[PATCH v3] riscv: entry: set a0 = -ENOSYS only when syscall != -1

[Guo Ren]

On Wed, Jul 19, 2023 at 5:01 AM Celeste Liu <coelacanthushex at gmail.com> wrote:
>
> When we test seccomp with 6.4 kernel, we found errno has wrong value.
> If we deny NETLINK_AUDIT with EAFNOSUPPORT, after f0bddf50586d, we will
> get ENOSYS instead. We got same result with commit 9c2598d43510 ("riscv: entry:
> Save a0 prior syscall_enter_from_user_mode()").
>
> After analysing code, we think that regs->a0 = -ENOSYS should only be executed
> when syscall != -1 In __seccomp_filter, when seccomp rejected this syscall with
> specified errno, they will set a0 to return number as syscall ABI, and then
> return -1. This return number is finally pass as return number of
> syscall_enter_from_user_mode, and then is compared with NR_syscalls after
> converted to ulong (so it will be ULONG_MAX). The condition
> syscall < NR_syscalls will always be false, so regs->a0 = -ENOSYS is always
> executed. It covered a0 set by seccomp, so we always get ENOSYS when match
> seccomp RET_ERRNO rule.
>
> Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
> Reported-by: Felix Yan <felixonmars at archlinux.org>
> Co-developed-by: Ruizhe Pan <c141028 at gmail.com>
> Signed-off-by: Ruizhe Pan <c141028 at gmail.com>
> Co-developed-by: Shiqi Zhang <shiqi at isrc.iscas.ac.cn>
> Signed-off-by: Shiqi Zhang <shiqi at isrc.iscas.ac.cn>
> Signed-off-by: Celeste Liu <CoelacanthusHex at gmail.com>
> Tested-by: Felix Yan <felixonmars at archlinux.org>
> ---
>
> v2 -> v3: use if-statement instead of set default value,
>           clarify the type of syscall
> v1 -> v2: added explanation on why always got ENOSYS
>
>  arch/riscv/kernel/traps.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index f910dfccbf5d2..5cef728745420 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -297,6 +297,10 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
>  asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>  {
>         if (user_mode(regs)) {
> +               /*
> +                * Convert negative numbers to very high and thus out of range
> +                * numbers for comparisons.
> +                */
>                 ulong syscall = regs->a7;
>
>                 regs->epc += 4;
> @@ -308,7 +312,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>
>                 if (syscall < NR_syscalls)
>                         syscall_handler(regs, syscall);
> -               else
> +               else if ((long)syscall != -1L)
Maybe we should define an explicit macro for this ERRNO in
__seccomp_filter, and this style obeys the coding convention.

For this patch:
Reviewed-by: Guo Ren <guoren at kernel.org>

Cc: loongarch guy, please check loongarch's code. :)

>                         regs->a0 = -ENOSYS;
>
>                 syscall_exit_to_user_mode(regs);
> --
> 2.41.0
>


-- 
Best Regards
 Guo Ren