Re: [PATCH v5] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run

[Edward Cree]

On 15/06/2021 17:42, Kurt Manucredo wrote:
> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run()
> kernel/bpf/core.c:1414:2.
> 
> The shift-out-of-bounds happens when we have BPF_X. This means we have
> to go the same way we go when we want to avoid a divide-by-zero. We do
> it in do_misc_fixups().

Shifts by more than insn_bitness are legal in the eBPF ISA; they are
 implementation-defined behaviour, rather than UB, and have been made
 legal for performance reasons.  Each of the JIT backends compiles the
 eBPF shift operations to machine instructions which produce
 implementation-defined results in such a case; the resulting contents
 of the register may be arbitrary but program behaviour as a whole
 remains defined.
Guard checks in the fast path (i.e. affecting JITted code) will thus
 not be accepted.
The case of division by zero is not truly analogous, as division
 instructions on many of the JIT-targeted architectures will raise a
 machine exception / fault on division by zero, whereas (to the best of
 my knowledge) none will do so on an out-of-bounds shift.
(That said, it would be possible to record from the verifier division
 instructions in the program which are known never to be passed zero as
 divisor, and eliding the fixup patch in those cases.  However, the
 extra complexity may not be worthwhile.)

As I understand it, the UBSAN report is coming from the eBPF interpreter,
 which is the *slow path* and indeed on many production systems is
 compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON).
Perhaps a better approach to the fix would be to change the interpreter
 to compute "DST = DST << (SRC & 63);" (and similar for other shifts and
 bitnesses), thus matching the behaviour of most chips' shift opcodes.
This would shut up UBSAN, without affecting JIT code generation.

-ed