Re: [PATCH v1] LoongArch: BPF: Fix off-by-one error for insn_is_cast_user

> LoongArch: BPF: Fix off-by-one error for insn_is_cast_user
>
> Fix a severe off-by-one error in the branch offset calculation for the
> user pointer cast helper insn_is_cast_user() inside build_insn().
>
> In the current code, an offset of 1 causes the "beq" to target the next
> "or" instruction, which means the "or" instruction is always executed,
> even if the pointer is NULL. Thus, a native NULL pointer is incorrectly
> combined with the arena base address and turns into a non-zero address,
> breaking nullable safety guards and causing silent memory corruption.
>
> Fix this by changing the branch offset from 1 to 2, which properly skips
> the "or" instruction and jumps directly to the "move_reg" instruction if
> the pointer is NULL, ensuring the destination register is safely cleared
> to 0.
>
> Fixes: 4fdb5dd8aeba ("LoongArch: BPF: Implement bpf_addr_space_cast instruction")
> Cc: [email protected]
> Signed-off-by: Tiezhu Yang <[email protected]>

This isn't a bug, but would it read more precisely to say the cast_user
JIT arm in build_insn() rather than "the user pointer cast helper
insn_is_cast_user()"?

insn_is_cast_user() in include/linux/filter.h is only the predicate that
recognises BPF_ALU64|BPF_MOV|BPF_X with off == BPF_ADDR_SPACE_CAST and
imm == 1U << 16; the cast itself is emitted by the JIT arm in build_insn().

Also, would describing the effect as "a zero arena offset no longer casts
to NULL" capture the same facts more concisely than "breaking nullable
safety guards and causing silent memory corruption"?


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