linux-kernel - Re: WARNING: suspicious RCU usage with PROVE_RCU

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Message-ID: <9bae445f-f971-f638-efa6-d948901843a2@canonical.com>
Date:   Mon, 18 May 2020 02:39:19 -0700
From:   John Johansen <john.johansen@...onical.com>
To:     Amol Grover <frextrite@...il.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: Re: WARNING: suspicious RCU usage with PROVE_RCU_LIST=y

On 4/6/20 4:41 AM, Amol Grover wrote:
> 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.
> 

The comment could be updated to

 * Requires: rcu_read_lock be held or the namespace->lock be held.


bascially there are reader paths which take the rcu_read_lock and then call
the __lookupn_profile() helper eg. aa_lookupn_profile() and the writer paths

the hold the ns lock, like in the splat
  aa_replace_profiles() takes the ns lock



> [   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
>