Re: [PATCH bpf v2 1/2] bpf, x86: Fix per-CPU address resolution into an extended register

On Fri, 2026-08-14 at 13:32 -0700, Vineet Gupta wrote:
> The destination of the per-CPU address MOV is encoded in ModRM.reg,
> which is extended by REX.R, but the REX prefix is built with
> add_1mod(), which sets REX.B. REX.B extends ModRM.rm and SIB.base, and
> this instruction addresses memory as disp32 with no base, so the bit
> has no effect at all and the high register bit is simply lost.
> 
> Every is_ereg() destination therefore resolves to the wrong register,
> picking whichever one shares the low three bits:
> 
>   R5 -> RAX    R7 -> RBP    R8 -> RSI    R9 -> RDI
> 
> With BPF_REG_5, whose reg2hex is 0, the emitted
> 
>   65 49 03 04 25 <off>	add %gs:<off>,%rax
> 
> adds the per-CPU offset to RAX rather than R8. The destination keeps
> the unadjusted address and RAX is clobbered, so the program goes on to
> dereference a pointer that was never made per-CPU:
> 
>   BUG: unable to handle page fault for address: 0000607e386a8894
>   RIP: bpf_prog_707837aafd2aa9ae_update_percpu_data+0x93/0xc9
>   Call Trace:
>    __bpf_prog_test_run_raw_tp+0x2dc/0x7d0
>    __flush_smp_call_function_queue+0x1e9/0xc80
>   Kernel panic - not syncing: Fatal exception in interrupt
> 
> R5 is the mildest of the four, aliasing a scratch register and faulting
> at the store. R7 aliases RBP and would corrupt the frame pointer, R8
> and R9 alias the argument registers.
> 
> Use add_2mod() so the register goes through REX.R, matching how
> add_2reg() places it in ModRM.reg and how emit_priv_frame_ptr()
> hardcodes 0x4c for the same instruction with R9. Encodings for the
> non-extended registers are unchanged.
> 
> Problem showed up when trying to resurrect BPF_GCC CI (selftests built
> with BPF_GCC).
> 
> This has gone unnoticed because clang reloads the address into R1
> before each per-CPU access, so the destination is never an extended
> register. GCC keeps several per-CPU addresses live at once, and
> test_progs-bpf_gcc panics the kernel in global_percpu_data/init, where
> the address of a .percpu variable ends up in R5.
> 
> Fixes: 7bdbf7446305 ("bpf: add special internal-only MOV instruction to resolve per-CPU addrs")
> Cc: [email protected]
> Signed-off-by: Vineet Gupta <[email protected]>
> ---

Reviewed-by: Eduard Zingerman <[email protected]>

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