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Date:   Sat, 7 Mar 2020 15:54:37 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     glider@...gle.com
Cc:     syzbot <syzbot+af962bf9e7e27bccd025@...kaller.appspotmail.com>,
        len.brown@...el.com, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, pavel@....cz, rjw@...ysocki.net,
        syzkaller-bugs@...glegroups.com
Subject: Re: KMSAN: uninit-value in snapshot_compat_ioctl

On Wed, Feb 26, 2020 at 07:59:13AM -0800, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    8bbbc5cf kmsan: don't compile memmove
> git tree:       https://github.com/google/kmsan.git master
> console output: https://syzkaller.appspot.com/x/log.txt?x=11514265e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=cd0e9a6b0e555cc3
> dashboard link: https://syzkaller.appspot.com/bug?extid=af962bf9e7e27bccd025
> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> userspace arch: i386
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16a89109e00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=176f774ee00000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+af962bf9e7e27bccd025@...kaller.appspotmail.com
> 
> =====================================================
> BUG: KMSAN: uninit-value in kmsan_check_memory+0xd/0x10 mm/kmsan/kmsan_hooks.c:413
> CPU: 1 PID: 11659 Comm: syz-executor923 Not tainted 5.6.0-rc2-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x1c9/0x220 lib/dump_stack.c:118
>  kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:118
>  kmsan_internal_check_memory+0x358/0x3d0 mm/kmsan/kmsan.c:457
>  kmsan_check_memory+0xd/0x10 mm/kmsan/kmsan_hooks.c:413
>  snapshot_compat_ioctl+0x559/0x650 kernel/power/user.c:422
>  __do_compat_sys_ioctl fs/ioctl.c:857 [inline]
>  __se_compat_sys_ioctl+0x57c/0xed0 fs/ioctl.c:808
>  __ia32_compat_sys_ioctl+0xd9/0x110 fs/ioctl.c:808
>  do_syscall_32_irqs_on arch/x86/entry/common.c:339 [inline]
>  do_fast_syscall_32+0x3c7/0x6e0 arch/x86/entry/common.c:410
>  entry_SYSENTER_compat+0x68/0x77 arch/x86/entry/entry_64_compat.S:139
> RIP: 0023:0xf7f70d99
> Code: 90 e8 0b 00 00 00 f3 90 0f ae e8 eb f9 8d 74 26 00 89 3c 24 c3 90 90 90 90 90 90 90 90 90 90 90 90 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
> RSP: 002b:00000000ffec145c EFLAGS: 00000213 ORIG_RAX: 0000000000000036
> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000080083313
> RDX: 0000000000000000 RSI: 00000000080ea078 RDI: 00000000ffec14b0
> 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:144 [inline]
>  kmsan_internal_chain_origin+0xad/0x130 mm/kmsan/kmsan.c:310
>  __msan_chain_origin+0x50/0x90 mm/kmsan/kmsan_instr.c:165
>  snapshot_compat_ioctl+0x5e0/0x650 kernel/power/user.c:422
>  __do_compat_sys_ioctl fs/ioctl.c:857 [inline]
>  __se_compat_sys_ioctl+0x57c/0xed0 fs/ioctl.c:808
>  __ia32_compat_sys_ioctl+0xd9/0x110 fs/ioctl.c:808
>  do_syscall_32_irqs_on arch/x86/entry/common.c:339 [inline]
>  do_fast_syscall_32+0x3c7/0x6e0 arch/x86/entry/common.c:410
>  entry_SYSENTER_compat+0x68/0x77 arch/x86/entry/entry_64_compat.S:139
> 
> Local variable ----offset@...pshot_compat_ioctl created at:
>  get_current arch/x86/include/asm/current.h:15 [inline]
>  snapshot_compat_ioctl+0x324/0x650 kernel/power/user.c:418
>  get_current arch/x86/include/asm/current.h:15 [inline]
>  snapshot_compat_ioctl+0x324/0x650 kernel/power/user.c:418
> 
> Bytes 0-7 of 8 are uninitialized
> Memory access of size 8 starts at ffff9946c156bd30
> =====================================================

Looks like a KMSAN false positive?  As far as I can tell, the memory is being
initialized by put_user() called under set_fs(KERNEL_DS).

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