syzbot reported a null-ptr-deref in pick_link. [1]
First, i_link and i_dir_seq are in the same union, they share the same memory
address, and i_dir_seq will be updated during the execution of walk_component,
which makes the value of i_link equal to i_dir_seq.
Secondly, the chmod execution failed, which resulted in setting the mode value
of file0's inode to REG when executing ntfs_bad_inode.
Third, during the execution of the link syscall, two things are done in
d_instantiate. One is to set the bad inode to the dentry of the bus. More
importantly, the symlink flag obtained in d_flags_for_inode(bad inode) is used
to update the d_flags of the dentry of the bus through __d_set_inode_and_type,
which will cause the dentry flag of the created bus to have symlink. The bus
should have been a hardlink, but it eventually became a symlink.
These two points constitute the root cause.
When executing the mount syscall, in walk_component(), when executing to
step_into() and using d_is_symlink() to judge, because the flag of the dentry
of the bus is symlink, it leads to the wrong process, and trigger null-ptr-deref
in pick_link().
Note: ("file0, bus" are defined in reproducer [2])
To avoid null-ptr-deref in pick_link, when creating a hardlink, first check
whether the inode of file is already bad.
[1]
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 0 UID: 0 PID: 5310 Comm: syz-executor255 Not tainted 6.12.0-rc6-syzkaller-00318-ga9cda7c0ffed #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
RIP: 0010:pick_link+0x51c/0xd50 fs/namei.c:1864
Code: c1 e8 03 42 80 3c 38 00 74 08 48 89 df e8 fc 00 e9 ff 48 8b 2b 48 85 ed 0f 84 92 00 00 00 e8 7b 36 7f ff 48 89 e8 48 c1 e8 03 <42> 0f b6 04 38 84 c0 0f 85 a2 05 00 00 0f b6 5d 00 bf 2f 00 00 00
RSP: 0018:ffffc9000d147998 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff88804558dec8 RCX: ffff88801ec7a440
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000002 R08: ffffffff8215a35f R09: 1ffffffff203a13d
R10: dffffc0000000000 R11: fffffbfff203a13e R12: 1ffff92001a28f93
R13: ffffc9000d147af8 R14: 1ffff92001a28f5f R15: dffffc0000000000
FS: 0000555577611380(0000) GS:ffff88801fc00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fcc0a595ed8 CR3: 0000000035760000 CR4: 0000000000352ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
step_into+0xca9/0x1080 fs/namei.c:1923
lookup_last fs/namei.c:2556 [inline]
path_lookupat+0x16f/0x450 fs/namei.c:2580
filename_lookup+0x256/0x610 fs/namei.c:2609
user_path_at+0x3a/0x60 fs/namei.c:3016
do_mount fs/namespace.c:3844 [inline]
__do_sys_mount fs/namespace.c:4057 [inline]
__se_sys_mount+0x297/0x3c0 fs/namespace.c:4034
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f4b18ad5b19
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 f1 17 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc2e486c48 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0030656c69662f2e RCX: 00007f4b18ad5b19
RDX: 0000000000000000 RSI: 00000000200000c0 RDI: 0000000000000000
RBP: 00007f4b18b685f0 R08: 0000000000000000 R09: 00005555776124c0
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffc2e486c70
R13: 00007ffc2e486e98 R14: 431bde82d7b634db R15: 00007f4b18b1e03b
</TASK>
[2]
move_mount(0xffffffffffffff9c, &(0x7f00000003c0)='./file0\x00', 0xffffffffffffff9c, &(0x7f0000000400)='./file0/file0\x00', 0x140)
chmod(&(0x7f0000000080)='./file0\x00', 0x0)
link(&(0x7f0000000200)='./file0\x00', &(0x7f0000000240)='./bus\x00')
mount$overlay(0x0, &(0x7f00000000c0)='./bus\x00', 0x0, 0x0, 0x0)
Reported-by: syzbot+73d8fc29ec7cba8286fa@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=73d8fc29ec7cba8286fa
Signed-off-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
---
V1 --> V2: add the root cause of the i_link not set issue and imporve the check
V2 --> V3: when creating a symbolic link, first check whether the inode of file is bad.
V3 --> V4: add comments for symlink use bad inode, it is the root cause
V4 --> V5: update comments for link command, it is creating hardlink
V5 --> V6: add comments for link syscall return a symlink and update subject
fs/ntfs3/inode.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/fs/ntfs3/inode.c b/fs/ntfs3/inode.c
index be04d2845bb7..fefbdcf75016 100644
--- a/fs/ntfs3/inode.c
+++ b/fs/ntfs3/inode.c
@@ -1719,6 +1719,9 @@ int ntfs_link_inode(struct inode *inode, struct dentry *dentry)
struct ntfs_sb_info *sbi = inode->i_sb->s_fs_info;
struct NTFS_DE *de;
+ if (is_bad_inode(inode))
+ return -EIO;
+
/* Allocate PATH_MAX bytes. */
de = __getname();
if (!de)
--
2.43.0
