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

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Message-ID: <cb6d724f-7afb-44c6-835c-6c8c229b70a0@I-love.SAKURA.ne.jp>
Date: Fri, 5 Apr 2024 23:02:05 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: syzbot <syzbot+cb76c2983557a07cdb14@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Kees Cook <keescook@...omium.org>
Subject: Re: [syzbot] [hardening?] [mm?] BUG: bad usercopy in fpa_set

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index c421a899fc84..347611ae762f 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -583,10 +583,15 @@ static int fpa_set(struct task_struct *target,
 		   const void *kbuf, const void __user *ubuf)
 {
 	struct thread_info *thread = task_thread_info(target);
+	const unsigned int pos0 = pos;
+	char buf[sizeof(struct user_fp)];
+	int ret;
 
-	return user_regset_copyin(&pos, &count, &kbuf, &ubuf,
-		&thread->fpstate,
-		0, sizeof(struct user_fp));
+	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
+				 buf, 0, sizeof(struct user_fp));
+	if (!ret)
+		memcpy(&thread->fpstate, buf, pos - pos0);
+	return ret;
 }
 
 #ifdef CONFIG_VFP