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

[Jann Horn <jannh@google.com>]

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.

Fixes: 48b32a3553a5 ("reiserfs: use generic xattr handlers")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
---
Triggering the bug:

root@debian:/home/user# mount -o user_xattr reiserimg reisermount/
root@debian:/home/user# cd reisermount/
root@debian:/home/user/reisermount# touch test_file
root@debian:/home/user/reisermount# setfattr -n user.foo1 -v A test_file
root@debian:/home/user/reisermount# setfattr -n user.foo2 -v A test_file
root@debian:/home/user/reisermount# setfattr -n user.foo3 -v A test_file
root@debian:/home/user/reisermount# setfattr -n user.foo4 -v A test_file
root@debian:/home/user/reisermount# setfattr -n user.foo5 -v A test_file
root@debian:/home/user/reisermount# setfattr -n user.foo6 -v A test_file
root@debian:/home/user/reisermount# cat xattr_test.c
#include <sys/types.h>
#include <attr/xattr.h>
#include <err.h>
#include <stdio.h>
#include <string.h>
int main(int argc, char **argv) {
  if (argc != 2) errx(1, "bad invocation");
  char list[10];
  int res = listxattr(argv[1], list, sizeof(list));
  if (res == -1)
    err(1, "listxattr failed");
  printf("listxattr returned %d\n", res);
  for (char *p = list; p < list+res-1; p = p + strlen(p) + 1) {
    printf("list entry: %s\n", p);
  }
}
root@debian:/home/user/reisermount# gcc -o xattr_test xattr_test.c
root@debian:/home/user/reisermount# ./xattr_test test_file
Segmentation fault
root@debian:/home/user/reisermount#

Result:

[  122.071318] ==================================================================
[  122.072334] BUG: KASAN: slab-out-of-bounds in listxattr_filler+0x170/0x1b0
[  122.073173] Write of size 9 at addr ffff8801c43b474a by task xattr_test/923
[  122.074030]
[  122.074223] CPU: 1 PID: 923 Comm: xattr_test Not tainted 4.18.0-rc7+ #67
[  122.075050] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[  122.076107] Call Trace:
[  122.076453]  dump_stack+0x71/0xab
[  122.076900]  print_address_description+0x6a/0x250
[  122.077514]  kasan_report+0x258/0x380
[  122.077961]  ? listxattr_filler+0x170/0x1b0
[  122.078469]  memcpy+0x34/0x50
[  122.078894]  listxattr_filler+0x170/0x1b0
[...]

 fs/reiserfs/xattr.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/reiserfs/xattr.c b/fs/reiserfs/xattr.c
index ff94fad477e4..48cdfc81fe10 100644
--- a/fs/reiserfs/xattr.c
+++ b/fs/reiserfs/xattr.c
@@ -792,8 +792,10 @@ static int listxattr_filler(struct dir_context *ctx, const char *name,
 			return 0;
 		size = namelen + 1;
 		if (b->buf) {
-			if (size > b->size)
+			if (b->pos + size > b->size) {
+				b->pos = -ERANGE;
 				return -ERANGE;
+			}
 			memcpy(b->buf + b->pos, name, namelen);
 			b->buf[b->pos + namelen] = 0;
 		}
-- 
2.18.0.597.ga71716f1ad-goog