[PATCH 3.16 155/328] reiserfs: fix broken xattr handling (heap corruption, bad retval)

[Ben Hutchings]

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@xxxxxxxxxx>

commit a13f085d111e90469faf2d9965eb39b11c114d7e upstream.

This fixes the following issues:

- When a buffer size is supplied to reiserfs_listxattr() such that each
  individual name fits, but the concatenation of all names doesn't fit,
  reiserfs_listxattr() overflows the supplied buffer.  This leads to a
  kernel heap overflow (verified using KASAN) followed by an out-of-bounds
  usercopy and is therefore a security bug.

- When a buffer size is supplied to reiserfs_listxattr() such that a
  name doesn't fit, -ERANGE should be returned.  But reiserfs instead just
  truncates the list of names; I have verified that if the only xattr on a
  file has a longer name than the supplied buffer length, listxattr()
  incorrectly returns zero.

With my patch applied, -ERANGE is returned in both cases and the memory
corruption doesn't happen anymore.

Credit for making me clean this code up a bit goes to Al Viro, who pointed
out that the ->actor calling convention is suboptimal and should be
changed.

Link: http://lkml.kernel.org/r/20180802151539.5373-1-jannh@xxxxxxxxxx
Fixes: 48b32a3553a5 ("reiserfs: use generic xattr handlers")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Acked-by: Jeff Mahoney <jeffm@xxxxxxxx>
Cc: Eric Biggers <ebiggers@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16:
 - The xattr handler's list operation does the copy, so also update the
   buffer size we pass to it
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/fs/reiserfs/xattr.c
+++ b/fs/reiserfs/xattr.c
@@ -822,10 +822,12 @@ static int listxattr_filler(void *buf, c
 			return 0;
 		if (b->buf) {
 			size = handler->list(b->dentry, b->buf + b->pos,
-					 b->size, name, namelen,
+					 b->size - b->pos, name, namelen,
 					 handler->flags);
-			if (size > b->size)
+			if (b->pos + size > b->size) {
+				b->pos = -ERANGE;
 				return -ERANGE;
+			}
 		} else {
 			size = handler->list(b->dentry, NULL, 0, name,
 					     namelen, handler->flags);