[PATCH 4.14 17/52] ext4: correctly handle a zero-length xattr with a non-zero e_value_offs

Greg Kroah-Hartman
2018-06-24T15:25:57Z

From: Theodore Ts'o <tytso@mit.edu>

commit 8a2b307c21d4b290e3cbe33f768f194286d07c23 upstream.

Ext4 will always create ext4 extended attributes which do not have a
value (where e_value_size is zero) with e_value_offs set to zero. In
most places e_value_offs will not be used in a substantive way if
e_value_size is zero.

There was one exception to this, which is in ext4_xattr_set_entry(),
where if there is a maliciously crafted file system where there is an
extended attribute with e_value_offs is non-zero and e_value_size is
0, the attempt to remove this xattr will result in a negative value
getting passed to memmove, leading to the following sadness:

[ 41.225365] EXT4-fs (loop0): mounted filesystem with ordered data mode. Opts: (null)
[ 44.538641] BUG: unable to handle kernel paging request at ffff9ec9a3000000
[ 44.538733] IP: __memmove+0x81/0x1a0
[ 44.538755] PGD 1249bd067 P4D 1249bd067 PUD 1249c1067 PMD 80000001230000e1
[ 44.538793] Oops: 0003 [#1] SMP PTI
[ 44.539074] CPU: 0 PID: 1470 Comm: poc Not tainted 4.16.0-rc1+ #1
...
[ 44.539475] Call Trace:
[ 44.539832] ext4_xattr_set_entry+0x9e7/0xf80
...
[ 44.539972] ext4_xattr_block_set+0x212/0xea0
...
[ 44.540041] ext4_xattr_set_handle+0x514/0x610
[ 44.540065] ext4_xattr_set+0x7f/0x120
[ 44.540090] __vfs_removexattr+0x4d/0x60
[ 44.540112] vfs_removexattr+0x75/0xe0
[ 44.540132] removexattr+0x4d/0x80
...
[ 44.540279] path_removexattr+0x91/0xb0
[ 44.540300] SyS_removexattr+0xf/0x20
[ 44.540322] do_syscall_64+0x71/0x120
[ 44.540344] entry_SYSCALL_64_after_hwframe+0x21/0x86

https://bugzilla.kernel.org/show_bug.cgi?id=199347

This addresses CVE-2018-10840.

Reported-by: "Xu, Wen" <wen.xu@gatech.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Cc: stable@kernel.org
Fixes: dec214d00e0d7 ("ext4: xattr inode deduplication")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/ext4/xattr.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -1687,7 +1687,7 @@ static int ext4_xattr_set_entry(struct e

/* No failures allowed past this point. */

- if (!s->not_found && here->e_value_offs) {
+ if (!s->not_found && here->e_value_size && here->e_value_offs) {
/* Remove the old value. */
void *first_val = s->base + min_offs;
size_t offs = le16_to_cpu(here->e_value_offs);