[PATCH v2 1/4] squashfs: Fix integer overflow in sqfs_resolve_symlink()

[Richard Weinberger]

A carefully crafted squashfs filesystem can exhibit an inode size of 0xffffffff,
as a consequence malloc() will do a zero allocation.
Later in the function the inode size is again used for copying data.
So an attacker can overwrite memory.
Avoid the overflow by using the __builtin_add_overflow() helper.

Signed-off-by: Richard Weinberger <richard at nod.at>
Reviewed-by: Miquel Raynal <miquel.raynal at bootlin.com>
---
 fs/squashfs/sqfs.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/squashfs/sqfs.c b/fs/squashfs/sqfs.c
index 1430e671a5..16a07c0622 100644
--- a/fs/squashfs/sqfs.c
+++ b/fs/squashfs/sqfs.c
@@ -422,8 +422,10 @@ static char *sqfs_resolve_symlink(struct squashfs_symlink_inode *sym,
 	char *resolved, *target;
 	u32 sz;
 
-	sz = get_unaligned_le32(&sym->symlink_size);
-	target = malloc(sz + 1);
+	if (__builtin_add_overflow(get_unaligned_le32(&sym->symlink_size), 1, &sz))
+		return NULL;
+
+	target = malloc(sz);
 	if (!target)
 		return NULL;
 
@@ -431,9 +433,9 @@ static char *sqfs_resolve_symlink(struct squashfs_symlink_inode *sym,
 	 * There is no trailling null byte in the symlink's target path, so a
 	 * copy is made and a '\0' is added at its end.
 	 */
-	target[sz] = '\0';
+	target[sz - 1] = '\0';
 	/* Get target name (relative path) */
-	strncpy(target, sym->symlink, sz);
+	strncpy(target, sym->symlink, sz - 1);
 
 	/* Relative -> absolute path conversion */
 	resolved = sqfs_get_abs_path(base_path, target);
-- 
2.35.3