[PATCH] Fix OOB write if err =3D=3D buflen in ntfs_readlink_hlp()

[Alexandro Calo]

ntfs_utf16_to_nls() may return buflen. The caller later uses the returned
length as the index for writing the trailing NUL byte,
so err =3D=3D buflen writes one byte past the end of buffer.

Fix this by limiting err to the last valid buffer index before writing
NUL.

ntfs_utf16_to_nls() returning a negative value is already handled by
if (err < 0) goto out;
As long as buflen is guaranteed to be nonzero the patch is fine.
As a defensive fix if(buflen=3D=3D0) could be added.

This heap out-of-bounds write requires a crafted filesystem image,
which is not in the kernel threat model, but fixing memory errors would
be nice to keep things secure.

Signed-off-by: Alexandro Calo <alexandro.calo@xxxxxxxxxxxxxxxxxx>
---
 fs/ntfs3/inode.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ntfs3/inode.c b/fs/ntfs3/inode.c
index 0c9bd669117d..d4803e1625fe 100644
--- a/fs/ntfs3/inode.c
+++ b/fs/ntfs3/inode.c
@@ -2029,6 +2029,9 @@ static noinline int ntfs_readlink_hlp(const struct de=
ntry *link_de,
 	if (err < 0)
 		goto out;
=20
+	if (err >=3D buflen)
+		err =3D buflen - 1;
+
 	/* Translate Windows '\' into Linux '/'. */
 	for (i =3D 0; i < err; i++) {
 		if (buffer[i] =3D=3D '\\')

base-commit: 0e35b9b6ec0ffcc5e23cbdec09f5c622ad532b53
--=20
2.47.3