| Message ID | 20241119112945.767118-1-lizhi.xu@windriver.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [V2] fs: improve the check of whether i_link has been set | expand |
On Tue, Nov 19, 2024 at 07:29:45PM +0800, Lizhi Xu wrote: > 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. > In this case, setting i_dir_seq is triggered by move_mount, and the calltrace > is as follows: > move_mount()-> > user_path_at()-> > filename_lookup()-> > path_lookupat()-> > lookup_last()-> > walk_component()-> > __lookup_slow()-> > ntfs_lookup()-> > d_splice_alias()-> > __d_add()-> > end_dir_add() > > In pick_link(), the simple "if (!i_link)" is used to determine whether i_link > has been set, which is not rigorous enough. > > On the other hand, the mode value of the symlink inode becomes REG because > attr_set_size() fails to set the attribute and calls ntfs_bad_inode(). > By confirming that the i_link pointer value is valid, the null-ptr-deref > problem in pick_link can be avoided. So basically your theory is that make_bad_inode() is called on a live directory inode (already reachable from dcache and remaining there), whereas the sucker somehow gets a new dentry alias which looks like a symlink. Right? NAK on the "mitigation", just in case anyone decides to pick that - no matter how we deal with the problem, sprinkling virt_addr_valid() is *NOT* a solution.
diff --git a/fs/namei.c b/fs/namei.c index 4a4a22a08ac2..49bd5ac16137 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1845,7 +1845,7 @@ static const char *pick_link(struct nameidata *nd, struct path *link, return ERR_PTR(error); res = READ_ONCE(inode->i_link); - if (!res) { + if (!virt_addr_valid(res)) { const char * (*get)(struct dentry *, struct inode *, struct delayed_call *); get = inode->i_op->get_link;
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. In this case, setting i_dir_seq is triggered by move_mount, and the calltrace is as follows: move_mount()-> user_path_at()-> filename_lookup()-> path_lookupat()-> lookup_last()-> walk_component()-> __lookup_slow()-> ntfs_lookup()-> d_splice_alias()-> __d_add()-> end_dir_add() In pick_link(), the simple "if (!i_link)" is used to determine whether i_link has been set, which is not rigorous enough. On the other hand, the mode value of the symlink inode becomes REG because attr_set_size() fails to set the attribute and calls ntfs_bad_inode(). By confirming that the i_link pointer value is valid, the null-ptr-deref problem in pick_link can be avoided. [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> Fixes: ad6cc4c338f4 ("finally fold get_link() into pick_link()") Reported-by: syzbot+73d8fc29ec7cba8286fa@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=73d8fc29ec7cba8286fa Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com> --- V1 --> V2: add the root cause of the i_link not set issue and imporve the check fs/namei.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)