atm: lec: fix null-ptr-deref in lec_arp_clear_vccs

Overview

In the Linux kernel, the following vulnerability has been resolved:

atm: lec: fix null-ptr-deref in lec_arp_clear_vccs

syzkaller reported a null-ptr-deref in lec_arp_clear_vccs(). This issue can be easily reproduced using the syzkaller reproducer.

In the ATM LANE (LAN Emulation) module, the same atm_vcc can be shared by multiple lec_arp_table entries (e.g., via entry->vcc or entry->recv_vcc). When the underlying VCC is closed, lec_vcc_close() iterates over all ARP entries and calls lec_arp_clear_vccs() for each matched entry.

For example, when lec_vcc_close() iterates through the hlists in priv->lec_arp_empty_ones or other ARP tables:

1. In the first iteration, for the first matched ARP entry sharing the VCC, lec_arp_clear_vccs() frees the associated vpriv (which is vcc->user_back) and sets vcc->user_back to NULL.
2. In the second iteration, for the next matched ARP entry sharing the same VCC, lec_arp_clear_vccs() is called again. It obtains a NULL vpriv from vcc->user_back (via LEC_VCC_PRIV(vcc)) and then attempts to dereference it via vcc->pop = vpriv->old_pop, leading to a null-ptr-deref crash.

Fix this by adding a null check for vpriv before dereferencing it. If vpriv is already NULL, it means the VCC has been cleared by a previous call, so we can safely skip the cleanup and just clear the entry's vcc/recv_vcc pointers.

The entire cleanup block (including vcc_release_async()) is placed inside the vpriv guard because a NULL vpriv indicates the VCC has already been fully released by a prior iteration — repeating the teardown would redundantly set flags and trigger callbacks on an already-closing socket.

The Fixes tag points to the initial commit because the entry->vcc path has been vulnerable since the original code. The entry->recv_vcc path was later added by commit 8d9f73c0ad2f ("atm: fix a memory leak of vcc->user_back") with the same pattern, and both paths are fixed here.

Affected

• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 8aff65a82b6389ec674d46e5b3d3ae6f07db5e3e
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 30c9744a989feb22cfbb84170eb0e038a7a2c1da
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < e9665986eb127290ceb535bd5d04d7a84265d94f
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 622062f24644b4536d3f437e0cf7a8c4bb421665
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 2d9f57ea29a1f1772373b98a509b44d49fda609e
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 7ea92ab075d809ec8a96669a5ecf00f752057875
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 5f1cfea7921f5c126a441d973690eeba52677b64
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 101bacb303e89dc2e0640ae6a5e0fb97c4eb45bb
• Linux 2.6.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.