Re: [PATCH V2] powerpc/syscall: Fix seccomp errno handling with GENERIC_ENTRY

[Michal Suchánek]

On Mon, Jun 29, 2026 at 11:59:46PM +0530, Mukesh Kumar Chaurasiya (IBM) wrote:
> After enabling GENERIC_ENTRY on PowerPC, seccomp filters using
> SCMP_ACT_ERRNO without an explicit errnoRet value return ENOSYS
> (Function not implemented) instead of the expected EPERM (Operation
> not permitted).
> 
> The issue occurs in system_call_exception() when syscall_enter_from_user_mode()
> returns -1 to indicate the syscall should be skipped (e.g., blocked by seccomp).
> The current code treats this -1 as a syscall number and compares it against
> NR_syscalls. Since -1 is greater than NR_syscalls,
> the code incorrectly returns -ENOSYS, overwriting the errno that seccomp
> already set via syscall_set_return_value().
> 
> The generic entry code in syscall_trace_enter() calls __secure_computing(),
> which sets the appropriate errno in regs->gpr[3] and returns -1 to signal
> that the syscall should be skipped. However, the PowerPC syscall handler
> was not checking for this -1 return value before validating the syscall
> number.
> 
> Fix this by explicitly checking if syscall_enter_from_user_mode() returns
> -1 and returning the value already set in regs->gpr[3] (the errno from
> seccomp) before performing the syscall number validation.
> 
> Also Move the syscall_enter_from_user_mode() call and the seccomp/ptrace
> skip check to after the NR_syscalls bounds check.
> 
> When syscall -1 was passed, the r0 == -1L check would trigger before
> the NR_syscalls check, causing syscall_get_error() to return 0 instead
> of -ENOSYS. This resulted in a silent success (ret=0, errno=0) instead
> of the expected ENOSYS error.
> 
> By moving syscall_enter_from_user_mode() after the bounds check, an
> initial syscall number of -1 is correctly rejected with -ENOSYS first.
> The seccomp/ptrace skip path still works correctly for valid syscall
> numbers that get overridden to -1 by seccomp or ptrace.
> 
> This aligns PowerPC's behavior with other architectures using GENERIC_ENTRY
> and restores correct seccomp errno handling.
> 
> Fixes: bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
> Reported-by: Michal Suchánek <msuchanek@xxxxxxx>

Tested-by: Michal Suchánek <msuchanek@xxxxxxx>

Reviewed-by: Michal Suchánek <msuchanek@xxxxxxx>

Thanks

Michal

> Closes: https://lore.kernel.org/all/ajpp-_XnbF3UTM_E@xxxxxxxxxxxxxx/
> Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@xxxxxxxxx>
> ---
> 
> v1 -> v2:
>  - Fix issues in the previous fix (Michal)
> v1: https://lore.kernel.org/all/20260624171520.772408-1-mkchauras@xxxxxxxxx
> 
>  arch/powerpc/kernel/syscall.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index a9da2af6efa8..36d73933a311 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -20,7 +20,6 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
>  	syscall_fn f;
>  
>  	add_random_kstack_offset();
> -	r0 = syscall_enter_from_user_mode(regs, r0);
>  
>  	if (unlikely(r0 >= NR_syscalls)) {
>  		if (unlikely(trap_is_unsupported_scv(regs))) {
> @@ -31,6 +30,12 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
>  		return -ENOSYS;
>  	}
>  
> +	r0 = syscall_enter_from_user_mode(regs, r0);
> +
> +	/* Seccomp or ptrace may have set return value, skip syscall */
> +	if (unlikely(r0 == -1L))
> +		return syscall_get_error(current, regs);
> +
>  	/* May be faster to do array_index_nospec? */
>  	barrier_nospec();
>  
> -- 
> 2.54.0
>