ntfs_inode_attr_pread(pos<0) can dereference bad pointer

ntfs_inode_attr_pread(pos<0) can dereference bad pointer

[rtm]

ntfs_inode_attr_pread()'s check on its pos argument is

        if (pos > isize) {
                mutex_unlock(&ni->mrec_lock);
                return -EINVAL;
        }

which allows pos to be negative, allowing this to potentially read from
a wild pointer:

                memcpy(buf, (u8 *)attr + pos, count);

A demo using a corrupt NTFS image:

# uname -a
Linux xxx 7.2.0-rc7-00016-g3d6d817622b0 #40 SMP PREEMPT_DYNAMIC Wed Aug 12 13:01:40 EDT 2026 x86_64 x86_64 x86_64 GNU/Linux
# wget http://www.rtmrtm.org/rtm/ntfs23d.img.gz
# gunzip ntfs23d.img.gz
# mount -t ntfs -o loop,rw junk /mnt
# ls /mnt
ntfs: (device loop0): ntfs_lookup_inode_by_name(): Corrupt directory.  Aborting lookup.
Oops: general protection fault, probably for non-canonical address 0x7fff88c0c159a850: 0000 [#1] SMP DEBUG_PAGEALLOC PTI
CPU: 3 UID: 0 PID: 1273 Comm: ls Not tainted 7.2.0-rc7-00016-g3d6d817622b0 #40 PREEMPT(full)
Hardware name: FreeBSD BHYVE/BHYVE, BIOS 14.0 10/17/2021
Call Trace:
 <TASK>
 ? ntfs_inode_attr_pread+0x26d/0x2f0
 ? __kvmalloc_node_noprof+0x1f0/0x5b0
 ntfs_ib_read+0x3d/0xd0
 ntfs_index_walk_down+0x76/0x120
 ntfs_readdir+0x7e5/0xa60
 ? selinux_file_permission+0x127/0x190
 iterate_dir+0x91/0x150
 __x64_sys_getdents64+0x77/0x110
 ? __pfx_filldir64+0x10/0x10
 x64_sys_call+0xfd0/0x2130

Robert Morris
rtm@mit.edu