CVE-2026-64448: smb: client: restrict implied bcc[0] exemption to responses without data area
Greg Kroah-Hartman <[email protected]> Sat, 25 Jul 2026 10:51:05 +0200
| Newsgroups | org.kernel.vger.linux-cve-announce |
|---|---|
| Message-ID | <2026072540-CVE-2026-64448-5817@gregkh> |
From: Greg Kroah-Hartman <[email protected]> Description =========== In the Linux kernel, the following vulnerability has been resolved: smb: client: restrict implied bcc[0] exemption to responses without data area smb2_check_message() has a long-standing quirk that accepts a response whose calculated length is one byte larger than the bytes actually received ("server can return one byte more due to implied bcc[0]"). This was introduced to accommodate servers that omit the trailing bcc[0] overlap byte when no data area is present. However, the exemption is applied unconditionally, regardless of whether the command actually carries a data area (has_smb2_data_area[]). When a response with a data area is subject to the +1 exemption, the reported data can extend one byte beyond the bytes actually received, yet smb2_check_message() still accepts it. The subsequent decoder then reads past the end of the receive buffer. This is reachable during NEGOTIATE and SESSION_SETUP, before the session is established. The resulting out-of-bounds reads are visible under KASAN when mounting against a non-conforming server; both the SPNEGO/negTokenInit and the NTLMSSP challenge decoders are affected: BUG: KASAN: slab-out-of-bounds in asn1_ber_decoder+0x16a7/0x1b00 Read of size 1 at addr ffff8880084d67c0 by task mount.cifs/81 CPU: 1 UID: 0 PID: 81 Comm: mount.cifs Not tainted 7.1.0-rc6 #1 Call Trace: <TASK> dump_stack_lvl+0x4e/0x70 print_report+0x157/0x4c9 kasan_report+0xce/0x100 asn1_ber_decoder+0x16a7/0x1b00 decode_negTokenInit+0x19/0x30 SMB2_negotiate+0x31d9/0x4c90 cifs_negotiate_protocol+0x1f2/0x3f0 cifs_get_smb_ses+0x93f/0x17e0 cifs_mount_get_session+0x7f/0x3a0 cifs_mount+0xb4/0xcf0 cifs_smb3_do_mount+0x23a/0x1500 smb3_get_tree+0x3b0/0x630 vfs_get_tree+0x82/0x2d0 fc_mount+0x10/0x1b0 path_mount+0x50d/0x1de0 __x64_sys_mount+0x20b/0x270 do_syscall_64+0xee/0x590 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK> Allocated by task 85: kmem_cache_alloc_noprof+0x106/0x380 mempool_alloc_noprof+0x116/0x1e0 cifs_small_buf_get+0x31/0x80 allocate_buffers+0x10d/0x2b0 cifs_demultiplex_thread+0x1d5/0x1d50 kthread+0x2c6/0x390 ret_from_fork+0x36e/0x5a0 ret_from_fork_asm+0x1a/0x30 The buggy address is located 0 bytes to the right of allocated 448-byte region [ffff8880084d6600, ffff8880084d67c0) which belongs to the cache cifs_small_rq of size 448 BUG: KASAN: slab-out-of-bounds in kmemdup_noprof+0x36/0x50 Read of size 329 at addr ffff88800726c678 by task mount.cifs/89 CPU: 0 UID: 0 PID: 89 Comm: mount.cifs Tainted: G B 7.1.0-rc6 #1 Call Trace: <TASK> dump_stack_lvl+0x4e/0x70 print_report+0x157/0x4c9 kasan_report+0xce/0x100 kasan_check_range+0x10f/0x1e0 __asan_memcpy+0x23/0x60 kmemdup_noprof+0x36/0x50 decode_ntlmssp_challenge+0x457/0x680 SMB2_sess_auth_rawntlmssp_negotiate+0x6f0/0xcb0 SMB2_sess_setup+0x219/0x4f0 cifs_setup_session+0x248/0xaf0 cifs_get_smb_ses+0xf79/0x17e0 cifs_mount_get_session+0x7f/0x3a0 cifs_mount+0xb4/0xcf0 cifs_smb3_do_mount+0x23a/0x1500 smb3_get_tree+0x3b0/0x630 vfs_get_tree+0x82/0x2d0 fc_mount+0x10/0x1b0 path_mount+0x50d/0x1de0 __x64_sys_mount+0x20b/0x270 do_syscall_64+0xee/0x590 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK> Allocated by task 93: kmem_cache_alloc_noprof+0x106/0x380 mempool_alloc_noprof+0x116/0x1e0 cifs_small_buf_get+0x31/0x80 allocate_buffers+0x10d/0x2b0 cifs_demultiplex_thread+0x1d5/0x1d50 kthread+0x2c6/0x390 ret_from_fork+0x36e/0x5a0 ret_from_fork_asm+0x1a/0x30 The buggy address is located 120 bytes inside of allocated 448-byte region [ffff88800726c600, ffff88800726c7c0) which belongs to the cache cifs_small_rq of size 448 Restrict the +1 exemption to responses that have no data area, so that it still covers the bcc[0] omission it was meant for. When a data area is present, the +1 discrepancy instead means the reported data length overruns the received buffer, so the response must be rejected. The Linux kernel CVE team has assigned CVE-2026-64448 to this issue. Affected and fixed versions =========================== Issue introduced in 3.6 with commit 093b2bdad3221e3fae3c26d89387e7297a157664 and fixed in 5.10.261 with commit 8d0bbc78046d264bbf6a574ea6f9072258a43e35 Issue introduced in 3.6 with commit 093b2bdad3221e3fae3c26d89387e7297a157664 and fixed in 5.15.212 with commit b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0 Issue introduced in 3.6 with commit 093b2bdad3221e3fae3c26d89387e7297a157664 and fixed in 6.1.178 with commit 31c6312608c60b72a1feb99a5afb680645a3e8a3 Issue introduced in 3.6 with commit 093b2bdad3221e3fae3c26d89387e7297a157664 and fixed in 6.6.145 with commit 573e502d14714d2947e22e7eff40ec20a6a44a42 Issue introduced in 3.6 with commit 093b2bdad3221e3fae3c26d89387e7297a157664 and fixed in 6.12.96 with commit 419ec1b604d7fb60c10aec2dc062371f9fcd4940 Issue introduced in 3.6 with commit 093b2bdad3221e3fae3c26d89387e7297a157664 and fixed in 6.18.39 with commit ceb875a375dedbf51c9425c1d13a2d7a8435c08c Issue introduced in 3.6 with commit 093b2bdad3221e3fae3c26d89387e7297a157664 and fixed in 7.1.4 with commit 6e9d10f62773b99bd927940fd9cbdfe7207e23ff Issue introduced in 3.6 with commit 093b2bdad3221e3fae3c26d89387e7297a157664 and fixed in 7.2-rc3 with commit 53b7c271f06be4dd5cfc8c6ef552a8355c891a7f Please see https://www.kernel.org for a full list of currently supported kernel versions by the kernel community. Unaffected versions might change over time as fixes are backported to older supported kernel versions. The official CVE entry at https://cve.org/CVERecord/?id=CVE-2026-64448 will be updated if fixes are backported, please check that for the most up to date information about this issue. Affected files ============== The file(s) affected by this issue are: fs/smb/client/smb2misc.c Mitigation ========== The Linux kernel CVE team recommends that you update to the latest stable kernel version for this, and many other bugfixes. Individual changes are never tested alone, but rather are part of a larger kernel release. Cherry-picking individual commits is not recommended or supported by the Linux kernel community at all. If however, updating to the latest release is impossible, the individual changes to resolve this issue can be found at these commits: https://git.kernel.org/stable/c/8d0bbc78046d264bbf6a574ea6f9072258a43e35 https://git.kernel.org/stable/c/b6a381c01e2ac98a48e32ac0f2a45bbadd9e26b0 https://git.kernel.org/stable/c/31c6312608c60b72a1feb99a5afb680645a3e8a3 https://git.kernel.org/stable/c/573e502d14714d2947e22e7eff40ec20a6a44a42 https://git.kernel.org/stable/c/419ec1b604d7fb60c10aec2dc062371f9fcd4940 https://git.kernel.org/stable/c/ceb875a375dedbf51c9425c1d13a2d7a8435c08c https://git.kernel.org/stable/c/6e9d10f62773b99bd927940fd9cbdfe7207e23ff https://git.kernel.org/stable/c/53b7c271f06be4dd5cfc8c6ef552a8355c891a7f