Re: [PATCH 5.4 29/33] net, sctp, filter: remap copy_from_user failure error

[Greg Kroah-Hartman]

On Fri, Jan 22, 2021 at 01:55:45PM -0300, Marcelo Ricardo Leitner wrote:
> On Fri, Jan 22, 2021 at 03:12:45PM +0100, Greg Kroah-Hartman wrote:
> > From: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > 
> > [ no upstream commit ]
> > 
> > Fix a potential kernel address leakage for the prerequisite where there is
> > a BPF program attached to the cgroup/setsockopt hook. The latter can only
> > be attached under root, however, if the attached program returns 1 to then
> > run the related kernel handler, an unprivileged program could probe for
> > kernel addresses that way. The reason this is possible is that we're under
> > set_fs(KERNEL_DS) when running the kernel setsockopt handler. Aside from
> > old cBPF there is also SCTP's struct sctp_getaddrs_old which contains
> > pointers in the uapi struct that further need copy_from_user() inside the
> > handler. In the normal case this would just return -EFAULT, but under a
> > temporary KERNEL_DS setting the memory would be copied and we'd end up at
> > a different error code, that is, -EINVAL, for both cases given subsequent
> > validations fail, which then allows the app to distinguish and make use of
> > this fact for probing the address space. In case of later kernel versions
> > this issue won't work anymore thanks to Christoph Hellwig's work that got
> > rid of the various temporary set_fs() address space overrides altogether.
> > One potential option for 5.4 as the only affected stable kernel with the
> > least complexity would be to remap those affected -EFAULT copy_from_user()
> > error codes with -EINVAL such that they cannot be probed anymore. Risk of
> > breakage should be rather low for this particular error case.
> > 
> > Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
> > Reported-by: Ryota Shiga (Flatt Security)
> > Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>
> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Cc: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> 
> For sctp bits,
> Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>

Thanks for the review!