linux-kernel - WARNING: suspicious RCU usage with PROVE_RCU

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Date:   Mon, 6 Apr 2020 17:11:34 +0530
From:   Amol Grover <frextrite@...il.com>
To:     John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Armijn Hemel <armijn@...ldur.nl>,
        Allison Randal <allison@...utok.net>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: WARNING: suspicious RCU usage with PROVE_RCU_LIST=y

Hello,

With respect to the patch https://lore.kernel.org/patchwork/patch/1202512/
I boot tested with CONFIG_PROVE_RCU_LIST=y and encountered a susppicious RCU
usage warning in "security/apparmor/include/lib.h". I thought of going forward
and fix it myself, however, while going through the stack trace and the actual
code, I found that the function (__lookupn_profile) is required to be called
with rcu_read_locK() but the splat proves it otherwise.

[   12.727582] =============================
[   12.727599] WARNING: suspicious RCU usage
[   12.727601] 5.5.4-stable #17 Tainted: G            E     
[   12.727602] -----------------------------
[   12.727604] security/apparmor/include/lib.h:191 RCU-list traversed in non-reader section!!
[   12.727605] 
               other info that might help us debug this:

[   12.727606] 
               rcu_scheduler_active = 2, debug_locks = 1 
[   12.727608] 2 locks held by apparmor_parser/506:
[   12.727609]  #0: ffff9f0687562490 (sb_writers#10){.+.+}, at: vfs_write+0x140/0x1a0
[   12.727614]  #1: ffff9f0687f09ca8 (&ns->lock){+.+.}, at: aa_replace_profiles+0x17a/0xdd0
[   12.727619] 
               stack backtrace:
[   12.727621] CPU: 3 PID: 506 Comm: apparmor_parser Tainted: G            E     5.5.4-stable #17 
[   12.727622] Hardware name: Gigabyte Technology Co., Ltd. Z170-D3H/Z170-D3H-CF, BIOS F21 03/06/2017
[   12.727623] Call Trace:
[   12.727627]  dump_stack+0x8f/0xd0
[   12.727630]  __lookupn_profile+0x19c/0x1a0
[   12.727632]  ? aa_unpack+0x51b/0x580
[   12.727636]  __lookup_replace+0x34/0xc0
[   12.727640]  aa_replace_profiles+0x2a0/0xdd0
[   12.727649]  policy_update+0x106/0x370
[   12.727653]  profile_replace+0xa3/0x110
[   12.727657]  vfs_write+0xb9/0x1a0
[   12.727661]  ksys_write+0x68/0xe0
[   12.727666]  do_syscall_64+0x5c/0xe0
[   12.727669]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[   12.727671] RIP: 0033:0x7ff83fec7f93
[   12.727673] Code: 75 05 48 83 c4 58 c3 e8 eb 41 ff ff 66 2e 0f 1f 84 00 00 00 00 00 90 64 8b 04 25 18 00 00 00 85 c0 75 14 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 55 c3 0f 1f 40 00 48 83 ec 28 48 89 54 24 18
[   12.727674] RSP: 002b:00007ffcebb5c398 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[   12.727676] RAX: ffffffffffffffda RBX: 0000000000007131 RCX: 00007ff83fec7f93
[   12.727677] RDX: 0000000000007131 RSI: 00005610fd804a40 RDI: 0000000000000006
[   12.727678] RBP: 00005610fd804a40 R08: 0000000000007131 R09: 00005610fd802f38
[   12.727680] R10: fffffffffffffa8a R11: 0000000000000246 R12: 0000000000000000
[   12.727681] R13: 0000000000000006 R14: 00005610fd7dd490 R15: 0000000000007131

Thanks
Amol