linux-kernel - fs/proc: Crash observed in next

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Date:   Mon, 15 Apr 2019 18:28:25 +0530
From:   Jitendra Sharma <shajit@...eaurora.org>
To:     keescook@...omium.org, mcgrof@...nel.org
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: fs/proc: Crash observed in next_tgid (fs/proc/base.c)

Hi Kees Cook/Luis,

We are observing one kernel crash in next_tgid function through 
getdents64 path. Call stack is as shown below:

-000|has_group_leader_pid(inline)
-000|next_tgid(
| [X20] ns = 0xFFFFFF87CABB1AC0,
| [locdesc] iter = (
| [locdesc] tgid = 424,
| [locdesc] task = ?))
| [X21] p = 0xFFFFFFD0FFFFF948
| [X21] task = 0xFFFFFFD0FFFFF948
-001|proc_pid_readdir(
| [X20] file = 0xFFFFFFD1AC60FC40,
| [X19] ctx = 0xFFFFFF8027363E40)
| [X21] ns = 0xFFFFFF87CABB1AC0
-002|proc_root_readdir(
| [X20] file = 0xFFFFFFD1AC60FC40,
| [X19] ctx = 0xFFFFFF8027363E40)
-003|iterate_dir(
| [X19] file = 0xFFFFFFD1AC60FC40,
| [X22] ctx = 0xFFFFFF8027363E40)
| [X23] inode = 0xFFFFFFD1F20246D0
-004|SYSC_getdents64(inline)
-004|sys_getdents64(
| ?,
| ?,
| [X19] count = 4200)
| [X19] count = 4200
| [X20] f = ([X20] file = 0xAC60FC43AC60FC40, [X20] flags = 1207898624)
| [X0] error = -1720
-005|el0_svc_naked(asm)
-->|exception
-006|NUX:0x78C5AD7D38(asm)
---|end of frame

 From this call stack,task: 0xFFFFFFD0FFFFF948, seems to be invalid. 
As(from ramdumps) it doesn't have any valid fields. And while trying to 
access the fields of this task struct in has_group_leader_pid, abort is 
happening.

 From the dumps, its not clear why the task struct is coming to be some 
invalid (Possibly task has already exited).  This issue is observed 
during normal monkey testing for long hours.

Could you please provide some pointers which could help in debugging 
this issue further.

Thanks,

Jitendra

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