[PATCH v3 2/2] usb: gadget: u_ether: fix out-of-bounds read in get_ether_addr

[Liu Chao]

get_ether_addr() walks the MAC address string with a fixed six iteration
loop, calling hex_to_bin() twice per iteration, and never checks for the
terminating NUL.  A short string therefore makes it read past the end of
the buffer.  The dev_addr and host_addr module parameters of the legacy
gadget drivers are copied into their own kmalloc'ed buffer by
param_set_charp(), so the overrun crosses a slab boundary and KASAN
reports it at insmod time:

  # insmod g_ether.ko dev_addr=ab

  BUG: KASAN: slab-out-of-bounds in get_ether_addr+0x1ec/0x230
  Read of size 1 at addr ffff888009f91f53 by task insmod/70
  ...
   get_ether_addr+0x1ec/0x230
   gether_set_dev_addr+0x6b/0x180
   eth_bind+0xfd/0x3b0 [g_ether]
   composite_bind+0x1bb/0x7b0
   gadget_bind_driver+0x229/0xb00
  ...
  Allocated by task 70:
   __kmalloc_noprof+0x1d1/0x4a0
   param_set_charp+0x4a/0x220
   parse_args+0x267/0x5c0
   load_module+0x4f6a/0x6600
  ...
  The buggy address is located 0 bytes to the right of
   allocated 19-byte region [ffff888009f91f40, ffff888009f91f53)

Besides reading past the end of the string, the loop ignores what
hex_to_bin() returns: it yields -1 for a character that is not a hex
digit, and that value is shifted and OR'ed into dev_addr unchecked, so a
malformed address can be accepted as a valid one and the interface comes
up with an address that nobody asked for:

  # insmod g_ether.ko dev_addr=aazz:cc:dd:ee:ff
  # cat /sys/class/net/usb0/address
  aa:ff:cc:dd:ee:ff

The configfs dev_addr and host_addr attributes of the ECM and NCM
functions reach the same parser through gether_set_dev_addr() and
gether_set_host_addr().  Their store handler passes the whole configfs
buffer on, so an overrun there stays inside the same object and is never
reported, but a malformed address is silently accepted all the same.

Replace the open coded loop with parse_mac_pton() for the delimited
notations and hex2bin() for the raw one, and keep the existing
eth_random_addr() fallback for everything both of them reject.
parse_mac_pton() checks the length and every digit before writing
anything to dev_addr, and hex2bin() stops at the first character that is
not a hex digit, which the NUL terminator is, so neither reads past the
end of the string.

The notations that keep working are the byte pair ones, ':' and '.', in
any mixture, and the separator-less "aabbccddeeff".  What stops working
is the Cisco notation "aabb.ccdd.eeff" and irregular groupings such as
"123456:789abc", which the old loop only accepted because it skipped a
delimiter wherever it happened to find one.  Supporting them would need
a parser per grouping, so such input now falls back to a random address.

Select GENERIC_NET_UTILS so that parse_mac_pton() is built in.

The loop is older than the git history: it is already present in
drivers/usb/gadget/ether.c in commit 1da177e4c3f4 ("Linux-2.6.12-rc2")
and was copied into u_ether.c by commit 2b3d942c4878 ("usb ethernet
gadget: split out network core"), hence no Fixes tag.

Suggested-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
Signed-off-by: Liu Chao <liuc63@xxxxxxxxxxxx>
---
v3: fall back to hex2bin() so that the separator-less notation keeps
    working, as suggested by Andy; drop the Fixes tag, the loop predates
    the git history.
v2: rework on top of parse_mac_pton(), add the KASAN report and the
    malformed address example, select GENERIC_NET_UTILS, drop Cc: stable
    and the Reviewed-by.
Link: https://lore.kernel.org/all/20261104025058.2650911-3-liuc63@xxxxxxxxxxxx/

 drivers/usb/gadget/Kconfig            |  1 +
 drivers/usb/gadget/function/u_ether.c | 18 ++++++++----------
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig
index 76521555e3c1..b83c42be686b 100644
--- a/drivers/usb/gadget/Kconfig
+++ b/drivers/usb/gadget/Kconfig
@@ -158,6 +158,7 @@ config USB_U_SERIAL
 
 config USB_U_ETHER
 	tristate
+	select GENERIC_NET_UTILS
 
 config USB_U_AUDIO
 	tristate
diff --git a/drivers/usb/gadget/function/u_ether.c b/drivers/usb/gadget/function/u_ether.c
index 59d85d6a84a8..deedea8a19fd 100644
--- a/drivers/usb/gadget/function/u_ether.c
+++ b/drivers/usb/gadget/function/u_ether.c
@@ -686,18 +686,16 @@ static int eth_stop(struct net_device *net)
 static int get_ether_addr(const char *str, u8 *dev_addr)
 {
 	if (str) {
-		unsigned	i;
+		bool valid;
 
-		for (i = 0; i < 6; i++) {
-			unsigned char num;
+		/* Try the delimited formats first */
+		valid = parse_mac_pton(str, dev_addr, ".:");
+		if (valid && is_valid_ether_addr(dev_addr))
+			return 0;
 
-			if ((*str == '.') || (*str == ':'))
-				str++;
-			num = hex_to_bin(*str++) << 4;
-			num |= hex_to_bin(*str++);
-			dev_addr [i] = num;
-		}
-		if (is_valid_ether_addr(dev_addr))
+		/* Fall back to the raw format */
+		valid = hex2bin(dev_addr, str, ETH_ALEN) == 0;
+		if (valid && is_valid_ether_addr(dev_addr))
 			return 0;
 	}
 	eth_random_addr(dev_addr);
-- 
2.50.1