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

[ZhengYuan Huang <gality369@gmail.com>]

[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.

Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
---
 fs/ntfs3/xattr.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/ntfs3/xattr.c b/fs/ntfs3/xattr.c
index 9eeac0a..7e51182 100644
--- a/fs/ntfs3/xattr.c
+++ b/fs/ntfs3/xattr.c
@@ -867,7 +867,9 @@ static noinline int ntfs_setxattr(const struct xattr_handler *handler,
 	if (!strcmp(name, SYSTEM_DOS_ATTRIB)) {
 		if (sizeof(u8) != size)
 			goto out;
-		new_fa = cpu_to_le32(*(u8 *)value);
+		/* system.dos_attrib only covers the low DOS attribute byte. */
+		new_fa = (ni->std_fa & ~cpu_to_le32(0xff)) |
+			 cpu_to_le32(*(u8 *)value);
 		goto set_new_fa;
 	}
 
-- 
2.43.0