Re: KASAN: use-after-free Read in blkdev_get

[Al Viro]

On Sun, Nov 24, 2019 at 11:07:00AM -0800, syzbot wrote:
> syzbot has bisected this bug to:
> 
> commit 77ef8f5177599efd0cedeb52c1950c1bd73fa5e3
> Author: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> Date:   Mon Jan 25 20:05:34 2016 +0000
> 
>     tile kgdb: fix bug in copy to gdb regs, and optimize memset
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1131bc0ee00000
> start commit:   f5b7769e Revert "debugfs: inode: debugfs_create_dir uses m..
> git tree:       upstream
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=1331bc0ee00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=1531bc0ee00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=709f8187af941e84
> dashboard link: https://syzkaller.appspot.com/bug?extid=eaeb616d85c9a0afec7d
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f898f800000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=147eb85f800000
> 
> Reported-by: syzbot+eaeb616d85c9a0afec7d@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 77ef8f517759 ("tile kgdb: fix bug in copy to gdb regs, and optimize
> memset")
> 
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection

Seriously?  How can the commit in question (limited to arch/tile/kernel/kgdb.c)
possibly affect a bug that manages to produce a crash report with
RSP: 0018:ffffffff82e03eb8  EFLAGS: 00000282                                    
RAX: 0000000000000000 RBX: ffffffff82e00000 RCX: 0000000000000000               
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81088779               
RBP: ffffffff82e03eb8 R08: 0000000000000000 R09: 0000000000000001               
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000               
R13: 0000000000000000 R14: 0000000000000000 R15: ffffffff82e00000               
FS:  0000000000000000(0000) GS:ffff88021fc00000(0000) knlGS:0000000000000000    
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033                               
CR2: 000000c420447ff8 CR3: 0000000213184000 CR4: 00000000001406f0               
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000               
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400               
in it?  Unless something very odd has happened to tile, this crash has
been observed on 64bit x86; the names of registers alone are enough
to be certain of that.

And the binaries produced by an x86 build should not be affected by any
changes in arch/tile; not unless something is very wrong with the build
system.  It's not even that this commit has fixed an earlier bug that
used to mask the one manifested here - it really should have had zero
impact on x86 builds, period.

So I'm sorry, but I'm calling bullshit.  Something's quite wrong with
the bot - either its build system or the bisection process.