On Fri, Aug 30, 2019 at 10:12:28PM -0500, Eric Biggers wrote:
> On Wed, Aug 21, 2019 at 10:16:33PM -0700, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@xxxxxxxxxx>
> >
> > fs_context::user_ns is used by fuse_parse_param(), even during remount,
> > so it needs to be set to the existing value for reconfigure.
> >
> > Reproducer:
> >
> > #include <fcntl.h>
> > #include <sys/mount.h>
> >
> > int main()
> > {
> > char opts[128];
> > int fd = open("/dev/fuse", O_RDWR);
> >
> > sprintf(opts, "fd=%d,rootmode=040000,user_id=0,group_id=0", fd);
> > mkdir("mnt", 0777);
> > mount("foo", "mnt", "fuse.foo", 0, opts);
> > mount("foo", "mnt", "fuse.foo", MS_REMOUNT, opts);
> > }
> >
> > Crash:
> > BUG: kernel NULL pointer dereference, address: 0000000000000000
> > #PF: supervisor read access in kernel mode
> > #PF: error_code(0x0000) - not-present page
> > PGD 0 P4D 0
> > Oops: 0000 [#1] SMP
> > CPU: 0 PID: 129 Comm: syz_make_kuid Not tainted 5.3.0-rc5-next-20190821 #3
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-20181126_142135-anatol 04/01/2014
> > RIP: 0010:map_id_range_down+0xb/0xc0 kernel/user_namespace.c:291
> > [...]
> > Call Trace:
> > map_id_down kernel/user_namespace.c:312 [inline]
> > make_kuid+0xe/0x10 kernel/user_namespace.c:389
> > fuse_parse_param+0x116/0x210 fs/fuse/inode.c:523
> > vfs_parse_fs_param+0xdb/0x1b0 fs/fs_context.c:145
> > vfs_parse_fs_string+0x6a/0xa0 fs/fs_context.c:188
> > generic_parse_monolithic+0x85/0xc0 fs/fs_context.c:228
> > parse_monolithic_mount_data+0x1b/0x20 fs/fs_context.c:708
> > do_remount fs/namespace.c:2525 [inline]
> > do_mount+0x39a/0xa60 fs/namespace.c:3107
> > ksys_mount+0x7d/0xd0 fs/namespace.c:3325
> > __do_sys_mount fs/namespace.c:3339 [inline]
> > __se_sys_mount fs/namespace.c:3336 [inline]
> > __x64_sys_mount+0x20/0x30 fs/namespace.c:3336
> > do_syscall_64+0x4a/0x1a0 arch/x86/entry/common.c:290
> > entry_SYSCALL_64_after_hwframe+0x49/0xbe
> >
> > Reported-by: syzbot+7d6a57304857423318a5@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: 408cbe695350 ("vfs: Convert fuse to use the new mount API")
> > Cc: David Howells <dhowells@xxxxxxxxxx>
> > Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
> > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> > ---
> > fs/fs_context.c | 4 +---
> > 1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/fs/fs_context.c b/fs/fs_context.c
> > index cc61d305dc4b..44c4174b250a 100644
> > --- a/fs/fs_context.c
> > +++ b/fs/fs_context.c
> > @@ -279,10 +279,8 @@ static struct fs_context *alloc_fs_context(struct file_system_type *fs_type,
> > fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);
> > break;
> > case FS_CONTEXT_FOR_RECONFIGURE:
> > - /* We don't pin any namespaces as the superblock's
> > - * subscriptions cannot be changed at this point.
> > - */
> > atomic_inc(&reference->d_sb->s_active);
> > + fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);
> > fc->root = dget(reference);
> > break;
> > }
> > --
> > 2.22.1
>
> Ping.
>
Ping. This is still broken in linux-next.
- Eric
