bpf, arm64: set up the frame pointer for the exception callback

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

bpf, arm64: set up the frame pointer for the exception callback

A program acting as exception boundary saves all callee-saved registers,
so build_prologue() takes the exception_cb path and never calls
push_callee_regs(). That is the only place find_used_callee_regs() runs,
and with it the only place ctx->fp_used is set, so the callback prologue
does not emit the

mov x25, sp

that points BPF_REG_FP at the frame the callback runs on. x25 keeps
whatever it held when bpf_throw() was called. If the throw came from a
subprogram that uses its own BPF stack, that is the subprogram's frame
pointer, and since the subprogram never returns it never restores x25
either.

Stack accesses through BPF_REG_FP are rewritten to be stack pointer
relative, so those still land in the callback's own frame. Materializing
the register does not: a callback that passes the address of a local
variable to a helper hands over an address in the dead subprogram's
frame. That address is below the callback's stack pointer by then, and
the helper's own call chain covers it, so the helper can write over its
own return address. 0x1234 below is the value the helper was asked to
store:

pc : 0x1234
lr : 0x1234
Call trace:
0x1234 (P)
bpf_test_run+0x188/0x3e0
bpf_prog_test_run_skb+0x47c/0x998
__sys_bpf+0xbdc/0xdd8
Kernel panic - not syncing: Oops: Fatal exception in interrupt

Set ctx->fp_used on the exception callback path so that the existing code
further down sets x25 from the stack pointer. The epilogue restores it
from the main program's save area along with the other callee-saved
registers, as it already does. x86 sets the frame pointer for the
callback from the argument it is passed, and powerpc computes it from
the stack pointer.