[PATCH v4] riscv: entry: set a0 = -ENOSYS only when syscall != -1

[Emil Renner Berthing]

On Thu, 20 Jul 2023 at 16:05, 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>

With this patch I can use groupmod, groupadd, usermod, useradd etc.
inside systemd-nspawn again. Thanks!
Tested-by: Emil Renner Berthing <emil.renner.berthing at canonical.com>

> Reviewed-by: Björn Töpel <bjorn at rivosinc.com>
> Reviewed-by: Guo Ren <guoren at kernel.org>
> ---
>
> v3 -> v4: use long instead of ulong to reduce type cast and avoid
>           implementation-defined behavior, and make the judgment of syscall
>           invalid more explicit
> 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, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index f910dfccbf5d2..729f79c97e2bf 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -297,7 +297,7 @@ 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)) {
> -               ulong syscall = regs->a7;
> +               long syscall = regs->a7;
>
>                 regs->epc += 4;
>                 regs->orig_a0 = regs->a0;
> @@ -306,9 +306,9 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>
>                 syscall = syscall_enter_from_user_mode(regs, syscall);
>
> -               if (syscall < NR_syscalls)
> +               if (syscall >= 0 && syscall < NR_syscalls)
>                         syscall_handler(regs, syscall);
> -               else
> +               else if (syscall != -1)
>                         regs->a0 = -ENOSYS;
>
>                 syscall_exit_to_user_mode(regs);
> --
> 2.41.0
>
>
> _______________________________________________
> linux-riscv mailing list
> linux-riscv at lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv