[PATCH 7.1.y 1/6] bpf: Support for hardening against JIT spraying

[bot+bpf-ci at kernel.org]

> commit 96cce16e26dd02a8678f1e87f88a4b5cdb63b995 upstream.
> The BPF JIT allocator packs many small programs into larger executable
> allocations and reuses space within those allocations as programs are
> loaded and freed. When fresh code is written into space that a previous
> program occupied, an indirect jump into the new program can reuse a branch
> prediction left behind by the old one.
> Flush the indirect branch predictors before reusing JIT memory so that
> indirect jumps into a newly written program don't reuse predictions from an
> old program that occupied the same space.
> Introduce bpf_arch_pred_flush_enabled static key and bpf_arch_pred_flush
> static call for flushing the branch predictors on JIT memory reuse.
> Architectures that need a flush, can update it to a predictor flush
> function. By default, its a NOP and does not emit any CALL.
> Allocations larger than a pack are not covered by this flush. That is safe
> because cBPF programs (the unprivileged attack surface) are bounded well
> below a pack size. Issue a warning if this assumption is ever violated
> while the flush is active.
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta at linux.intel.com>
> Acked-by: Daniel Borkmann <daniel at iogearbox.net>
> Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>

The reuse path this patch hardens (bpf_prog_pack_alloc() finding a free
area within a pack, and the size > BPF_PROG_PACK_SIZE branch) comes from
the pack allocator that first packed many programs into shared executable
allocations and reused freed space within a pack.

Should this carry a Fixes: tag pointing at that commit?

  Fixes: 57631054fae6 ("bpf: Introduce bpf_prog_pack allocator")


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