Re: BUG : PowerPC RCU: torture test failed with __stack_chk_fail

Boqun Feng
2023-04-24T00:33:32Z

On Sat, Apr 22, 2023 at 09:28:39PM +0200, Joel Fernandes wrote:
> On Sat, Apr 22, 2023 at 2:47 PM Zhouyi Zhou <zhouzhouyi@gmail.com> wrote:
> >
> > Dear PowerPC and RCU developers:
> > During the RCU torture test on mainline (on the VM of Opensource Lab
> > of Oregon State University), SRCU-P failed with __stack_chk_fail:
> > [ 264.381952][ T99] [c000000006c7bab0] [c0000000010c67c0]
> > dump_stack_lvl+0x94/0xd8 (unreliable)
> > [ 264.383786][ T99] [c000000006c7bae0] [c00000000014fc94] panic+0x19c/0x468
> > [ 264.385128][ T99] [c000000006c7bb80] [c0000000010fca24]
> > __stack_chk_fail+0x24/0x30
> > [ 264.386610][ T99] [c000000006c7bbe0] [c0000000002293b4]
> > srcu_gp_start_if_needed+0x5c4/0x5d0
> > [ 264.388188][ T99] [c000000006c7bc70] [c00000000022f7f4]
> > srcu_torture_call+0x34/0x50
> > [ 264.389611][ T99] [c000000006c7bc90] [c00000000022b5e8]
> > rcu_torture_fwd_prog+0x8c8/0xa60
> > [ 264.391439][ T99] [c000000006c7be00] [c00000000018e37c] kthread+0x15c/0x170
> > [ 264.392792][ T99] [c000000006c7be50] [c00000000000df94]
> > ret_from_kernel_thread+0x5c/0x64
> > The kernel config file can be found in [1].
> > And I write a bash script to accelerate the bug reproducing [2].
> > After a week's debugging, I found the cause of the bug is because the
> > register r10 used to judge for stack overflow is not constant between
> > context switches.
> > The assembly code for srcu_gp_start_if_needed is located at [3]:
> > c000000000226eb4: 78 6b aa 7d mr r10,r13
> > c000000000226eb8: 14 42 29 7d add r9,r9,r8
> > c000000000226ebc: ac 04 00 7c hwsync
> > c000000000226ec0: 10 00 7b 3b addi r27,r27,16
> > c000000000226ec4: 14 da 29 7d add r9,r9,r27
> > c000000000226ec8: a8 48 00 7d ldarx r8,0,r9
> > c000000000226ecc: 01 00 08 31 addic r8,r8,1
> > c000000000226ed0: ad 49 00 7d stdcx. r8,0,r9
> > c000000000226ed4: f4 ff c2 40 bne- c000000000226ec8
> > <srcu_gp_start_if_needed+0x1c8>
> > c000000000226ed8: 28 00 21 e9 ld r9,40(r1)
> > c000000000226edc: 78 0c 4a e9 ld r10,3192(r10)
> > c000000000226ee0: 79 52 29 7d xor. r9,r9,r10
> > c000000000226ee4: 00 00 40 39 li r10,0
> > c000000000226ee8: b8 03 82 40 bne c0000000002272a0
> > <srcu_gp_start_if_needed+0x5a0>
> > by debugging, I see the r10 is assigned with r13 on c000000000226eb4,
> > but if there is a context-switch before c000000000226edc, a false
> > positive will be reported.
> >
> > [1] http://154.220.3.115/logs/0422/configformainline.txt
> > [2] 154.220.3.115/logs/0422/whilebash.sh
> > [3] http://154.220.3.115/logs/0422/srcu_gp_start_if_needed.txt
> >
> > My analysis and debugging may not be correct, but the bug is easily
> > reproducible.
>
> If this is a bug in the stack smashing protection as you seem to hint,
> I wonder if you see the issue with a specific gcc version and is a
> compiler-specific issue. It's hard to say, but considering this I

Very likely, more asm code from Zhouyi's link:

This is the __srcu_read_unlock_nmisafe(), since "hwsync" is
smp_mb__{after,before}_atomic(), and the following code is first
barrier then atomic, so it's the unlock.

c000000000226eb4: 78 6b aa 7d mr r10,r13

^ r13 is the pointer to percpu data on PPC64 kernel, and it's also
the pointer to TLS data for userspace code.

c000000000226eb8: 14 42 29 7d add r9,r9,r8
c000000000226ebc: ac 04 00 7c hwsync
c000000000226ec0: 10 00 7b 3b addi r27,r27,16
c000000000226ec4: 14 da 29 7d add r9,r9,r27
c000000000226ec8: a8 48 00 7d ldarx r8,0,r9
c000000000226ecc: 01 00 08 31 addic r8,r8,1
c000000000226ed0: ad 49 00 7d stdcx. r8,0,r9
c000000000226ed4: f4 ff c2 40 bne- c000000000226ec8 <srcu_gp_start_if_needed+0x1c8>
c000000000226ed8: 28 00 21 e9 ld r9,40(r1)
c000000000226edc: 78 0c 4a e9 ld r10,3192(r10)

here I think that the compiler is using r10 as an alias to r13, since
for userspace program, it's safe to assume the TLS pointer doesn't
change. However this is not true for kernel percpu pointer.

The real intention here is to compare 40(r1) vs 3192(r13) for stack
guard checking, however since r13 is the percpu pointer in kernel, so
the value of r13 can be changed if the thread gets scheduled to a
different CPU after reading r13 for r10.

__srcu_read_unlock_nmisafe() triggers this issue, because:

* it contains a read from r13
* it locates at the very end of srcu_gp_start_if_needed().

This gives the compiler more opportunity to "optimize" a read from r13
away.

c000000000226ee0: 79 52 29 7d xor. r9,r9,r10
c000000000226ee4: 00 00 40 39 li r10,0
c000000000226ee8: b8 03 82 40 bne c0000000002272a0 <srcu_gp_start_if_needed+0x5a0>

As a result, here triggers __stack_chk_fail if mis-match.

If I'm correct, the following should be a workaround:

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index ab4ee58af84b..f5ae3be3d04d 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -747,6 +747,7 @@ void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx)

smp_mb__before_atomic(); /* C */ /* Avoid leaking the critical section. */
atomic_long_inc(&sdp->srcu_unlock_count[idx]);
+ asm volatile("" : : : "r13", "memory");
}
EXPORT_SYMBOL_GPL(__srcu_read_unlock_nmisafe);

Zhouyi, could you give a try? Note I think the "memory" clobber here is
unnecesarry, but I just add it in case I'm wrong.

Needless to say, the correct fix is to make ppc stack protector aware of
r13 is volatile.

Regards,
Boqun

> think it's important for you to mention the compiler version in your
> report (along with kernel version, kernel logs etc.)
>
> thanks,
>
> - Joel
>
>
> - Joel