On Wed, Nov 14, 2018 at 08:52:46AM -0800, 'Dmitry Vyukov' via syzkaller-bugs wrote: > On Wed, Nov 14, 2018 at 4:20 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote: > > Hey > > > > On Wed, Nov 14, 2018 at 1:25 AM syzbot > > <syzbot+72473edc9bf4eb1c6556@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > >> syzbot has found a reproducer for the following crash on: > >> > >> HEAD commit: ccda4af0f4b9 Linux 4.20-rc2 > >> git tree: upstream > >> console output: https://syzkaller.appspot.com/x/log.txt?x=13b4e77b400000 > >> kernel config: https://syzkaller.appspot.com/x/.config?x=4a0a89f12ca9b0f5 > >> dashboard link: https://syzkaller.appspot.com/bug?extid=72473edc9bf4eb1c6556 > >> compiler: gcc (GCC) 8.0.1 20180413 (experimental) > >> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1646a225400000 > >> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=108a6533400000 > >> > >> IMPORTANT: if you fix the bug, please add the following tag to the commit: > >> Reported-by: syzbot+72473edc9bf4eb1c6556@xxxxxxxxxxxxxxxxxxxxxxxxx > >> > > [...] > >> BUG: GPF in non-whitelisted uaccess (non-canonical address?) > > > > This uses sendpage(2) to feed data from a file into a uhid chardev. > > The default behavior of the kernel is to create a temporary pipe, then > > splice from the file into the pipe, and then splice again from the > > pipe into uhid. > > > > The kernel provides default implementations for splicing between files > > and any other file. The default implementation of `.splice_write()` > > uses kmap() to map the page from the pipe and then uses the > > __kernel_write() (which uses .f_op->write()) to push the data into the > > target file. The problem is, __kernel_write() sets the address-space > > to KERNEL_DS `set_fs(get_ds())`, thus granting the UHID request access > > to kernel memory. > > > > I see several ways to fix that, the most simple solution is to simply > > prevent splice/sendpage on uhid (by setting f_op.splice_write to a > > dummy). Alternatively, we can implement a proper splice helper that > > takes the page directly, rather than through the __kernel_write() > > default implementation. > > also +dtor for uhid > Well, the problem is that uhid_char_write() reads from a user pointer embedded in the write() payload. (Which really is abusing write(), but I assume it cannot be changed at this point...) Thus it's unsafe to be called under KERNEL_DS. So it needs: if (uaccess_kernel()) return -EACCES; See sg_check_file_access(), called from sg_read() and sg_write(), for another example of this in the kernel. - Eric
