linux-kernel - Re: general protection fault in override

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Message-ID: <39ea5e00-3278-a84f-25ae-c7a93a4b8ab5@kernel.dk>
Date:   Mon, 2 Dec 2019 08:40:45 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+5320383e16029ba057ff@...kaller.appspotmail.com>,
        Al Viro <viro@...iv.linux.org.uk>, io-uring@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Cc:     Anna.Schumaker@...app.com,
        Casey Schaufler <casey@...aufler-ca.com>,
        David Howells <dhowells@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>, NeilBrown <neilb@...e.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: general protection fault in override_creds

On 12/1/19 10:40 PM, Dmitry Vyukov wrote:
> On Mon, Dec 2, 2019 at 7:35 AM syzbot
> <syzbot+5320383e16029ba057ff@...kaller.appspotmail.com> wrote:
>>
>> Hello,
>>
>> syzbot found the following crash on:
>>
>> HEAD commit:    b94ae8ad Merge tag 'seccomp-v5.5-rc1' of git://git.kernel...
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=10f9ffcee00000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=ff560c3de405258c
>> dashboard link: https://syzkaller.appspot.com/bug?extid=5320383e16029ba057ff
>> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12dd682ae00000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16290abce00000
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+5320383e16029ba057ff@...kaller.appspotmail.com
> 
> I think this relates to fs/io_uring.c rather than kernel/cred.c.
> +io_uring maintainers

Yeah that's my fault, guessing the below will fix it. I'll test and
queue it up.


diff --git a/fs/io_uring.c b/fs/io_uring.c
index ec53aa7cdc94..afafe0463d2d 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4361,7 +4361,8 @@ static void io_ring_ctx_free(struct io_ring_ctx *ctx)
  		io_unaccount_mem(ctx->user,
  				ring_pages(ctx->sq_entries, ctx->cq_entries));
  	free_uid(ctx->user);
-	put_cred(ctx->creds);
+	if (ctx->creds)
+		put_cred(ctx->creds);
  	kfree(ctx->completions);
  	kmem_cache_free(req_cachep, ctx->fallback_req);
  	kfree(ctx);
@@ -4759,7 +4760,12 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p)
  	ctx->compat = in_compat_syscall();
  	ctx->account_mem = account_mem;
  	ctx->user = user;
+
  	ctx->creds = prepare_creds();
+	if (!ctx->creds) {
+		ret = -ENOMEM;
+		goto err;
+	}
  
  	ret = io_allocate_scq_urings(ctx, p);
  	if (ret)

-- 
Jens Axboe