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

["Michal Suchánek" <msuchanek@suse.de>]

On Wed, Jul 01, 2026 at 09:41:57AM +0200, Michal Suchánek wrote:
> On Wed, Jul 01, 2026 at 11:57:00AM +0530, Mukesh Kumar Chaurasiya wrote:
> > On Wed, Jul 01, 2026 at 01:41:09AM +0530, Shrikanth Hegde wrote:
> > > Hi Mukesh.
> > > 
> > > On 6/29/26 11:59 PM, 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@suse.de>
> > > > Closes: https://lore.kernel.org/all/ajpp-_XnbF3UTM_E@kunlun.suse.cz/
> > > > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> > > > ---
> > > > 
> > > > v1 -> v2:
> > > >   - Fix issues in the previous fix (Michal)
> > > > v1: https://lore.kernel.org/all/20260624171520.772408-1-mkchauras@gmail.com
> > > > 
> > > >   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);
> > > > +
> > > 
> > > I see many arch first do syscall_enter_from_user_mode and then check for return value.
> > > take x86 for example,
> > > 
> > > __visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
> > > {
> > >         nr = syscall_enter_from_user_mode(regs, nr);
> > > 
> > >         if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr) && nr != -1) {
> > >                 /* Invalid system call, but still a system call. */
> > >                 regs->ax = __x64_sys_ni_syscall(regs);
> > >         }
> > > 
> > > }
> > > 
> > > So seccomp fails silently there if initial nr was -1?
> > > 
> > Hey,
> > 
> > No the -1 syscall ignores the error silently and returns 0.
> > 
> 
> There seems to be some inconsistency with the invalid syscalls.
> 
> Adapting the example from seccomp man page to ignore architecture I get
> on x86_64 (presumably with GENERIC_ENTRY since long ago):
> 
> ./a.out -2 55 /usr/bin/perl -MPOSIX -e '$!=0; my $r = syscall(-2, 0); print "ret=$r errno=".($!+0)." ($!)\n"'
> ret=-1 errno=55 (No anode)
> 
> but on ppc64le (with GENEREC_ENTRY):
> 
> ./a.out -2 55 /usr/bin/perl -MPOSIX -e '$!=0; my $r = syscall(-2, 0); print "ret=$r errno=".($!+0)." ($!)\n"'
> ret=-1 errno=38 (Function not implemented)
> 
> That said, behavior of seccomp on invalid syscalls is not particularly
> concerning. The tools that people typically use for constructing those
> filters typically require a valid syscall number.
> 
> It would be nice to align, though.

It is more concerning for SECCOMP_SET_MODE_STRICT or similar. So it
should be resolved to correctly execute seccomp even on invalid
syscalls. The syscall_enter_from_user_mode API is not particularly
well-suited for that, though.

Thanks

Michal