s390/bpf: Fix bpf_plt pointer arithmetic

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

s390/bpf: Fix bpf_plt pointer arithmetic

Kui-Feng Lee reported a crash on s390x triggered by the

dummy_st_ops/dummy_init_ptr_arg test [1]:

[<0000000000000002>] 0x2

[<00000000009d5cde>] bpf_struct_ops_test_run+0x156/0x250

[<000000000033145a>] __sys_bpf+0xa1a/0xd00

[<00000000003319dc>] __s390x_sys_bpf+0x44/0x50

[<0000000000c4382c>] __do_syscall+0x244/0x300

[<0000000000c59a40>] system_call+0x70/0x98

This is caused by GCC moving memcpy() after assignments in

bpf_jit_plt(), resulting in NULL pointers being written instead of

the return and the target addresses.

Looking at the GCC internals, the reordering is allowed because the

alias analysis thinks that the memcpy() destination and the assignments'

left-hand-sides are based on different objects: new_plt and

bpf_plt_ret/bpf_plt_target respectively, and therefore they cannot

alias.

This is in turn due to a violation of the C standard:

When two pointers are subtracted, both shall point to elements of the

same array object, or one past the last element of the array object

...

From the C's perspective, bpf_plt_ret and bpf_plt are distinct objects

and cannot be subtracted. In the practical terms, doing so confuses the

GCC's alias analysis.

The code was written this way in order to let the C side know a few

offsets defined in the assembly. While nice, this is by no means

necessary. Fix the noncompliance by hardcoding these offsets.

[1] https://lore.kernel.org/bpf/[email protected]/