Re: [PATCH] cifs: Fix encryption of cleared, but unset rq_iter data buffers

[Paulo Alcantara <pc@manguebit.com>]

David Howells <dhowells@redhat.com> writes:

> cifs: Fix encryption of cleared, but unset rq_iter data buffers
>
> Each smb_rqst struct contains two things: an array of kvecs (rq_iov) that
> contains the protocol data for an RPC op and an iterator (rq_iter) that
> contains the data payload of an RPC op.  When an smb_rqst is allocated
> rq_iter is it always cleared, but we don't set it up unless we're going to
> use it.
>
> The functions that determines the size of the ciphertext buffer that will
> be needed to encrypt a request, cifs_get_num_sgs(), assumes that rq_iter is
> always initialised - and employs user_backed_iter() to check that the
> iterator isn't user-backed.  This used to incidentally work, because
> ->user_backed was set to false because the iterator has never been
> initialised, but with commit f1b4cb650b9a0eeba206d8f069fcdc532bfbcd74[1]
> which changes user_backed_iter() to determine this based on the iterator
> type insted, a warning is now emitted:
>
>         WARNING: CPU: 7 PID: 4584 at fs/smb/client/cifsglob.h:2165 smb2_get_aead_req+0x3fc/0x420 [cifs]
>         ...
>         RIP: 0010:smb2_get_aead_req+0x3fc/0x420 [cifs]
>         ...
>          crypt_message+0x33e/0x550 [cifs]
>          smb3_init_transform_rq+0x27d/0x3f0 [cifs]
>          smb_send_rqst+0xc7/0x160 [cifs]
>          compound_send_recv+0x3ca/0x9f0 [cifs]
>          cifs_send_recv+0x25/0x30 [cifs]
>          SMB2_tcon+0x38a/0x820 [cifs]
>          cifs_get_smb_ses+0x69c/0xee0 [cifs]
>          cifs_mount_get_session+0x76/0x1d0 [cifs]
>          dfs_mount_share+0x74/0x9d0 [cifs]
>          cifs_mount+0x6e/0x2e0 [cifs]
>          cifs_smb3_do_mount+0x143/0x300 [cifs]
>          smb3_get_tree+0x15e/0x290 [cifs]
>          vfs_get_tree+0x2d/0xe0
>          do_new_mount+0x124/0x340
>          __se_sys_mount+0x143/0x1a0
>
> The problem is that rq_iter was never set, so the type is 0 (ie. ITER_UBUF)
> which causes user_backed_iter() to return true.  The code doesn't
> malfunction because it checks the size of the iterator - which is 0.
>
> Fix cifs_get_num_sgs() to ignore rq_iter if its count is 0, thereby
> bypassing the warnings.
>
> It might be better to explicitly initialise rq_iter to a zero-length
> ITER_BVEC, say, as it can always be reinitialised later.

Agreed.  We could probably set those in ->init_transform_rq().

> Fixes: d08089f649a0 ("cifs: Change the I/O paths to use an iterator rather than a page list")
> Reported-by: Damian Tometzki <damian@riscv-rocks.de>
> Link: https://lore.kernel.org/r/ZUfQo47uo0p2ZsYg@fedora.fritz.box/
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Steve French <smfrench@gmail.com>
> cc: Shyam Prasad N <sprasad@microsoft.com>
> cc: Rohith Surabattula <rohiths.msft@gmail.com>
> cc: Paulo Alcantara <pc@manguebit.com>
> cc: Namjae Jeon <linkinjeon@kernel.org>
> cc: Tom Talpey <tom@talpey.com>
> cc: Jeff Layton <jlayton@kernel.org>
> cc: Eric Biggers <ebiggers@kernel.org>
> cc: linux-cifs@vger.kernel.org
> cc: linux-fsdevel@vger.kernel.org
> Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f1b4cb650b9a0eeba206d8f069fcdc532bfbcd74 [1]
> ---
>  fs/smb/client/cifsglob.h |   12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)

FWIW, the patch fixes mounts of shares with encryption enabled.

Reviewed-by: Paulo Alcantara (SUSE) <pc@manguebit.com>