Re: [PATCH -fixes] riscv: Fix out-of-bounds when accessing Andes per hart vendor extension array

[Charlie Jenkins <charlie@rivosinc.com>]

On Sun, Aug 11, 2024 at 05:02:29PM +0200, Alexandre Ghiti wrote:
> 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
> 
> This happens because we unconditionally use the cpu parameter to access
> this array. But if -1 is passed, that means we should not and we don't
> need to access this array, so simply prevent accessing the array in that case.
> 
> Fixes: 23c996fc2bc1 ("riscv: Extend cpufeature.c to detect vendor extensions")
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
>  arch/riscv/kernel/vendor_extensions.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/kernel/vendor_extensions.c b/arch/riscv/kernel/vendor_extensions.c
> index b6c1e7b5d34b..01dc79b1d17b 100644
> --- a/arch/riscv/kernel/vendor_extensions.c
> +++ b/arch/riscv/kernel/vendor_extensions.c
> @@ -27,7 +27,7 @@ const size_t riscv_isa_vendor_ext_list_size = ARRAY_SIZE(riscv_isa_vendor_ext_li
>   * @bit: bit position of the desired extension
>   * Return: true or false
>   *
> - * NOTE: When cpu is -1, will check if extension is available on all cpus
> + * NOTE: When cpu is VENDOR_EXT_ALL_CPUS, will check if extension is available on all cpus
>   */
>  bool __riscv_isa_vendor_extension_available(int cpu, unsigned long vendor, unsigned int bit)
>  {
> @@ -38,14 +38,15 @@ bool __riscv_isa_vendor_extension_available(int cpu, unsigned long vendor, unsig
>  	#ifdef CONFIG_RISCV_ISA_VENDOR_EXT_ANDES
>  	case ANDES_VENDOR_ID:
>  		bmap = &riscv_isa_vendor_ext_list_andes.all_harts_isa_bitmap;
> -		cpu_bmap = &riscv_isa_vendor_ext_list_andes.per_hart_isa_bitmap[cpu];
> +		if (cpu != VENDOR_EXT_ALL_CPUS)
> +			cpu_bmap = &riscv_isa_vendor_ext_list_andes.per_hart_isa_bitmap[cpu];
>  		break;
>  	#endif
>  	default:
>  		return false;
>  	}
>  
> -	if (cpu != -1)
> +	if (cpu != VENDOR_EXT_ALL_CPUS)
>  		bmap = &cpu_bmap[cpu];
>  
>  	if (bit >= RISCV_ISA_VENDOR_EXT_MAX)
> -- 
> 2.39.2
> 
> 
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The line that is setting the cpu_bmap shouldn't be indexing into it at
all. It is supposed to be:

cpu_bmap = &riscv_isa_vendor_ext_list_andes.per_hart_isa_bitmap;

The indexing is handled later on by the if-statement.

Thank you for looking into this.

- Charlie