[PATCH net-next v2 0/8] net: skbuff: replace most BUG_ON()s with error returns

[Josef Bacik <josef@toxicpanda.com>]

v1: https://lore.kernel.org/all/20261006-b4-skbuff-bug-on-v1-0-1b4434c5357c@toxicpanda.com/

v1->v2:
- Use DEBUG_NET_WARN_ON_ONCE() instead of WARN_ON_ONCE(), so
  panic_on_warn systems take the error path instead of panicking
  (Willem, Fernando).
- Split out the skb_copy_and_csum_bits() unreadable-frags fix and sent
  it to net on its own with Cc: stable (Willem):
  https://lore.kernel.org/all/20261007-b4-skb-copy-csum-stale-bytes-v1-1-adbbde033fb3@toxicpanda.com/
- Rebased onto current net-next.

--- Original email (v1) ---

I'm going through and reducing BUG_ON() usage in areas that have created
the most problems for us.  89 commits in the tree quote "kernel BUG at
net/core/skbuff.c", 31 of them since 2024, and some of those could be
triggered from inside a user namespace.

The first patch is a fix: skb_copy_and_csum_bits() leaves stale bytes
in a buffer headed for the wire when it hits unreadable frags.  The
BUG_ON() conversion of the same function needs the same handling, so
it's here rather than sent separately.

The rest of the series converts 17 of the 19 BUG_ON()s in skbuff.c.
Each one becomes

	if (WARN_ON_ONCE(cond))
		<error path>;

where the error path is a failure return the function already has and
its callers already handle: -EINVAL from pskb_expand_head() and
skb_segment(), NULL from skb_copy(), 0 from skb_shift(), and so on.
Each patch says what its error path is and why it's safe.  Anybody who
wants the old behaviour, syzbot included, gets it with panic_on_warn.

A few don't have an obvious error return:

 - skb_copy_and_csum_bits() zeroes the part of the caller's buffer it
   couldn't fill instead of leaving stale bytes in it.
 - skb_copy_and_csum_dev() copies the frame without a checksum.  It
   also now catches a csum_start before the head and a csum_offset past
   the end of the frame, which the BUG_ON() missed.
 - skb_shift()'s second check ran after the shift had been committed.
   It moves up to just before the commit, where nothing has changed
   yet, and returns 0 there.

Two BUG_ON()s are left on purpose.  __pskb_pull_tail() and
skb_pull_rcsum() have callers that can't otherwise fail, so they don't
check the return.  Some of them would carry on and BUG() somewhere
else, or push back a pull that never happened.  Those need their
callers fixed first and will come as separate series.
skb_over_panic() and skb_under_panic() keep their BUG() as well; that's
overflow hardening and should stay fatal.

skbuff.o text on x86_64 defconfig grows by 114 bytes.  The fast path
takes the same branch it does today; the extra bytes are the error
paths that BUG() used to replace.

Testing: x86_64 defconfig with CONFIG_WERROR boots, and every patch
builds net/core/skbuff.o on its own with allmodconfig, W=1 and
CONFIG_DEBUG_NET.  A test module drives 12 of the 17 converted
BUG_ON()s with a bad argument or a malformed skb, skb_shift() through
a test-only export.  Each one warns once and returns its documented
error, and the skbs are left alone.  The same module covers the
unreadable-frags fix.  test_bpf's skb_segment tests pass.  The other
five were only reviewed: skb_crc32c() isn't built in defconfig, and
the four skb_segment() layout checks need a crafted frag_list.

Thanks,
Josef

---
Josef Bacik (8):
      net: skbuff: don't BUG() on bad arguments to pskb_expand_head()
      net: skbuff: don't BUG() on a bad frag_list layout in skb_segment()
      net: skbuff: don't BUG() when skb_copy_bits() fails in copy helpers
      net: skbuff: don't BUG() on leftover length in skb_checksum() and friends
      net: skbuff: don't BUG() on a bad csum_start in skb_copy_and_csum_dev()
      net: skbuff: don't BUG() on leftover length in skb_copy_and_csum_bits()
      net: skbuff: don't BUG() on a missing head_frag in skb_zerocopy()
      net: skbuff: remove the BUG_ON()s from skb_shift()

 net/core/skbuff.c | 118 +++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 98 insertions(+), 20 deletions(-)
---
base-commit: 45ad84d2800e4a092fb8d96006a533b2d0ab13f6
change-id: 20261006-b4-skbuff-bug-on-b2844a487925