Re: NULL pointer deref in put_fs_context with unprivileged LXC
From: Christian Brauner
Date: Fri Oct 11 2019 - 10:14:11 EST
On Thu, Oct 10, 2019 at 11:35:12PM +0200, Thibaut Sautereau wrote:
> Since v5.1 and as of v5.3.5, I get the following oops every single time
> I start an *unprivileged* LXC container:
>
> BUG: kernel NULL pointer dereference, address: 0000000000000043
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 0 P4D 0
> Oops: 0000 [#1] SMP PTI
> CPU: 3 PID: 3789 Comm: systemd Tainted: G T 5.3.5 #5
> RIP: 0010:put_fs_context+0x13/0x180
> Code: e4 31 c9 eb c8 e8 1d d6 dc ff 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 41 54 55 48 89 fd 53 48 8b b>
> RSP: 0018:ffffc90000777e10 EFLAGS: 00010286
> RAX: 00000000fffffff3 RBX: 0000000000000000 RCX: ffffc90000777d6c
> RDX: 0000000000000000 RSI: ffff8884062331e8 RDI: fffffffffffffff3
> RBP: ffff8883e772dc00 R08: ffff88840d6bc680 R09: 0000000000000001
> R10: 0000000000000000 R11: 0000000000000001 R12: fffffffffffffff3
> R13: ffff888405ad2860 R14: ffff8883e772dc00 R15: 0000000000000027
> FS: 00007998d1444980(0000) GS:ffff88840f980000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000043 CR3: 000000040d236003 CR4: 00000000001606e0
> Call Trace:
> do_mount+0x2f6/0xab0
> ksys_mount+0x79/0xc0
> __x64_sys_mount+0x1d/0x30
> do_syscall_64+0x68/0x666
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x7998d23aafea
> Code: 48 8b 0d a9 0e 0c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 4>
> RSP: 002b:00007ffd4b0c8bc8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
> RAX: ffffffffffffffda RBX: 00005ae352a55a30 RCX: 00007998d23aafea
> RDX: 00005ae3529fe0b3 RSI: 00005ae3529fe0d5 RDI: 00005ae3529fe0b3
> RBP: 0000000000000007 R08: 00005ae3529fe0ca R09: 00005ae35433fb20
> R10: 000000000000000e R11: 0000000000000246 R12: 00000000fffffffe
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000001
> Modules linked in:
> CR2: 0000000000000043
> ---[ end trace 66de701522a6be46 ]---
> RIP: 0010:put_fs_context+0x13/0x180
> Code: e4 31 c9 eb c8 e8 1d d6 dc ff 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 41 54 55 48 89 fd 53 48 8b b>
> RSP: 0018:ffffc90000777e10 EFLAGS: 00010286
> RAX: 00000000fffffff3 RBX: 0000000000000000 RCX: ffffc90000777d6c
> RDX: 0000000000000000 RSI: ffff8884062331e8 RDI: fffffffffffffff3
> RBP: ffff8883e772dc00 R08: ffff88840d6bc680 R09: 0000000000000001
> R10: 0000000000000000 R11: 0000000000000001 R12: fffffffffffffff3
> R13: ffff888405ad2860 R14: ffff8883e772dc00 R15: 0000000000000027
> FS: 00007998d1444980(0000) GS:ffff88840f980000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000043 CR3: 000000040d236003 CR4: 00000000001606e0
>
> According to GDB:
> $ gdb fs/fs_context.o
> (gdb) l *put_fs_context+0x13
> 0xa53 is in put_fs_context (fs/fs_context.c:494).
> 489 void put_fs_context(struct fs_context *fc)
> 490 {
> 491 struct super_block *sb;
> 492
> 493 if (fc->root) {
> 494 sb = fc->root->d_sb;
> 495 dput(fc->root);
> 496 fc->root = NULL;
> 497 deactivate_super(sb);
> 498 }
>
> $ gdb fs/namespace.o
> (gdb) l *do_mount+0x2f6
> 0x5506 is in do_mount (fs/namespace.c:2796).
> 2791 err = vfs_get_tree(fc);
> 2792 if (!err)
> 2793 err = do_new_mount_fc(fc, path, mnt_flags);
> 2794
> 2795 put_fs_context(fc);
> 2796 return err;
> 2797 }
> 2798
> 2799 int finish_automount(struct vfsmount *m, struct path *path)
> 2800 {
>
>
> I don't face this issue when starting the same container as a
> privileged one. I tried to strace the container when launched in
> foreground and the following snippet may be related to the problem:
>
> [pid 35813] openat(AT_FDCWD, "/sys/fs", O_RDONLY|O_CLOEXEC|O_PATH|O_DIRECTORY) = 4
> [pid 35813] name_to_handle_at(4, "cgroup", {handle_bytes=128}, 0x7ffcdf6ebac4, AT_SYMLINK_FOLLOW) = -1 EOPNOTSUPP (Operation not supported)
> [pid 35813] name_to_handle_at(4, "", {handle_bytes=128}, 0x7ffcdf6ebac4, AT_EMPTY_PATH) = -1 EOPNOTSUPP (Operation not supported)
> [pid 35813] openat(4, "cgroup", O_RDONLY|O_CLOEXEC|O_PATH) = 5
> [pid 35813] openat(AT_FDCWD, "/proc/self/fdinfo/5", O_RDONLY|O_CLOEXEC) = 6
> [pid 35813] fstat(6, {st_mode=S_IFREG|0400, st_size=0, ...}) = 0
> [pid 35813] fstat(6, {st_mode=S_IFREG|0400, st_size=0, ...}) = 0
> [pid 35813] read(6, "pos:\t0\nflags:\t012000000\nmnt_id:\t"..., 2048) = 36
> [pid 35813] read(6, "", 1024) = 0
> [pid 35813] close(6) = 0
> [pid 35813] close(5) = 0
> [pid 35813] openat(AT_FDCWD, "/proc/self/fdinfo/4", O_RDONLY|O_CLOEXEC) = 5
> [pid 35813] fstat(5, {st_mode=S_IFREG|0400, st_size=0, ...}) = 0
> [pid 35813] fstat(5, {st_mode=S_IFREG|0400, st_size=0, ...}) = 0
> [pid 35813] read(5, "pos:\t0\nflags:\t012200000\nmnt_id:\t"..., 2048) = 36
> [pid 35813] read(5, "", 1024) = 0
> [pid 35813] close(5) = 0
> [pid 35813] newfstatat(4, "cgroup", {st_mode=S_IFDIR|0555, st_size=0, ...}, 0) = 0
> [pid 35813] newfstatat(4, "", {st_mode=S_IFDIR|0755, st_size=0, ...}, AT_EMPTY_PATH) = 0
> [pid 35813] close(4) = 0
> [pid 35813] stat("/sys/fs", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
> [pid 35813] mkdir("/sys/fs/cgroup", 0755) = -1 EEXIST (File exists)
> [pid 35813] stat("/sys/fs/cgroup", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
> [pid 35813] mount("tmpfs", "/sys/fs/cgroup", "tmpfs", MS_NOSUID|MS_NODEV|MS_NOEXEC|MS_STRICTATIME, "mode=755") = 0
> [pid 35813] statfs("/sys/fs/cgroup/", {f_type=TMPFS_MAGIC, f_bsize=4096, f_blocks=2032290, f_bfree=2032290, f_bavail=2032290, f_files=2032290, f_ffree=2032289, f_fsid={val=[0, 0]}, f_namelen=255, f_frsize=4096, f_flags=ST_VALID|ST_NOSUID|ST_NODEV|ST_NOEXEC}) = 0
> [pid 35813] statfs("/sys/fs/cgroup/unified/", 0x7ffcdf6ebc10) = -1 ENOENT (No such file or directory)
> [pid 35813] statfs("/sys/fs/cgroup/systemd/", 0x7ffcdf6ebc10) = -1 ENOENT (No such file or directory)
> [pid 35813] openat(AT_FDCWD, "/proc/1/cmdline", O_RDONLY|O_CLOEXEC) = 4
> [pid 35813] prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0
> [pid 35813] mmap(NULL, 2101248, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7999ced5b000
> [pid 35813] fstat(4, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
> [pid 35813] read(4, "/sbin/init\0", 1024) = 11
> [pid 35813] read(4, "", 1024) = 0
> [pid 35813] mremap(0x7999ced5b000, 2101248, 4096, MREMAP_MAYMOVE) = 0x7999ced5b000
> [pid 35813] close(4) = 0
> [pid 35813] munmap(0x7999ced5b000, 4096) = 0
> [pid 35813] openat(AT_FDCWD, "/", O_RDONLY|O_NOFOLLOW|O_CLOEXEC|O_PATH) = 4
> [pid 35813] openat(4, "sys", O_RDONLY|O_NOFOLLOW|O_CLOEXEC|O_PATH) = 5
> [pid 35813] fstat(5, {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
> [pid 35813] close(4) = 0
> [pid 35813] openat(5, "fs", O_RDONLY|O_NOFOLLOW|O_CLOEXEC|O_PATH) = 4
> [pid 35813] fstat(4, {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
> [pid 35813] close(5) = 0
> [pid 35813] openat(4, "cgroup", O_RDONLY|O_NOFOLLOW|O_CLOEXEC|O_PATH) = 5
> [pid 35813] fstat(5, {st_mode=S_IFDIR|0755, st_size=40, ...}) = 0
> [pid 35813] close(4) = 0
> [pid 35813] openat(5, "unified", O_RDONLY|O_NOFOLLOW|O_CLOEXEC|O_PATH) = -1 ENOENT (No such file or directory)
> [pid 35813] close(5) = 0
> [pid 35813] stat("/sys/fs/cgroup", {st_mode=S_IFDIR|0755, st_size=40, ...}) = 0
> [pid 35813] mkdir("/sys/fs/cgroup/unified", 0755) = 0
> [pid 35813] mount("cgroup2", "/sys/fs/cgroup/unified", "cgroup2", MS_NOSUID|MS_NODEV|MS_NOEXEC, "nsdelegate") = ?
> [pid 35813] +++ killed by SIGKILL +++
>
> I've been trying to reproduce by playing with user namespaces and
> cgroup2 mounts but I didn't succeed. Only an lxc-start of an
> unprivileged container causes an oops (every single time). I wanted to
> dive into the code but I hadn't looked at this part of the kernel since
> the recent rework of file system mounting internals, thus I've been
> postponing that for weeks now and I thought it was time to report the
> bug anyway. Sorry for the lack of more detailed info :/
No worries.
Let's add David to this.
Thanks
Christian