[PATCH wireless-next v2] wext: use flex array destination for memcpy()

Hawkins Jiawei
2022-09-26T23:35:20Z

Syzkaller reports buffer overflow false positive as follows:
------------[ cut here ]------------
memcpy: detected field-spanning write (size 8) of single field
"&compat_event->pointer" at net/wireless/wext-core.c:623 (size 4)
WARNING: CPU: 0 PID: 3607 at net/wireless/wext-core.c:623
wireless_send_event+0xab5/0xca0 net/wireless/wext-core.c:623
Modules linked in:
CPU: 1 PID: 3607 Comm: syz-executor659 Not tainted
6.0.0-rc6-next-20220921-syzkaller #0
[...]
Call Trace:
<TASK>
ioctl_standard_call+0x155/0x1f0 net/wireless/wext-core.c:1022
wireless_process_ioctl+0xc8/0x4c0 net/wireless/wext-core.c:955
wext_ioctl_dispatch net/wireless/wext-core.c:988 [inline]
wext_ioctl_dispatch net/wireless/wext-core.c:976 [inline]
wext_handle_ioctl+0x26b/0x280 net/wireless/wext-core.c:1049
sock_ioctl+0x285/0x640 net/socket.c:1220
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:870 [inline]
__se_sys_ioctl fs/ioctl.c:856 [inline]
__x64_sys_ioctl+0x193/0x200 fs/ioctl.c:856
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
[...]
</TASK>

Wireless events will be sent on the appropriate channels in
wireless_send_event(). Different wireless events may have different
payload structure and size, so kernel uses **len** and **cmd** field
in struct __compat_iw_event as wireless event common LCP part, uses
**pointer** as a label to mark the position of remaining different part.

Yet the problem is that, **pointer** is a compat_caddr_t type, which may
be smaller than the relative structure at the same position. So during
wireless_send_event() tries to parse the wireless events payload, it may
trigger the memcpy() run-time destination buffer bounds checking when the
relative structure's data is copied to the position marked by **pointer**.

This patch solves it by introducing flexible-array field **ptr_bytes**,
to mark the position of the wireless events remaining part next to
LCP part. What's more, this patch also adds **ptr_len** variable in
wireless_send_event() to improve its maintainability.

Reported-and-tested-by: syzbot+473754e5af963cf014cf@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/00000000000070db2005e95a5984@google.com/
Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
---
v2: correct the typo error pointed out by Eric Dumazet and
Kees Cook

v1: https://lore.kernel.org/all/20220926150907.8551-1-yin31149@gmail.com/

include/linux/wireless.h | 10 +++++++++-
net/wireless/wext-core.c | 17 ++++++++++-------
2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/include/linux/wireless.h b/include/linux/wireless.h
index 2d1b54556eff..e6e34d74dda0 100644
--- a/include/linux/wireless.h
+++ b/include/linux/wireless.h
@@ -26,7 +26,15 @@ struct compat_iw_point {
struct __compat_iw_event {
__u16 len; /* Real length of this stuff */
__u16 cmd; /* Wireless IOCTL */
- compat_caddr_t pointer;
+
+ union {
+ compat_caddr_t pointer;
+
+ /* we need ptr_bytes to make memcpy() run-time destination
+ * buffer bounds checking happy, nothing special
+ */
+ DECLARE_FLEX_ARRAY(__u8, ptr_bytes);
+ };
};
#define IW_EV_COMPAT_LCP_LEN offsetof(struct __compat_iw_event, pointer)
#define IW_EV_COMPAT_POINT_OFF offsetof(struct compat_iw_point, length)
diff --git a/net/wireless/wext-core.c b/net/wireless/wext-core.c
index 76a80a41615b..fe8765c4075d 100644
--- a/net/wireless/wext-core.c
+++ b/net/wireless/wext-core.c
@@ -468,6 +468,7 @@ void wireless_send_event(struct net_device * dev,
struct __compat_iw_event *compat_event;
struct compat_iw_point compat_wrqu;
struct sk_buff *compskb;
+ int ptr_len;
#endif

/*
@@ -582,6 +583,9 @@ void wireless_send_event(struct net_device * dev,
nlmsg_end(skb, nlh);
#ifdef CONFIG_COMPAT
hdr_len = compat_event_type_size[descr->header_type];
+
+ /* ptr_len is remaining size in event header apart from LCP */
+ ptr_len = hdr_len - IW_EV_COMPAT_LCP_LEN;
event_len = hdr_len + extra_len;

compskb = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_ATOMIC);
@@ -612,16 +616,15 @@ void wireless_send_event(struct net_device * dev,
if (descr->header_type == IW_HEADER_TYPE_POINT) {
compat_wrqu.length = wrqu->data.length;
compat_wrqu.flags = wrqu->data.flags;
- memcpy(&compat_event->pointer,
- ((char *) &compat_wrqu) + IW_EV_COMPAT_POINT_OFF,
- hdr_len - IW_EV_COMPAT_LCP_LEN);
+ memcpy(compat_event->ptr_bytes,
+ ((char *)&compat_wrqu) + IW_EV_COMPAT_POINT_OFF,
+ ptr_len);
if (extra_len)
- memcpy(((char *) compat_event) + hdr_len,
- extra, extra_len);
+ memcpy(&compat_event->ptr_bytes[ptr_len],
+ extra, extra_len);
} else {
/* extra_len must be zero, so no if (extra) needed */
- memcpy(&compat_event->pointer, wrqu,
- hdr_len - IW_EV_COMPAT_LCP_LEN);
+ memcpy(compat_event->ptr_bytes, wrqu, ptr_len);
}

nlmsg_end(compskb, nlh);
--
2.25.1