On Fri, Oct 11, 2019 at 04:14:05PM +0200, Christian Brauner wrote: > 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 Thanks. Has anyone been able to reproduce? Using the attached patch, I got the following logs: fs_context: DEBUG: fc = ffff8883c50e3180 fs_context: DEBUG: fc->root = ffff888364552580 fs_context: DEBUG: fc->source = none fs_context: DEBUG: fc->root->d_sb = ffff88837ce95500 fs_context: DEBUG: fc = ffff8883c50e2f00 fs_context: DEBUG: fc->root = ffff8883cb8d06e0 fs_context: DEBUG: fc->source = proc fs_context: DEBUG: fc->root->d_sb = ffff88837f01a200 fs_context: DEBUG: fc = ffff8883c50e3540 fs_context: DEBUG: fc->root = ffff8883645534a0 fs_context: DEBUG: fc->source = sysfs fs_context: DEBUG: fc->root->d_sb = ffff88837ce90000 fs_context: DEBUG: fc = ffff8883c50e2640 fs_context: DEBUG: fc->root = ffff88833cbf94a0 fs_context: DEBUG: fc->source = devpts fs_context: DEBUG: fc->root->d_sb = ffff88837ea61100 fs_context: DEBUG: fc = ffff88837c0b7e00 fs_context: DEBUG: fc->root = ffff888342247b80 fs_context: DEBUG: fc->source = tmpfs fs_context: DEBUG: fc->root->d_sb = ffff88837eb8a200 fs_context: DEBUG: fc = ffff88837c0b7680 fs_context: DEBUG: fc->root = ffff888342247080 fs_context: DEBUG: fc->source = tmpfs fs_context: DEBUG: fc->root->d_sb = ffff88837c500000 fs_context: DEBUG: fc = ffff88837c0b7b80 fs_context: DEBUG: fc->root = ffff888342246160 fs_context: DEBUG: fc->source = tmpfs fs_context: DEBUG: fc->root->d_sb = ffff88837c51b300 fs_context: DEBUG: fc = ffff88837c0b72c0 fs_context: DEBUG: fc->root = ffff888342247a20 fs_context: DEBUG: fc->source = tmpfs fs_context: DEBUG: fc->root->d_sb = ffff88837c361100 fs_context: DEBUG: fc = ffff88837c0b7900 fs_context: DEBUG: fc->root = fffffffffffffff3 fs_context: DEBUG: fc->source = cgroup2 kasan: CONFIG_KASAN_INLINE enabled kasan: GPF could be caused by NULL-ptr deref or user memory access general protection fault: 0000 [#2] SMP KASAN PTI CPU: 1 PID: 3959 Comm: systemd Tainted: G D T 5.3.8+ #12 Hardware name: Gigabyte Technology Co., Ltd. Z97P-D3/Z97P-D3, BIOS F5 05/30/2014 RIP: 0010:put_fs_context.cold+0xa9/0x179 Code: 03 48 c1 e0 2a 80 3c 02 00 0f 85 b9 00 00 00 48 8b 9d 98 00 00 00 b8 ff ff 37 00 48 c1 e0 2a 48 8d 7b 50 48 89 fa 48 c1 ea 03 <80> 3c 02 00 74 05 e8 d5 c1 ef ff 48 8b 73 50 48 c7 c7 e0 35 50 83 RSP: 0018:ffff88837f18fcb8 EFLAGS: 00010202 RAX: dffffc0000000000 RBX: fffffffffffffff3 RCX: 0000000000000000 RDX: 0000000000000008 RSI: ffffed107a015ee2 RDI: 0000000000000043 RBP: ffff88837c0b7900 R08: 0000000000000027 R09: ffffed107a015ee2 R10: ffffed107a015ee3 R11: 0000000000000000 R12: ffff88837c0b7998 R13: ffff88837c0b79c0 R14: ffff88837c0b7900 R15: ffff88837f18fd68 FS: 0000741e8147a980(0000) GS:ffff8883d0080000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000741e81279000 CR3: 00000003c1bd0002 CR4: 00000000001606e0 Call Trace: do_mount+0x739/0x1660 ? copy_mount_string+0x20/0x20 ? __kasan_kmalloc.constprop.0+0xc3/0xd0 ? __kasan_kmalloc.constprop.0+0xc3/0xd0 ksys_mount+0x79/0xc0 __x64_sys_mount+0xb5/0x150 do_syscall_64+0xe7/0x1287 ? syscall_return_slowpath+0x8f0/0x8f0 ? trace_hardirqs_off_thunk+0x1a/0x20 entry_SYSCALL_64_after_hwframe+0x49/0xbe RIP: 0033:0x741e823e0fea 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 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 76 0e 0c 00 f7 d8 64 89 01 48 RSP: 002b:00007ffe0e738948 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5 RAX: ffffffffffffffda RBX: 000057eea0dd9a30 RCX: 0000741e823e0fea RDX: 000057eea0d820b3 RSI: 000057eea0d820d5 RDI: 000057eea0d820b3 RBP: 0000000000000007 R08: 000057eea0d820ca R09: 000057eea1dd3b20 R10: 000000000000000e R11: 0000000000000246 R12: 00000000fffffffe R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000001 Modules linked in: ---[ end trace 914fc1428888c43a ]--- RIP: 0010:put_fs_context.cold+0xa9/0x179 Code: 03 48 c1 e0 2a 80 3c 02 00 0f 85 b9 00 00 00 48 8b 9d 98 00 00 00 b8 ff ff 37 00 48 c1 e0 2a 48 8d 7b 50 48 89 fa 48 c1 ea 03 <80> 3c 02 00 74 05 e8 d5 c1 ef ff 48 8b 73 50 48 c7 c7 e0 35 50 83 RSP: 0018:ffff88837f17fcb8 EFLAGS: 00010202 RAX: dffffc0000000000 RBX: fffffffffffffff3 RCX: 0000000000000000 RDX: 0000000000000008 RSI: ffffed107a015ee2 RDI: 0000000000000043 RBP: ffff88833bcd03c0 R08: 0000000000000027 R09: ffffed107a015ee2 R10: ffffed107a015ee3 R11: 0000000000000000 R12: ffff88833bcd0458 R13: ffff88833bcd0480 R14: ffff88833bcd03c0 R15: ffff88837f17fd68 FS: 0000741e8147a980(0000) GS:ffff8883d0080000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000741e81279000 CR3: 00000003c1bd0002 CR4: 00000000001606e0 0xfffffffffffffff3 seems to be a weird value for fc->root. This would imply that the fc->root dereference to get fc->root->d_sb is causing the bug, and confirm that the issue is related to cgroup2 mounting. -- Thibaut Sautereau