CVE-2023-52920

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

bpf: support non-r10 register spill/fill to/from stack in precision tracking

Use instruction (jump) history to record instructions that performed

register spill/fill to/from stack, regardless if this was done through

read-only r10 register, or any other register after copying r10 into it

*and* potentially adjusting offset.

To make this work reliably, we push extra per-instruction flags into

instruction history, encoding stack slot index (spi) and stack frame

number in extra 10 bit flags we take away from prev_idx in instruction

history. We don't touch idx field for maximum performance, as it's

checked most frequently during backtracking.

This change removes basically the last remaining practical limitation of

precision backtracking logic in BPF verifier. It fixes known

deficiencies, but also opens up new opportunities to reduce number of

verified states, explored in the subsequent patches.

There are only three differences in selftests' BPF object files

according to veristat, all in the positive direction (less states).

File Program Insns (A) Insns (B) Insns (DIFF) States (A) States (B) States (DIFF)

-------------------------------------- ------------- --------- --------- ------------- ---------- ---------- -------------

test_cls_redirect_dynptr.bpf.linked3.o cls_redirect 2987 2864 -123 (-4.12%) 240 231 -9 (-3.75%)

xdp_synproxy_kern.bpf.linked3.o syncookie_tc 82848 82661 -187 (-0.23%) 5107 5073 -34 (-0.67%)

xdp_synproxy_kern.bpf.linked3.o syncookie_xdp 85116 84964 -152 (-0.18%) 5162 5130 -32 (-0.62%)

Note, I avoided renaming jmp_history to more generic insn_hist to

minimize number of lines changed and potential merge conflicts between

bpf and bpf-next trees.

Notice also cur_hist_entry pointer reset to NULL at the beginning of

instruction verification loop. This pointer avoids the problem of

relying on last jump history entry's insn_idx to determine whether we

already have entry for current instruction or not. It can happen that we

added jump history entry because current instruction is_jmp_point(), but

also we need to add instruction flags for stack access. In this case, we

don't want to entries, so we need to reuse last added entry, if it is

present.

Relying on insn_idx comparison has the same ambiguity problem as the one

that was fixed recently in [0], so we avoid that.

[0] https://patchwork.kernel.org/project/netdevbpf/patch/[email protected]/