[PATCH 4.19 083/114] netfilter: ebtables: compat: reject all padding in matches/watchers

Greg Kroah-Hartman
2020-01-02T22:53:17Z

From: Florian Westphal <fw@strlen.de>

commit e608f631f0ba5f1fc5ee2e260a3a35d13107cbfe upstream.

syzbot reported following splat:

BUG: KASAN: vmalloc-out-of-bounds in size_entry_mwt net/bridge/netfilter/ebtables.c:2063 [inline]
BUG: KASAN: vmalloc-out-of-bounds in compat_copy_entries+0x128b/0x1380 net/bridge/netfilter/ebtables.c:2155
Read of size 4 at addr ffffc900004461f4 by task syz-executor267/7937

CPU: 1 PID: 7937 Comm: syz-executor267 Not tainted 5.5.0-rc1-syzkaller #0
size_entry_mwt net/bridge/netfilter/ebtables.c:2063 [inline]
compat_copy_entries+0x128b/0x1380 net/bridge/netfilter/ebtables.c:2155
compat_do_replace+0x344/0x720 net/bridge/netfilter/ebtables.c:2249
compat_do_ebt_set_ctl+0x22f/0x27e net/bridge/netfilter/ebtables.c:2333
[..]

Because padding isn't considered during computation of ->buf_user_offset,
"total" is decremented by fewer bytes than it should.

Therefore, the first part of

if (*total < sizeof(*entry) || entry->next_offset < sizeof(*entry))

will pass, -- it should not have. This causes oob access:
entry->next_offset is past the vmalloced size.

Reject padding and check that computed user offset (sum of ebt_entry
structure plus all individual matches/watchers/targets) is same
value that userspace gave us as the offset of the next entry.

Reported-by: syzbot+f68108fed972453a0ad4@syzkaller.appspotmail.com
Fixes: 81e675c227ec ("netfilter: ebtables: add CONFIG_COMPAT support")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
net/bridge/netfilter/ebtables.c | 33 ++++++++++++++++-----------------
1 file changed, 16 insertions(+), 17 deletions(-)

--- a/net/bridge/netfilter/ebtables.c
+++ b/net/bridge/netfilter/ebtables.c
@@ -1876,7 +1876,7 @@ static int ebt_buf_count(struct ebt_entr
}

static int ebt_buf_add(struct ebt_entries_buf_state *state,
- void *data, unsigned int sz)
+ const void *data, unsigned int sz)
{
if (state->buf_kern_start == NULL)
goto count_only;
@@ -1910,7 +1910,7 @@ enum compat_mwt {
EBT_COMPAT_TARGET,
};

-static int compat_mtw_from_user(struct compat_ebt_entry_mwt *mwt,
+static int compat_mtw_from_user(const struct compat_ebt_entry_mwt *mwt,
enum compat_mwt compat_mwt,
struct ebt_entries_buf_state *state,
const unsigned char *base)
@@ -1988,22 +1988,23 @@ static int compat_mtw_from_user(struct c
/* return size of all matches, watchers or target, including necessary
* alignment and padding.
*/
-static int ebt_size_mwt(struct compat_ebt_entry_mwt *match32,
+static int ebt_size_mwt(const struct compat_ebt_entry_mwt *match32,
unsigned int size_left, enum compat_mwt type,
struct ebt_entries_buf_state *state, const void *base)
{
+ const char *buf = (const char *)match32;
int growth = 0;
- char *buf;

if (size_left == 0)
return 0;

- buf = (char *) match32;
-
- while (size_left >= sizeof(*match32)) {
+ do {
struct ebt_entry_match *match_kern;
int ret;

+ if (size_left < sizeof(*match32))
+ return -EINVAL;
+
match_kern = (struct ebt_entry_match *) state->buf_kern_start;
if (match_kern) {
char *tmp;
@@ -2040,22 +2041,18 @@ static int ebt_size_mwt(struct compat_eb
if (match_kern)
match_kern->match_size = ret;

- /* rule should have no remaining data after target */
- if (type == EBT_COMPAT_TARGET && size_left)
- return -EINVAL;
-
match32 = (struct compat_ebt_entry_mwt *) buf;
- }
+ } while (size_left);

return growth;
}

/* called for all ebt_entry structures. */
-static int size_entry_mwt(struct ebt_entry *entry, const unsigned char *base,
+static int size_entry_mwt(const struct ebt_entry *entry, const unsigned char *base,
unsigned int *total,
struct ebt_entries_buf_state *state)
{
- unsigned int i, j, startoff, new_offset = 0;
+ unsigned int i, j, startoff, next_expected_off, new_offset = 0;
/* stores match/watchers/targets & offset of next struct ebt_entry: */
unsigned int offsets[4];
unsigned int *offsets_update = NULL;
@@ -2141,11 +2138,13 @@ static int size_entry_mwt(struct ebt_ent
return ret;
}

- startoff = state->buf_user_offset - startoff;
+ next_expected_off = state->buf_user_offset - startoff;
+ if (next_expected_off != entry->next_offset)
+ return -EINVAL;

- if (WARN_ON(*total < startoff))
+ if (*total < entry->next_offset)
return -EINVAL;
- *total -= startoff;
+ *total -= entry->next_offset;
return 0;
}