The out-of-bounds access is reported by UBSAN:

[    0.000000] UBSAN: array-index-out-of-bounds in ../arch/riscv/kernel/vendor_extensions.c:41:66
[    0.000000] index -1 is out of range for type 'riscv_isavendorinfo [32]'
[    0.000000] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 6.11.0-rc2ubuntu-defconfig #2
[    0.000000] Hardware name: riscv-virtio,qemu (DT)
[    0.000000] Call Trace:
[    0.000000] [<ffffffff94e078ba>] dump_backtrace+0x32/0x40
[    0.000000] [<ffffffff95c83c1a>] show_stack+0x38/0x44
[    0.000000] [<ffffffff95c94614>] dump_stack_lvl+0x70/0x9c
[    0.000000] [<ffffffff95c94658>] dump_stack+0x18/0x20
[    0.000000] [<ffffffff95c8bbb2>] ubsan_epilogue+0x10/0x46
[    0.000000] [<ffffffff95485a82>] __ubsan_handle_out_of_bounds+0x94/0x9c
[    0.000000] [<ffffffff94e09442>] __riscv_isa_vendor_extension_available+0x90/0x92
[    0.000000] [<ffffffff94e043b6>] riscv_cpufeature_patch_func+0xc4/0x148
[    0.000000] [<ffffffff94e035f8>] _apply_alternatives+0x42/0x50
[    0.000000] [<ffffffff95e04196>] apply_boot_alternatives+0x3c/0x100
[    0.000000] [<ffffffff95e05b52>] setup_arch+0x85a/0x8bc
[    0.000000] [<ffffffff95e00ca0>] start_kernel+0xa4/0xfb6

The dereferencing using cpu should actually not happen, so remove it.

Fixes: 23c996fc ("riscv: Extend cpufeature.c to detect vendor extensions")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240814192619.276794-1-alexghiti@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>