get a stack with a set_fs call from inside do_work_pending and got
> the following:
>
> [ 227.582402] CPU: 0 PID: 829 Comm: sleep Not tainted 4.12.0-01057-g93af8f7-dirty #332
> [ 227.590171] Hardware name: Freescale i.MX6 SoloLite (Device Tree)
> [ 227.596275] Backtrace:
> [ 227.598754] [<c010cbb4>] (dump_backtrace) from [<c010ce60>] (show_stack+0x18/0x1c)
> [ 227.606339] r7:00000000 r6:60070113 r5:00000000 r4:c105a958
> [ 227.612016] [<c010ce48>] (show_stack) from [<c0493498>] (dump_stack+0xb4/0xe8)
> [ 227.619258] [<c04933e4>] (dump_stack) from [<c010c350>] (mydbg_set_fs+0x40/0x48)
> [ 227.626671] r9:c08cf35c r8:ee1cda7c r7:ee1e3dce r6:bf000000 r5:00000000 r4:ffffe000
> [ 227.634433] [<c010c310>] (mydbg_set_fs) from [<c021f0b8>] (__probe_kernel_read+0x44/0xd0)
> [ 227.642629] [<c021f074>] (__probe_kernel_read) from [<c011b8d8>] (do_alignment+0x8c/0x75c)
> [ 227.650909] r10:ef085000 r9:c08cf35c r8:00000001 r7:ee1e3dce r6:c011b84c r5:ee1cdbe0
> [ 227.658748] r4:00000000 r3:00000000
> [ 227.662338] [<c011b84c>] (do_alignment) from [<c0101394>] (do_DataAbort+0x40/0xc0)
> [ 227.669921] r10:ef085000 r9:ee1cc000 r8:ee1cdbe0 r7:ee1e3dce r6:c011b84c r5:00000001
> [ 227.677760] r4:c100dd3c
> [ 227.680308] [<c0101354>] (do_DataAbort) from [<c010da44>] (__dabt_svc+0x64/0xa0)
> [ 227.687714] Exception stack(0xee1cdbe0 to 0xee1cdc28)
> [ 227.692780] dbe0: 9064a8c0 ee1e3de2 d82727d8 00000000 ee1b20c0 ee1e3dce 00000000 ef08572c
> [ 227.700971] dc00: c0bb2034 c10c75ea ef085000 ee1cdc74 ee1cdc00 ee1cdc30 c01761a8 c08cf35c
> [ 227.709158] dc20: 40070113 ffffffff
> [ 227.712661] r8:c0bb2034 r7:ee1cdc14 r6:ffffffff r5:40070113 r4:c08cf35c
> [ 227.719382] [<c08cf16c>] (inet_gro_receive) from [<c084a8ec>] (dev_gro_receive+0x2f0/0x618)
> [ 227.727746] r10:ef085000 r9:00000001 r8:00000000 r7:ef085710 r6:c1008b88 r5:ee1b20c0
> [ 227.735585] r4:c1009f78
> [ 227.738132] [<c084a5fc>] (dev_gro_receive) from [<c084ac8c>] (napi_gro_receive+0x78/0x1f4)
> [ 227.746410] r10:ef085000 r9:00000001 r8:c10d15ec r7:c100792c r6:ef085710 r5:c10c744e
> [ 227.754249] r4:ee1b20c0
> [ 227.756801] [<c084ac14>] (napi_gro_receive) from [<c06a2784>] (fec_enet_rx_napi+0x39c/0x988)
> [ 227.765253] r9:00000001 r8:f0c8a960 r7:00000000 r6:00000000 r5:ef086000 r4:ee1b20c0
> [ 227.773010] [<c06a23e8>] (fec_enet_rx_napi) from [<c084a3a4>] (net_rx_action+0x21c/0x474)
> [ 227.781201] r10:ee1cdd78 r9:c0fa7b80 r8:ef7dab80 r7:0000012c r6:00000040 r5:00000001
> [ 227.789039] r4:ef085710
> [ 227.791593] [<c084a188>] (net_rx_action) from [<c012f2d4>] (__do_softirq+0x158/0x534)
> [ 227.799437] r10:00000008 r9:ee1cc000 r8:c10ce568 r7:c100792c r6:c10247bd r5:00000003
> [ 227.807275] r4:c100208c
> [ 227.809824] [<c012f17c>] (__do_softirq) from [<c012fa68>] (irq_exit+0xec/0x168)
> [ 227.817147] r10:c1007ea0 r9:ef010400 r8:00000001 r7:00000000 r6:c1007d3c r5:00000000
> [ 227.824984] r4:c0fa534c
> [ 227.827534] [<c012f97c>] (irq_exit) from [<c01883f4>] (__handle_domain_irq+0x74/0xe8)
> [ 227.835377] [<c0188380>] (__handle_domain_irq) from [<c01015fc>] (gic_handle_irq+0x58/0xbc)
> [ 227.843742] r9:f080b100 r8:c105ae80 r7:ee1cde80 r6:000003ff r5:000003eb r4:f080b10c
> [ 227.851498] [<c01015a4>] (gic_handle_irq) from [<c010daf0>] (__irq_svc+0x70/0x98)
> [ 227.858990] Exception stack(0xee1cde80 to 0xee1cdec8)
> [ 227.864056] de80: ee7a1140 00000001 00000000 000012a9 ee7a1140 ee9d9f10 ee76edc0 ee9d9f60
> [ 227.872248] dea0: 00000000 ee9d9f10 00000010 ee1cdeec ee1cdeb8 ee1cded0 c038a77c c0389688
> [ 227.880434] dec0: 60070013 ffffffff
> [ 227.883937] r10:00000010 r9:ee1cc000 r8:00000000 r7:ee1cdeb4 r6:ffffffff r5:60070013
> [ 227.891775] r4:c0389688
> [ 227.894327] [<c038a6f8>] (nfs_file_clear_open_context) from [<c03860e8>] (nfs_file_release+0x54/0x60)
> [ 227.903558] r7:ee9a78a0 r6:ee68f010 r5:ee9d9f10 r4:ee76edc0
> [ 227.909235] [<c0386094>] (nfs_file_release) from [<c0276cb4>] (__fput+0x94/0x1e0)
> [ 227.916734] [<c0276c20>] (__fput) from [<c0276e60>] (____fput+0x10/0x14)
> [ 227.923448] r10:c10d4298 r9:00000000 r8:00000000 r7:ef2ed780 r6:ef2edc00 r5:c10d5180
> [ 227.931286] r4:ef2edbd4
> [ 227.933839] [<c0276e50>] (____fput) from [<c014c534>] (task_work_run+0xc8/0xec)
> [ 227.941166] [<c014c46c>] (task_work_run) from [<c010c484>] (do_work_pending+0x12c/0x1c4)
> [ 227.949271] r9:ee1cdfb0 r8:00000000 r7:00000000 r6:ee1cc000 r5:00000000 r4:00000000
> [ 227.957029] [<c010c358>] (do_work_pending) from [<c0107c90>] (slow_work_pending+0xc/0x20)
> [ 227.965219] r10:00000000 r9:ee1cc000 r8:c0107e24 r7:0000005b r6:b6f76568 r5:b6f741f0
> [ 227.973058] r4:b6f76904
>
> Maybe the reason this reproduces easily in this particular setup is
> that ethernet causes lots of alignment faults?

Can you try this change?

diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index 3a48b54c6405..bc6ad7789568 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -573,12 +573,11 @@ do_work_pending(struct pt_regs *regs, unsigned
int thread_flags, int syscall)
*/
trace_hardirqs_off();

- /* Check valid user FS if needed */
- addr_limit_user_check();
-
do {
if (likely(thread_flags & _TIF_NEED_RESCHED)) {
schedule();
+ } else if (thread_flags & _TIF_FSCHECK) {
+ addr_limit_user_check();
} else {
if (unlikely(!user_mode(regs)))
return 0;

>
> --
> Regards,
> Leonard



--
Thomas