Linux: fs/ntfs3: preserve non-DOS attribute bits in system.dos_attrib

In the Linux kernel, the following vulnerability has been resolved:

fs/ntfs3: preserve non-DOS attribute bits in system.dos_attrib

[BUG] A corrupted ntfs3 image can hit a NULL function pointer call in generic_perform_write() after toggling system.ntfs_attrib and then overwriting system.dos_attrib on the same file.

BUG: kernel NULL pointer dereference, address: 0000000000000000 \#PF: supervisor instruction fetch in kernel mode \#PF: error_code(0x0010) - not-present page PGD bed5067 P4D bed5067 PUD 0 Oops: Oops: 0010 [#1] SMP KASAN NOPTI RIP: 0010:0x0 Code: Unable to access opcode bytes at 0xffffffffffffffd6. RSP: 0018:ffff88801025f988 EFLAGS: 00010246 Call Trace: generic_perform_write+0x409/0x8c0 mm/filemap.c:4255 __generic_file_write_iter+0x1bb/0x200 mm/filemap.c:4372 ntfs_file_write_iter+0xcd9/0x1c20 fs/ntfs3/file.c:1253 new_sync_write fs/read_write.c:593 [inline] vfs_write+0x63b/0xf70 fs/read_write.c:686 ksys_write+0x133/0x250 fs/read_write.c:738 __do_sys_write fs/read_write.c:749 [inline] __se_sys_write fs/read_write.c:746 [inline] __x64_sys_write+0x77/0xc0 fs/read_write.c:746 ...

[CAUSE] system.ntfs_attrib updates ATTR_DATA flags via ni_new_attr_flags() and switches i_mapping->a_ops to ntfs_aops_cmpr when FILE_ATTRIBUTE_COMPRESSED is set. system.dos_attrib then overwrites ni->std_fa from a one-byte DOS attribute value, clearing the compression bit without updating ATTR_DATA or the mapping operations.

Old buffered writes use is_compressed(ni) to choose __generic_file_write_iter(). That leaves generic_perform_write() calling a NULL write_begin callback from ntfs_aops_cmpr.

[FIX] Treat system.dos_attrib as a low-byte DOS attribute update and preserve the existing non-DOS attribute bits in ni->std_fa. This keeps compressed and sparse state consistent with ATTR_DATA and the mapping operations while keeping the existing DOS attribute semantics intact.