In the Linux kernel, the following vulnerability has been resolved: slip: reject VJ receive...

In the Linux kernel, the following vulnerability has been resolved:

slip: reject VJ receive packets on instances with no rstate array

slhc_init() accepts rslots == 0 as a valid configuration, with the
documented meaning of 'no receive compression'. In that case the
allocation loop in slhc_init() is skipped, so comp->rstate stays
NULL and comp->rslot_limit stays 0 (from the kzalloc of struct
slcompress).

The receive helpers do not defend against that configuration.
slhc_uncompress() dereferences comp->rstate[x] when the VJ header
carries an explicit connection ID, and slhc_remember() later assigns
cs = &comp->rstate[...] after only comparing the packet's slot number
to comp->rslot_limit. Because rslot_limit is 0, slot 0 passes the
range check, and the code dereferences a NULL rstate.

The configuration is reachable in-tree through PPP. PPPIOCSMAXCID
stores its argument in a signed int, and (val >> 16) uses arithmetic
shift. Passing 0xffff0000 therefore sign-extends to -1, so val2 + 1
is 0 and ppp_generic.c ends up calling slhc_init(0, 1). Because
/dev/ppp open is gated by ns_capable(CAP_NET_ADMIN), the whole path
is reachable from an unprivileged user namespace. Once the malformed
VJ state is installed, any inbound VJ-compressed or VJ-uncompressed
frame that selects slot 0 crashes the kernel in softirq context:

Oops: general protection fault, probably for non-canonical
address 0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
RIP: 0010:slhc_uncompress (drivers/net/slip/slhc.c:519)
Call Trace:
<TASK>
ppp_receive_nonmp_frame (drivers/net/ppp/ppp_generic.c:2466)
ppp_input (drivers/net/ppp/ppp_generic.c:2359)
ppp_async_process (drivers/net/ppp/ppp_async.c:492)
tasklet_action_common (kernel/softirq.c:926)
handle_softirqs (kernel/softirq.c:623)
run_ksoftirqd (kernel/softirq.c:1055)
smpboot_thread_fn (kernel/smpboot.c:160)
kthread (kernel/kthread.c:436)
ret_from_fork (arch/x86/kernel/process.c:164)
</TASK>

Reject the receive side on such instances instead of touching rstate.
slhc_uncompress() falls through to its existing 'bad' label, which
bumps sls_i_error and enters the toss state. slhc_remember() mirrors
that with an explicit sls_i_error increment followed by slhc_toss();
the sls_i_runt counter is not used here because a missing rstate is
an internal configuration state, not a runt packet.

The transmit path is unaffected: the only in-tree caller that picks
rslots from userspace (ppp_generic.c) still supplies tslots >= 1, and
slip.c always calls slhc_init(16, 16), so comp->tstate remains valid
and slhc_compress() continues to work.