• Eric Biggers's avatar
    vfs: set fs_context::user_ns for reconfigure · 1dd9bc08
    Eric Biggers authored
    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@syzkaller.appspotmail.com
    Fixes: 408cbe695350 ("vfs: Convert fuse to use the new mount API")
    Cc: David Howells <dhowells@redhat.com>
    Cc: Miklos Szeredi <miklos@szeredi.hu>
    Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
    Reviewed-by: default avatarDavid Howells <dhowells@redhat.com>
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    1dd9bc08
fs_context.c 18.5 KB