Re: [PATCH] smb: client: fix OOB write when compounding SMB2_IOCTL with encryption

[Namjae Jeon]

On Sat, Oct 31, 2026 at 6:38 AM Andrey Misiurov
<andrey.misyrov@xxxxxxxxx> wrote:
>
> On an encrypted ("seal") mount, SMB2_ioctl_init() keeps the IOCTL input
> in a separate iov (iov[1]); iov[0] is a 448-byte buffer from
> cifs_small_buf_get().  When such an IOCTL is not the last request of a
> compound, smb2_set_next_command() pads it for alignment by copying the
> whole request into iov[0].  A large enough input overflows that 448-byte
> buffer and corrupts the slab; the copy also sets rq_nvec to 1, so the
> input buffer is leaked too.
>
> An unprivileged user can trigger it by creating a symlink with a long
> target on such a mount: the native symlink is sent as
> FSCTL_SET_REPARSE_POINT + QUERY_INFO + CLOSE in one compound, so the
> IOCTL is not the last request.
>
> With slub_debug red zones on the cifs_small_rq cache, the overflow is
> caught on the next allocation from that cache in the same compound (the
> 0x61 bytes are the 'a' characters of the symlink target):
>
>   BUG cifs_small_rq (Not tainted): Object corrupt
>   [Left Redzone overwritten] 0xffff8a59c7790500-0xffff8a59c779053f First byte 0x61 instead of 0xbb
>   Call Trace:
>    check_object+0x1a8/0x350
>    alloc_debug_processing+0xab/0x110
>    ___slab_alloc+0x7e/0x2f0
>    kmem_cache_alloc_noprof+0x28d/0x4b0
>    mempool_alloc_slab+0x16/0x30
>    cifs_small_buf_get+0x1a/0x30 [cifs]
>    SMB2_close_init+0x8a/0x100 [cifs]
>    smb2_compound_op+0x2571/0x2b50 [cifs]
>    smb2_create_reparse_inode+0x27e/0x490 [cifs]
>    mknod_nfs+0x1ce/0x2e0 [cifs]
>    create_reparse_symlink+0x119/0x180 [cifs]
>    cifs_symlink+0x19f/0x8e0 [cifs]
>    vfs_symlink+0xa3/0x1b0
>    __x64_sys_symlink+0x41/0x70
>
> Pad the input to a multiple of 8 bytes when encryption is required.  The
> request is then already aligned and smb2_set_next_command() no longer
> copies it; InputCount still carries the real length.
>
> Fixes: 90f7c100d2dd ("smb: client: fix compound alignment with encryption")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Andrey Misiurov <andrey.misyrov@xxxxxxxxx>
Reviewed-by: Namjae Jeon <linkinjeon@xxxxxxxxxx>
Thanks.