Re: general protection fault in syscall_return_slowpath

From: Jann Horn <jannh@google.com>
To: syzbot <syzbot+cd66e43794b178bb5cd6@syzkaller.appspotmail.com>
Cc: Borislav Petkov <bp@alien8.de>, "H . Peter Anvin" <hpa@zytor.com>,
	kernel list <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"the arch/x86 maintainers" <x86@kernel.org>
Subject: Re: general protection fault in syscall_return_slowpath
Date: Sun, 8 Mar 2020 18:20:41 +0100	[thread overview]
Message-ID: <CAG48ez2TnPppWonr8K3POyg3hGosgNXU2-ZMjyaYN4tLYRiJiQ@mail.gmail.com> (raw)
In-Reply-To: <000000000000ff323f05a053100c@google.com>

[-- Attachment #1: Type: text/plain, Size: 2256 bytes --]

On Sun, Mar 8, 2020 at 5:40 PM syzbot
<syzbot+cd66e43794b178bb5cd6@syzkaller.appspotmail.com> wrote:
> syzbot found the following crash on:
>
> HEAD commit:    63623fd4 Merge tag 'for-linus' of git://git.kernel.org/pub..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=16cfeac3e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=5d2e033af114153f
> dashboard link: https://syzkaller.appspot.com/bug?extid=cd66e43794b178bb5cd6
> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12a42329e00000
[...]
> general protection fault, probably for non-canonical address 0x1ffffffff1255a6b: 0000 [#1] PREEMPT SMP KASAN
> CPU: 0 PID: 8742 Comm: syz-executor.2 Not tainted 5.6.0-rc3-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:arch_local_irq_disable arch/x86/include/asm/paravirt.h:757 [inline]
> RIP: 0010:syscall_return_slowpath+0xeb/0x4a0 arch/x86/entry/common.c:277
> Code: 00 10 0f 85 de 00 00 00 e8 b2 a3 76 00 48 c7 c0 58 d3 2a 89 48 c1 e8 03 80 3c 18 00 74 0c 48 c7 c7 58 d3 2a 89 e8 05 00 00 00 <00> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> RSP: 0018:ffffc900020a7ed0 EFLAGS: 00010246
> RAX: 1ffffffff1255a6b RBX: dffffc0000000000 RCX: ffff88808c512380
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> RBP: ffffc900020a7f10 R08: ffffffff810075bb R09: fffffbfff14d9182
> R10: fffffbfff14d9182 R11: 0000000000000000 R12: 1ffff110118a2470
> R13: 0000000000004000 R14: ffff88808c512380 R15: ffff88808c512380
> FS:  000000000154f940(0000) GS:ffff8880aea00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000000000076c000 CR3: 00000000a6b05000 CR4: 00000000001406f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

Let's see if we can get syzkaller to tell us which fbcon
implementation it's hitting...

#syz test: https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux.git
63623fd44972d1ed2bfb6e0fb631dfcf547fd1e7

[-- Attachment #2: 0001-FOR-TESTING-ONLY-tell-us-which-fbcon-implementation-.patch --]
[-- Type: text/x-patch, Size: 1194 bytes --]

From 8442144fe582ab38985378cf21ca88de1594b5df Mon Sep 17 00:00:00 2001
From: Jann Horn <jannh@google.com>
Date: Sun, 8 Mar 2020 18:15:11 +0100
Subject: [PATCH] FOR TESTING ONLY: tell us which fbcon implementation is used

---
 drivers/tty/vt/vt.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 0cfbb7182b5a..b3dd44802208 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -4590,7 +4590,7 @@ static int con_font_copy(struct vc_data *vc, struct console_font_op *op)
 {
 	int con = op->height;
 	int rc;
-
+	static void *LAST;
 
 	console_lock();
 	if (vc->vc_mode != KD_TEXT)
@@ -4601,8 +4601,14 @@ static int con_font_copy(struct vc_data *vc, struct console_font_op *op)
 		rc = -ENOTTY;
 	else if (con == vc->vc_num)	/* nothing to do */
 		rc = 0;
-	else
+	else {
+		if (LAST != vc->vc_sw) {
+			pr_warn("con_font_copy(): vc_sw=%pS, vc_sw->con_font_copy=%pS\n",
+				vc->vc_sw, vc->vc_sw->con_font_copy);
+			LAST = vc->vc_sw;
+		}
 		rc = vc->vc_sw->con_font_copy(vc, con);
+	}
 	console_unlock();
 	return rc;
 }

base-commit: 63623fd44972d1ed2bfb6e0fb631dfcf547fd1e7
-- 
2.25.1.481.gfbce0eb801-goog

