Re: NULL pointer deref in put_fs_context with unprivileged LXC

From: Thibaut Sautereau
Date: Tue Nov 05 2019 - 15:58:38 EST

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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

Next message: Lina Iyer: "Re: [PATCH RFC v2 06/14] dt-bindings/interrupt-controller: pdc: add SPI config register"
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