CVE-2026-89637smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2

Official description

In the Linux kernel, the following vulnerability has been resolved: smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2 When a valid primary TRANSACT2 response has been received (mid->resp_buf set, mid->multiRsp true) and a subsequent secondary response causes cifs_check_trans2() to return false -- either because the SMB header is invalid (malformed != 0) or because check2ndT2() rejects the PDU -- handle_mid() overwrites mid->resp_buf with the new buffer (leaking the primary buffer) and, because mid->multiRsp is set, skips the server->smallbuf/bigbuf NULL-out. When the user thread frees mid->resp_buf, server->smallbuf or server->bigbuf is left dangling; the demux thread reuses it for the next packet, resulting in a use-after-free. Combine both early-exit conditions and, when mid->multiRsp is already set, abort the pending transaction inline: set multiEnd, call dequeue_mid() with malformed=true, and return true so handle_mid() exits without touching mid->resp_buf or the server buffer pointers.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/9eed72e9534b10a6d9f8f5146feff3db53aebdba
• https://git.kernel.org/stable/c/5e6533a683f6a851158d9f33fb4ea8f4f25d7f84
• https://git.kernel.org/stable/c/730d0bb19507b9e19c2fe5343109ac618e2fbce5