[PATCH net-next net] netdev: Hide the netdev_priv() origin from GCC in netdev_from_priv()

GCC 15.1 and later can compute an object size of 0 for a pointer that
container_of() derives from a pointer into a __counted_by flexible array
member[1]; Clang gets it right. netdev_from_priv() does this with struct
net_device's "priv" array, so when GCC can see that its argument came
from netdev_priv(), FORTIFY_SOURCE sees a zero-sized net_device. Since
commit 9027497a25e3c ("team: use netdev_from_priv()"), adding a port of
another type to a team device can then fail the memcpy() to
"dev->broadcast" in team_setup_by_port(), which GCC sees through after
inlining team_port_add() into team_add_slave():

  memcpy: detected buffer overflow: 4 byte write of buffer size 0
  ...
  kernel BUG at lib/string_helpers.c:1044!
  ...
  Call Trace:
   <TASK>
   team_add_slave.cold+0x88/0x201
   do_set_master+0x11c/0x180
   do_setlink.isra.0+0x3a4/0x1410

Whether GCC inlines that far depends on the architecture and the
configuration. It does on ppc64le, where this was reported, and on
x86_64 with defconfig plus hardening.config or with Debian's
configuration, but not with allmodconfig.

Hide where "priv" came from with OPTIMIZER_HIDE_VAR(), so GCC sees a
net_device of unknown size whose members have their declared sizes. (An
"unknown" size is considered uncheckable by FORTIFY.)  FORTIFY still
bounds writes to members; only the whole-object size becomes unknown,
as it already is for any net_device pointer GCC can't trace to its
allocation. In an extracted PoC, the __builtin_dynamic_object_size()
of "dev->broadcast" for the two types FORTIFY sees, before and after
this patch:

                                   GCC 16.2.0      Clang 22 and 24
                                 before   after
  plain pointer, whole object   unknown  unknown        44
  container_of(), whole object        0  unknown        44
  container_of(), member              0       32        32

In addition to getting the bogus "0" fixed in GCC, I'll be following up
to solve the whole-object visiblity disparity as well.

Build tested with GCC 16.2.0: ARCH=x86_64 defconfig with hardening.config,
CONFIG_NET_TEAM=y, and CONFIG_NET_IPGRE=y; ARCH=arm64 defconfig with
hardening.config, CONFIG_NET_TEAM=y, CONFIG_PPP=y, and CONFIG_NET_AIROHA=m;
and drivers/net/team/, drivers/net/ppp/, and drivers/net/ethernet/airoha/
in ARCH=x86_64 allmodconfig. Under QEMU, the x86_64 kernel hits the BUG
above when adding the GRE fallback device "gre0" as a port of a new team
device without this patch, and adds it with the patch.

Reported-by: Yeswanth Krishna Tellakula <redacted>
Closes: https://lore.kernel.org/all/3ad19e86-234c-408f-896e-0d6c774fea49@linux.ibm.com/ (local)
Fixes: 9027497a25e3c ("team: use netdev_from_priv()")
Cc: stable@vger.kernel.org
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127875 [1]
Signed-off-by: Kees Cook <kees@kernel.org>
---
 include/linux/netdevice.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 87cafc932e9e..e1dedecd796a 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h

@@ -2838,6 +2838,13 @@ static inline void *netdev_priv(const struct net_device *dev)
  */
 static inline struct net_device *netdev_from_priv(const void *priv)
 {
+	/*
+	 * When GCC can see that @priv came from netdev_priv(), it can
+	 * compute an object size of 0 for the net_device and its members,
+	 * which makes FORTIFY_SOURCE fail valid accesses through the result:
+	 * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127875
+	 */
+	OPTIMIZER_HIDE_VAR(priv);
 	return container_of(priv, struct net_device, priv);
 }
 

-- 
2.55.0