linux-kernel - Re: [syzbot] [pci?] KCSAN: data-race in vga_arb_write / vga_arb

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Message-ID: <CACT4Y+YAXoB+GeA2dQODzDz0u3Z2Dmh98bFHT4Y0Tdi7ZFUT9A@mail.gmail.com>
Date: Fri, 10 Jan 2025 16:18:10 +0100
From: Dmitry Vyukov <dvyukov@...gle.com>
To: syzbot <syzbot+2699c160471dfeee2644@...kaller.appspotmail.com>
Cc: bhelgaas@...gle.com, linux-kernel@...r.kernel.org, 
	linux-pci@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [pci?] KCSAN: data-race in vga_arb_write / vga_arb_write (4)

On Fri, 10 Jan 2025 at 16:04, syzbot
<syzbot+2699c160471dfeee2644@...kaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    9b2ffa6148b1 Merge tag 'mtd/fixes-for-6.13-rc5' of git://g..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=12aac0b0580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=87d3cfca6847d1fa
> dashboard link: https://syzkaller.appspot.com/bug?extid=2699c160471dfeee2644
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/8f3f5cceed80/disk-9b2ffa61.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/1ba00b70d91e/vmlinux-9b2ffa61.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/b99c1bb4d63f/bzImage-9b2ffa61.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+2699c160471dfeee2644@...kaller.appspotmail.com

This is a race between io_cnt++ and io_cnt--.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/vgaarb.c?id=9b2ffa6148b1e4468d08f7e0e7e371c43cac9ffe#n1204
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/vgaarb.c?id=9b2ffa6148b1e4468d08f7e0e7e371c43cac9ffe#n1263

Later io_cnt is used to drop references:

while (uc->io_cnt--)
    vga_put(uc->pdev, VGA_RSRC_LEGACY_IO);

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/vgaarb.c?id=9b2ffa6148b1e4468d08f7e0e7e371c43cac9ffe#n1454

So this will corrupt memory.





> ==================================================================
> BUG: KCSAN: data-race in vga_arb_write / vga_arb_write
>
> read-write to 0xffff88814f612424 of 4 bytes by task 17707 on cpu 0:
>  vga_arb_write+0x11fa/0x1410 drivers/pci/vgaarb.c:1204
>  vfs_write+0x281/0x920 fs/read_write.c:677
>  ksys_write+0xe8/0x1b0 fs/read_write.c:731
>  __do_sys_write fs/read_write.c:742 [inline]
>  __se_sys_write fs/read_write.c:739 [inline]
>  __x64_sys_write+0x42/0x50 fs/read_write.c:739
>  x64_sys_call+0x287e/0x2dc0 arch/x86/include/generated/asm/syscalls_64.h:2
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> read-write to 0xffff88814f612424 of 4 bytes by task 17708 on cpu 1:
>  vga_arb_write+0xf45/0x1410 drivers/pci/vgaarb.c:1263
>  vfs_write+0x281/0x920 fs/read_write.c:677
>  ksys_write+0xe8/0x1b0 fs/read_write.c:731
>  __do_sys_write fs/read_write.c:742 [inline]
>  __se_sys_write fs/read_write.c:739 [inline]
>  __x64_sys_write+0x42/0x50 fs/read_write.c:739
>  x64_sys_call+0x287e/0x2dc0 arch/x86/include/generated/asm/syscalls_64.h:2
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> value changed: 0x00000001 -> 0x00000007
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 UID: 0 PID: 17708 Comm: syz.4.3325 Not tainted 6.13.0-rc4-syzkaller-00012-g9b2ffa6148b1 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
> ==================================================================
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@...glegroups.com.
>
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>
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