Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail

[Ben Dooks]

On 10/03/2021 17:16, Dmitry Vyukov wrote:
> On Wed, Mar 10, 2021 at 5:46 PM syzbot
> <syzbot+e74b94fe601ab9552d69@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    0d7588ab riscv: process: Fix no prototype for arch_dup_tas..
> > git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git fixes
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1212c6e6d00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=e3c595255fb2d136
> > dashboard link: https://syzkaller.appspot.com/bug?extid=e74b94fe601ab9552d69
> > userspace arch: riscv64
> >
> > 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+e74b94fe601ab9552d69@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> +riscv maintainers
>
> This is riscv64-specific.
> I've seen similar crashes in put_user in other places. It looks like
> put_user crashes in the user address is not mapped/protected (?).

I've been having a look, and this seems to be down to access of the
tsk->set_child_tid variable. I assume the fuzzing here is to pass a
bad address to clone?

From looking at the code, the put_user() code should have set the
relevant SR_SUM bit (the value for this, which is 1<<18 is in the
s2 register in the crash report) and from looking at the compiler
output from my gcc-10, the code looks to be dong the relevant csrs
and then csrc around the put_user

So currently I do not understand how the above could have happened
over than something re-tried the code seqeunce and ended up retrying
the faulting instruction without the SR_SUM bit set.

--
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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