linux-kernel - Re: [syzbot] [hardening?] [mm?] BUG: bad usercopy in fpa

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Message-ID: <0b49d91b-511f-449e-b7c3-93b2ccce6c49@I-love.SAKURA.ne.jp>
Date: Fri, 5 Apr 2024 23:28:20 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: "Russell King (Oracle)" <linux@...linux.org.uk>,
        Kees Cook <keescook@...omium.org>
Cc: Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        syzbot <syzbot+cb76c2983557a07cdb14@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [hardening?] [mm?] BUG: bad usercopy in fpa_set

On 2024/04/04 1:12, Russell King (Oracle) wrote:
> Therefore, there is _no way_ for fpa_set() to overwrite anything
> outside of thread_info->fpstate, because sizeof(struct user_fp)
> is smaller than sizeof(thread->fpstate).
> 
> Syzbot appears to be wrong in this instance.
> 

Thanks for clarification.

I came to suspect that commit 08626a6056aa ("arm: Implement thread_struct
whitelist for hardened usercopy") missed that ptrace(PTRACE_SETFPREGS)
needs to declare a usercopy whitelist. It seems to me that
https://syzkaller.appspot.com/text?tag=Patch&x=14c42099180000 can fix
this problem, but I'm not sure whether this is safe/correct. Can you check?