ufs: Rework shortlink handling to avoid subobject overflows

Shortlinks occupy the space of both di_db and di_ib when used. However,
everywhere that wants to read or write a shortlink takes a pointer do
di_db and promptly runs off the end of it into di_ib. This is fine on
most architectures, if a little dodgy. However, on CHERI, the compiler
can optionally restrict the bounds on pointers to subobjects to just
that subobject, in order to mitigate intra-object buffer overflows, and
this is enabled in CheriBSD's pure-capability kernels.

Instead, clean this up by inserting a union such that a new di_shortlink
can be added with the right size and element type, avoiding the need to
cast and allowing the use of the DIP macro to access the field. This
also mirrors how the ext2fs code implements extents support, with the
exact same structure other than having a uint32_t i_data[] instead of a
char di_shortlink[].

Reviewed by:	mckusick, jhb
Differential Revision:	https://reviews.freebsd.org/D33650

4 files changed, 23 insertions, 9 deletions

diff --git a/sys/ufs/ffs/ffs_inode.c b/sys/ufs/ffs/ffs_inode.c
index 4b31b4febcbd..d8b86df27121 100644
--- a/sys/ufs/ffs/ffs_inode.c
+++ b/sys/ufs/ffs/ffs_inode.c
@@ -342,7 +342,7 @@ ffs_truncate(vp, length, flags, cred)
 		if (length != 0)
 			panic("ffs_truncate: partial truncate of symlink");
 #endif
-		bzero(SHORTLINK(ip), (u_int)ip->i_size);
+		bzero(DIP(ip, i_shortlink), (u_int)ip->i_size);
 		ip->i_size = 0;
 		DIP_SET(ip, i_size, 0);
 		UFS_INODE_SET_FLAG(ip, IN_SIZEMOD | IN_CHANGE | IN_UPDATE);
diff --git a/sys/ufs/ufs/dinode.h b/sys/ufs/ufs/dinode.h
index 1f0f25c4d5ec..840a4cc7d40f 100644
--- a/sys/ufs/ufs/dinode.h
+++ b/sys/ufs/ufs/dinode.h
@@ -145,8 +145,16 @@ struct ufs2_dinode {
 	u_int32_t	di_flags;	/*  88: Status flags (chflags). */
 	u_int32_t	di_extsize;	/*  92: External attributes size. */
 	ufs2_daddr_t	di_extb[UFS_NXADDR];/* 96: External attributes block. */
-	ufs2_daddr_t	di_db[UFS_NDADDR]; /* 112: Direct disk blocks. */
-	ufs2_daddr_t	di_ib[UFS_NIADDR]; /* 208: Indirect disk blocks. */
+	union {
+		struct {
+			ufs2_daddr_t	di_db /* 112: Direct disk blocks. */
+			    [UFS_NDADDR];
+			ufs2_daddr_t	di_ib /* 208: Indirect disk blocks. */
+			    [UFS_NIADDR];
+		};
+		char	di_shortlink	/* 112: Embedded symbolic link. */
+		    [(UFS_NDADDR + UFS_NIADDR) * sizeof(ufs2_daddr_t)];
+	};
 	u_int64_t	di_modrev;	/* 232: i_modrev for NFSv4 */
 	uint32_t	di_freelink;	/* 240: SUJ: Next unlinked inode. */
 	uint32_t	di_ckhash;	/* 244: if CK_INODE, its check-hash */
@@ -179,8 +187,16 @@ struct ufs1_dinode {
 	int32_t		di_mtimensec;	/*  28: Last modified time. */
 	int32_t		di_ctime;	/*  32: Last inode change time. */
 	int32_t		di_ctimensec;	/*  36: Last inode change time. */
-	ufs1_daddr_t	di_db[UFS_NDADDR]; /*  40: Direct disk blocks. */
-	ufs1_daddr_t	di_ib[UFS_NIADDR]; /*  88: Indirect disk blocks. */
+	union {
+		struct {
+			ufs1_daddr_t	di_db /*  40: Direct disk blocks. */
+			    [UFS_NDADDR];
+			ufs1_daddr_t	di_ib /*  88: Indirect disk blocks. */
+			    [UFS_NIADDR];
+		};
+		char	di_shortlink	/*  40: Embedded symbolic link. */
+		    [(UFS_NDADDR + UFS_NIADDR) * sizeof(ufs1_daddr_t)];
+	};
 	u_int32_t	di_flags;	/* 100: Status flags (chflags). */
 	u_int32_t	di_blocks;	/* 104: Blocks actually held. */
 	u_int32_t	di_gen;		/* 108: Generation number. */
diff --git a/sys/ufs/ufs/inode.h b/sys/ufs/ufs/inode.h
index a82b4658c0af..7ea88d15b1e3 100644
--- a/sys/ufs/ufs/inode.h
+++ b/sys/ufs/ufs/inode.h
@@ -246,8 +246,6 @@ I_IS_UFS2(const struct inode *ip)
 		(ip)->i_din2->d##field = (val); 		\
 	} while (0)
 
-#define	SHORTLINK(ip)	(I_IS_UFS1(ip) ?			\
-    (caddr_t)(ip)->i_din1->di_db : (caddr_t)(ip)->i_din2->di_db)
 #define	IS_SNAPSHOT(ip)		((ip)->i_flags & SF_SNAPSHOT)
 
 /*
diff --git a/sys/ufs/ufs/ufs_vnops.c b/sys/ufs/ufs/ufs_vnops.c
index 89454d0effff..1d0525bcbe0c 100644
--- a/sys/ufs/ufs/ufs_vnops.c
+++ b/sys/ufs/ufs/ufs_vnops.c
@@ -2311,7 +2311,7 @@ ufs_symlink(ap)
 	len = strlen(ap->a_target);
 	if (len < VFSTOUFS(vp->v_mount)->um_maxsymlinklen) {
 		ip = VTOI(vp);
-		bcopy(ap->a_target, SHORTLINK(ip), len);
+		bcopy(ap->a_target, DIP(ip, i_shortlink), len);
 		ip->i_size = len;
 		DIP_SET(ip, i_size, len);
 		UFS_INODE_SET_FLAG(ip, IN_SIZEMOD | IN_CHANGE | IN_UPDATE);
@@ -2481,7 +2481,7 @@ ufs_readlink(ap)
 
 	isize = ip->i_size;
 	if (isize < VFSTOUFS(vp->v_mount)->um_maxsymlinklen)
-		return (uiomove(SHORTLINK(ip), isize, ap->a_uio));
+		return (uiomove(DIP(ip, i_shortlink), isize, ap->a_uio));
 	return (VOP_READ(vp, ap->a_uio, 0, ap->a_cred));
 }