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To:     syzbot <syzbot+620cf21140fc7e772a5d@syzkaller.appspotmail.com>,
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References: <00000000000056cda705bbe93d11@google.com>
From:   =?UTF-8?Q?Michel_D=c3=a4nzer?= <michel@daenzer.net>
Subject: Re: KMSAN: kernel-infoleak in compat_drm_wait_vblank
Message-ID: <79146a29-a05d-f5b9-e81b-bda1db95b952@daenzer.net>
Date:   Wed, 3 Mar 2021 16:37:18 +0100
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On 2021-02-22 10:15 a.m., syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    29ad81a1 arch/x86: add missing include to sparsemem.h
> git tree:       https://github.com/google/kmsan.git master
> console output: https://syzkaller.appspot.com/x/log.txt?x=111e6312d00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=c8e3b38ca92283e
> dashboard link: https://syzkaller.appspot.com/bug?extid=620cf21140fc7e772a5d
> compiler:       Debian clang version 11.0.1-2
> userspace arch: i386
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+620cf21140fc7e772a5d@syzkaller.appspotmail.com
> 
> =====================================================
> BUG: KMSAN: kernel-infoleak in kmsan_copy_to_user+0x9c/0xb0 mm/kmsan/kmsan_hooks.c:249
> CPU: 1 PID: 26999 Comm: syz-executor.2 Not tainted 5.11.0-rc7-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>   __dump_stack lib/dump_stack.c:79 [inline]
>   dump_stack+0x21c/0x280 lib/dump_stack.c:120
>   kmsan_report+0xfb/0x1e0 mm/kmsan/kmsan_report.c:118
>   kmsan_internal_check_memory+0x484/0x520 mm/kmsan/kmsan.c:437
>   kmsan_copy_to_user+0x9c/0xb0 mm/kmsan/kmsan_hooks.c:249
>   instrument_copy_to_user include/linux/instrumented.h:121 [inline]
>   _copy_to_user+0x1ac/0x270 lib/usercopy.c:33
>   copy_to_user include/linux/uaccess.h:209 [inline]
>   compat_drm_wait_vblank+0x36f/0x450 drivers/gpu/drm/drm_ioc32.c:866
>   drm_compat_ioctl+0x3f6/0x590 drivers/gpu/drm/drm_ioc32.c:995
>   __do_compat_sys_ioctl fs/ioctl.c:842 [inline]
>   __se_compat_sys_ioctl+0x53d/0x1100 fs/ioctl.c:793
>   __ia32_compat_sys_ioctl+0x4a/0x70 fs/ioctl.c:793
>   do_syscall_32_irqs_on arch/x86/entry/common.c:79 [inline]
>   __do_fast_syscall_32+0x102/0x160 arch/x86/entry/common.c:141
>   do_fast_syscall_32+0x6a/0xc0 arch/x86/entry/common.c:166
>   do_SYSENTER_32+0x73/0x90 arch/x86/entry/common.c:209
>   entry_SYSENTER_compat_after_hwframe+0x4d/0x5c
> RIP: 0023:0xf7f47549
> Code: 03 74 c0 01 10 05 03 74 b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 10 08 03 74 d8 01 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 8d b4 26 00 00 00 00 8d b4 26 00 00 00 00
> RSP: 002b:00000000f55415fc EFLAGS: 00000296 ORIG_RAX: 0000000000000036
> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00000000c018643a
> RDX: 0000000020000100 RSI: 0000000000000000 RDI: 0000000000000000
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> 
> Uninit was stored to memory at:
>   kmsan_save_stack_with_flags mm/kmsan/kmsan.c:121 [inline]
>   kmsan_internal_chain_origin+0xad/0x130 mm/kmsan/kmsan.c:289
>   __msan_chain_origin+0x57/0xa0 mm/kmsan/kmsan_instr.c:147
>   compat_drm_wait_vblank+0x43c/0x450 drivers/gpu/drm/drm_ioc32.c:865
>   drm_compat_ioctl+0x3f6/0x590 drivers/gpu/drm/drm_ioc32.c:995
>   __do_compat_sys_ioctl fs/ioctl.c:842 [inline]
>   __se_compat_sys_ioctl+0x53d/0x1100 fs/ioctl.c:793
>   __ia32_compat_sys_ioctl+0x4a/0x70 fs/ioctl.c:793
>   do_syscall_32_irqs_on arch/x86/entry/common.c:79 [inline]
>   __do_fast_syscall_32+0x102/0x160 arch/x86/entry/common.c:141
>   do_fast_syscall_32+0x6a/0xc0 arch/x86/entry/common.c:166
>   do_SYSENTER_32+0x73/0x90 arch/x86/entry/common.c:209
>   entry_SYSENTER_compat_after_hwframe+0x4d/0x5c
> 
> Local variable ----req@compat_drm_wait_vblank created at:
>   compat_drm_wait_vblank+0x7b/0x450 drivers/gpu/drm/drm_ioc32.c:849
>   compat_drm_wait_vblank+0x7b/0x450 drivers/gpu/drm/drm_ioc32.c:849
> 
> Bytes 12-15 of 16 are uninitialized
> Memory access of size 16 starts at ffff88814ffe3c98
> Data copied to user address 0000000020000100
> =====================================================

compat_drm_wait_vblank would need to initialize

	req.reply.tval_usec = req32.reply.tval_usec;

before calling drm_ioctl_kernel, since it's not aliased by any req.request.* member, and drm_wait_vblank_ioctl doesn't always write to it.


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer