Re: linux-next boot error: general protection fault in __x64_sys_settimeofday

From: Thomas Gleixner <tglx@linutronix.de>
To: syzbot <syzbot+dccce9b26ba09ca49966@syzkaller.appspotmail.com>
Cc: John Stultz <john.stultz@linaro.org>,
	LKML <linux-kernel@vger.kernel.org>,
	sboyd@kernel.org, syzkaller-bugs@googlegroups.com,
	x86@kernel.org, kasan-dev@googlegroups.com
Subject: Re: linux-next boot error: general protection fault in __x64_sys_settimeofday
Date: Thu, 14 Nov 2019 13:35:35 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.21.1911141210410.2507@nanos.tec.linutronix.de> (raw)
In-Reply-To: <0000000000007ce85705974c50e5@google.com>

On Thu, 14 Nov 2019, syzbot wrote:

From the full console output:

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
RIP: 0010:__x64_sys_settimeofday+0x170/0x320 

Code: 85 50 ff ff ff 85 c0 0f 85 50 01 00 00 e8 b8 cd 10 00 48 8b 85 48 ff ff ff 48 c1 e8 03 48 89 c2 48 b8 00 00 00 00 00 fc ff df <80> 3c 02 00 0f 85 8a 01 00 00 49 8b 74 24 08 bf 40 42 0f 00 48 89

      80 3c 02 00             cmpb   $0x0,(%rdx,%rax,1)

RSP: 0018:ffff888093d0fe58 EFLAGS: 00010206
RAX: dffffc0000000000 RBX: 1ffff110127a1fcd RCX: ffffffff8162e915
RDX: 00000fff820fb94b RSI: ffffffff8162e928 RDI: 0000000000000005

i.e.

     *(0x00000fff820fb94b + 0xdffffc0000000000 * 1) == 0

     *(0xe0000bff820fb94b) == 0

So base == 0x00000fff820fb94b and index == 0xdffffc0000000000 and scale =
1. As scale is 1, base and index might be swapped, but that still does not
make any sense.

0xdffffc0000000000 is explicitely loaded into RAX according to the
disassembly, but I can't find the corresponding source as this is in the
middle of the function prologue and looks KASAN related.

RBP: ffff888093d0ff10 R08: ffff8880a8904380 R09: ffff8880a8904c18
R10: fffffbfff1390d30 R11: ffffffff89c86987 R12: 00007ffc107dca50
R13: ffff888093d0fee8 R14: 00007ffc107dca10 R15: 0000000000087a85
FS:  00007f614c01b700(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f4440cdf000 CR3: 00000000a5236000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 ? do_sys_settimeofday64+0x250/0x250
 ? trace_hardirqs_on_thunk+0x1a/0x1c
 ? do_syscall_64+0x26/0x760
 ? entry_SYSCALL_64_after_hwframe+0x49/0xbe
 ? do_syscall_64+0x26/0x760
 ? lockdep_hardirqs_on+0x421/0x5e0
 ? trace_hardirqs_on+0x67/0x240
 do_syscall_64+0xfa/0x760
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The below is the user code which triggered that:

RIP: 0033:0x7f614bb16047

Code: ff ff 73 05 48 83 c4 08 c3 48 8b 0d eb 7d 2e 00 31 d2 48 29 c2 64 89 11 48 83 c8 ff eb e6 90 90 90 90 90 b8 a4 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d c1 7d 2e 00 31 d2 48 29 c2 64

  23:   b8 a4 00 00 00          mov    $0xa4,%eax
  28:   0f 05                   syscall
  2a:*  48 3d 01 f0 ff ff       cmp    $0xfffffffffffff001,%rax
  30:   73 01                   jae    0x33
  32:   c3                      retq

RSP: 002b:00007ffc107dc978 EFLAGS: 00000206 ORIG_RAX: 00000000000000a4
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f614bb16047
RDX: 000000005dcd1ee0 RSI: 00007ffc107dca10 RDI: 00007ffc107dca50
RBP: 0000000000000000 R08: 00007ffc107e6080 R09: 0000000000000eca
R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000

So RAX is obviously the syscall number and the arguments are in RDI (tv()
and RSI (tz), which both look like legit user space addresses.

As this is deep in the function prologue compiler/KASAN people might want
to have a look at that.

Thanks,

	tglx