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Subject[PATCH 4.19 181/234] powerpc/kasan: Silence KASAN warnings in __get_wchan()
Date
From: He Ying <heying24@huawei.com>

[ Upstream commit a1b29ba2f2c171b9bea73be993bfdf0a62d37d15 ]

The following KASAN warning was reported in our kernel.

BUG: KASAN: stack-out-of-bounds in get_wchan+0x188/0x250
Read of size 4 at addr d216f958 by task ps/14437

CPU: 3 PID: 14437 Comm: ps Tainted: G O 5.10.0 #1
Call Trace:
[daa63858] [c0654348] dump_stack+0x9c/0xe4 (unreliable)
[daa63888] [c035cf0c] print_address_description.constprop.3+0x8c/0x570
[daa63908] [c035d6bc] kasan_report+0x1ac/0x218
[daa63948] [c00496e8] get_wchan+0x188/0x250
[daa63978] [c0461ec8] do_task_stat+0xce8/0xe60
[daa63b98] [c0455ac8] proc_single_show+0x98/0x170
[daa63bc8] [c03cab8c] seq_read_iter+0x1ec/0x900
[daa63c38] [c03cb47c] seq_read+0x1dc/0x290
[daa63d68] [c037fc94] vfs_read+0x164/0x510
[daa63ea8] [c03808e4] ksys_read+0x144/0x1d0
[daa63f38] [c005b1dc] ret_from_syscall+0x0/0x38
--- interrupt: c00 at 0x8fa8f4
LR = 0x8fa8cc

The buggy address belongs to the page:
page:98ebcdd2 refcount:0 mapcount:0 mapping:00000000 index:0x2 pfn:0x1216f
flags: 0x0()
raw: 00000000 00000000 01010122 00000000 00000002 00000000 ffffffff 00000000
raw: 00000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
d216f800: 00 00 00 00 00 f1 f1 f1 f1 00 00 00 00 00 00 00
d216f880: f2 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>d216f900: 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00
^
d216f980: f2 f2 f2 f2 f2 f2 f2 00 00 00 00 00 00 00 00 00
d216fa00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

After looking into this issue, I find the buggy address belongs
to the task stack region. It seems KASAN has something wrong.
I look into the code of __get_wchan in x86 architecture and
find the same issue has been resolved by the commit
f7d27c35ddff ("x86/mm, kasan: Silence KASAN warnings in get_wchan()").
The solution could be applied to powerpc architecture too.

As Andrey Ryabinin said, get_wchan() is racy by design, it may
access volatile stack of running task, thus it may access
redzone in a stack frame and cause KASAN to warn about this.

Use READ_ONCE_NOCHECK() to silence these warnings.

Reported-by: Wanming Hu <huwanming@huaweil.com>
Signed-off-by: He Ying <heying24@huawei.com>
Signed-off-by: Chen Jingwen <chenjingwen6@huawei.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220121014418.155675-1-heying24@huawei.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/powerpc/kernel/process.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 02b69a68139c..56c33285b1df 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -2017,12 +2017,12 @@ unsigned long get_wchan(struct task_struct *p)
return 0;

do {
- sp = *(unsigned long *)sp;
+ sp = READ_ONCE_NOCHECK(*(unsigned long *)sp);
if (!validate_sp(sp, p, STACK_FRAME_OVERHEAD) ||
p->state == TASK_RUNNING)
return 0;
if (count > 0) {
- ip = ((unsigned long *)sp)[STACK_FRAME_LR_SAVE];
+ ip = READ_ONCE_NOCHECK(((unsigned long *)sp)[STACK_FRAME_LR_SAVE]);
if (!in_sched_functions(ip))
return ip;
}
--
2.35.1


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