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

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

[Mukesh Kumar Chaurasiya (IBM)]

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 (when cast to unsigned long) 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.

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>
Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
---
 arch/powerpc/kernel/syscall.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
index a9da2af6efa8..5b58c8d396c8 100644
--- a/arch/powerpc/kernel/syscall.c
+++ b/arch/powerpc/kernel/syscall.c
@@ -22,6 +22,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
 	add_random_kstack_offset();
 	r0 = syscall_enter_from_user_mode(regs, r0);
 
+	/* Seccomp or ptrace may have set return value, skip syscall */
+	if (unlikely(r0 == -1L))
+		return regs->gpr[3];
+
 	if (unlikely(r0 >= NR_syscalls)) {
 		if (unlikely(trap_is_unsupported_scv(regs))) {
 			/* Unsupported scv vector */
-- 
2.54.0

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

[Michal Suchánek]

On Wed, Jun 24, 2026 at 10:45:20PM +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 (when cast to unsigned long) 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.
> 
> 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>

Tested-by: Michal Suchánek <msuchanek@suse.de>

Thanks

Michal

> Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> ---
>  arch/powerpc/kernel/syscall.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index a9da2af6efa8..5b58c8d396c8 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -22,6 +22,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
>  	add_random_kstack_offset();
>  	r0 = syscall_enter_from_user_mode(regs, r0);
>  
> +	/* Seccomp or ptrace may have set return value, skip syscall */
> +	if (unlikely(r0 == -1L))
> +		return regs->gpr[3];
> +
>  	if (unlikely(r0 >= NR_syscalls)) {
>  		if (unlikely(trap_is_unsupported_scv(regs))) {
>  			/* Unsupported scv vector */
> -- 
> 2.54.0
> 

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

[Michal Suchánek]

Hello,

On Wed, Jun 24, 2026 at 10:45:20PM +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 (when cast to unsigned long) 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.
> 
> 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>
> Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> ---
>  arch/powerpc/kernel/syscall.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index a9da2af6efa8..5b58c8d396c8 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -22,6 +22,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
>  	add_random_kstack_offset();
>  	r0 = syscall_enter_from_user_mode(regs, r0);
>  
> +	/* Seccomp or ptrace may have set return value, skip syscall */
> +	if (unlikely(r0 == -1L))
> +		return regs->gpr[3];

this should use syscall_get_error instead of accessing the register
value directly. The error is represented differently in the scv and
non-scv case, and for non-scv case this will return wrong value.

Thanks

Michal

> +
>  	if (unlikely(r0 >= NR_syscalls)) {
>  		if (unlikely(trap_is_unsupported_scv(regs))) {
>  			/* Unsupported scv vector */
> -- 
> 2.54.0
> 

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

[Kees Cook]

On Wed, Jun 24, 2026 at 10:45:20PM +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).

Was this noticed by the seccomp_bpf kselftest? (If not, I'd love a
regression test added for this case...)

-Kees

-- 
Kees Cook

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

[Christophe Leroy (CS GROUP)]

Le 26/06/2026 à 09:50, Kees Cook a écrit :
> On Wed, Jun 24, 2026 at 10:45:20PM +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).
> 
> Was this noticed by the seccomp_bpf kselftest? (If not, I'd love a
> regression test added for this case...)
> 

Was reported here:

https://lore.kernel.org/all/ajpp-_XnbF3UTM_E@kunlun.suse.cz/

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

[Christophe Leroy (CS GROUP)]

Le 24/06/2026 à 19:15, Mukesh Kumar Chaurasiya (IBM) a écrit :
> 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 (when cast to unsigned long) 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.
> 
> 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>
> ---
>   arch/powerpc/kernel/syscall.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index a9da2af6efa8..5b58c8d396c8 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -22,6 +22,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
>   	add_random_kstack_offset();
>   	r0 = syscall_enter_from_user_mode(regs, r0);
>   
> +	/* Seccomp or ptrace may have set return value, skip syscall */
> +	if (unlikely(r0 == -1L))

Is it really needed to add the L after 1 ?

> +		return regs->gpr[3];
> +
>   	if (unlikely(r0 >= NR_syscalls)) {
>   		if (unlikely(trap_is_unsupported_scv(regs))) {
>   			/* Unsupported scv vector */

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

[Mukesh Kumar Chaurasiya]

On Fri, Jun 26, 2026 at 07:54:08AM +0200, Michal Suchánek wrote:
> Hello,
> 
> On Wed, Jun 24, 2026 at 10:45:20PM +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 (when cast to unsigned long) 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.
> > 
> > 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>
> > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> > ---
> >  arch/powerpc/kernel/syscall.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> > index a9da2af6efa8..5b58c8d396c8 100644
> > --- a/arch/powerpc/kernel/syscall.c
> > +++ b/arch/powerpc/kernel/syscall.c
> > @@ -22,6 +22,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
> >  	add_random_kstack_offset();
> >  	r0 = syscall_enter_from_user_mode(regs, r0);
> >  
> > +	/* Seccomp or ptrace may have set return value, skip syscall */
> > +	if (unlikely(r0 == -1L))
> > +		return regs->gpr[3];
> 
> this should use syscall_get_error instead of accessing the register
> value directly. The error is represented differently in the scv and
> non-scv case, and for non-scv case this will return wrong value.
> 
> Thanks
> 
> Michal
> 
Yeah, Thanks.

Will send out a new version.

Regards,
Mukesh
> > +
> >  	if (unlikely(r0 >= NR_syscalls)) {
> >  		if (unlikely(trap_is_unsupported_scv(regs))) {
> >  			/* Unsupported scv vector */
> > -- 
> > 2.54.0
> > 

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

[Mukesh Kumar Chaurasiya]

On Fri, Jun 26, 2026 at 04:31:47PM +0200, Christophe Leroy (CS GROUP) wrote:
> 
> 
> Le 24/06/2026 à 19:15, Mukesh Kumar Chaurasiya (IBM) a écrit :
> > 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 (when cast to unsigned long) 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.
> > 
> > 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>
> > ---
> >   arch/powerpc/kernel/syscall.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> > index a9da2af6efa8..5b58c8d396c8 100644
> > --- a/arch/powerpc/kernel/syscall.c
> > +++ b/arch/powerpc/kernel/syscall.c
> > @@ -22,6 +22,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
> >   	add_random_kstack_offset();
> >   	r0 = syscall_enter_from_user_mode(regs, r0);
> > +	/* Seccomp or ptrace may have set return value, skip syscall */
> > +	if (unlikely(r0 == -1L))
> 
> Is it really needed to add the L after 1 ?
> 
I guess it's not needed, but syscall_trace_enter is returning -1L, and i
wanted to keep it consistent to that.

Regards,
Mukesh
> > +		return regs->gpr[3];
> > +
> >   	if (unlikely(r0 >= NR_syscalls)) {
> >   		if (unlikely(trap_is_unsupported_scv(regs))) {
> >   			/* Unsupported scv vector */
> 

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

[Michal Suchánek]

Hello,

there is yet another bug identified.

When the initial syscall number is -1 the new condition bypasses setting
the ENOSYS below in if (unlikely(r0 >= NR_syscalls)) and returns 0.

perl -MPOSIX -e '$!=0; my $r = syscall(-1, 0); print "ret=$r errno=".($!+0)." ($!)\n"'

Normally the result is

ret=-1 errno=38 (Function not implemented)

but with this patch the result is

ret=0 errno=0 ()

fixup below.

On Wed, Jun 24, 2026 at 10:45:20PM +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 (when cast to unsigned long) 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.
> 
> 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>
> Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> ---
>  arch/powerpc/kernel/syscall.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index a9da2af6efa8..5b58c8d396c8 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -22,6 +22,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
	unsigned long r0_initial = r0;
>  	add_random_kstack_offset();
>  	r0 = syscall_enter_from_user_mode(regs, r0);
>  
> +	/* Seccomp or ptrace may have set return value, skip syscall */
> +	if (unlikely(r0 == -1L)
				&& (r0_initial != -1L))
> +		return regs->gpr[3];
> +
>  	if (unlikely(r0 >= NR_syscalls)) {
>  		if (unlikely(trap_is_unsupported_scv(regs))) {
>  			/* Unsupported scv vector */

Thanks

Michal

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

[Mukesh Kumar Chaurasiya]

On Mon, Jun 29, 2026 at 03:31:36PM +0200, Michal Suchánek wrote:
> Hello,
> 
> there is yet another bug identified.
> 
> When the initial syscall number is -1 the new condition bypasses setting
> the ENOSYS below in if (unlikely(r0 >= NR_syscalls)) and returns 0.
> 
> perl -MPOSIX -e '$!=0; my $r = syscall(-1, 0); print "ret=$r errno=".($!+0)." ($!)\n"'
> 
> Normally the result is
> 
> ret=-1 errno=38 (Function not implemented)
> 
> but with this patch the result is
> 
> ret=0 errno=0 ()
> 
> fixup below.
> 
> On Wed, Jun 24, 2026 at 10:45:20PM +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 (when cast to unsigned long) 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.
> > 
> > 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>
> > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> > ---
> >  arch/powerpc/kernel/syscall.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> > index a9da2af6efa8..5b58c8d396c8 100644
> > --- a/arch/powerpc/kernel/syscall.c
> > +++ b/arch/powerpc/kernel/syscall.c
> > @@ -22,6 +22,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
> 	unsigned long r0_initial = r0;
> >  	add_random_kstack_offset();
> >  	r0 = syscall_enter_from_user_mode(regs, r0);
> >  
> > +	/* Seccomp or ptrace may have set return value, skip syscall */
> > +	if (unlikely(r0 == -1L)
> 				&& (r0_initial != -1L))
> > +		return regs->gpr[3];
> > +
> >  	if (unlikely(r0 >= NR_syscalls)) {
> >  		if (unlikely(trap_is_unsupported_scv(regs))) {
> >  			/* Unsupported scv vector */
> 
> Thanks
> 
> Michal

What do you think about this diff?
This seems much cleaner.

diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
index a9da2af6efa8..a6c89052e8c5 100644
--- a/arch/powerpc/kernel/syscall.c
+++ b/arch/powerpc/kernel/syscall.c
@@ -20,8 +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))) {
                        /* Unsupported scv vector */
@@ -30,6 +28,11 @@ 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();

Regards,
Mukesh

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

[Michal Suchánek]

On Mon, Jun 29, 2026 at 10:32:47PM +0530, Mukesh Kumar Chaurasiya wrote:
> On Mon, Jun 29, 2026 at 03:31:36PM +0200, Michal Suchánek wrote:
> > Hello,
> > 
> > there is yet another bug identified.
> > 
> > When the initial syscall number is -1 the new condition bypasses setting
> > the ENOSYS below in if (unlikely(r0 >= NR_syscalls)) and returns 0.
> > 
> > perl -MPOSIX -e '$!=0; my $r = syscall(-1, 0); print "ret=$r errno=".($!+0)." ($!)\n"'
> > 
> > Normally the result is
> > 
> > ret=-1 errno=38 (Function not implemented)
> > 
> > but with this patch the result is
> > 
> > ret=0 errno=0 ()
> > 
> > fixup below.
> > 
> > On Wed, Jun 24, 2026 at 10:45:20PM +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 (when cast to unsigned long) 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.
> > > 
> > > 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>
> > > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> > > ---
> > >  arch/powerpc/kernel/syscall.c | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > > 
> > > diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> > > index a9da2af6efa8..5b58c8d396c8 100644
> > > --- a/arch/powerpc/kernel/syscall.c
> > > +++ b/arch/powerpc/kernel/syscall.c
> > > @@ -22,6 +22,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
> > 	unsigned long r0_initial = r0;
> > >  	add_random_kstack_offset();
> > >  	r0 = syscall_enter_from_user_mode(regs, r0);
> > >  
> > > +	/* Seccomp or ptrace may have set return value, skip syscall */
> > > +	if (unlikely(r0 == -1L)
> > 				&& (r0_initial != -1L))
> > > +		return regs->gpr[3];
> > > +
> > >  	if (unlikely(r0 >= NR_syscalls)) {
> > >  		if (unlikely(trap_is_unsupported_scv(regs))) {
> > >  			/* Unsupported scv vector */
> > 
> > Thanks
> > 
> > Michal
> 
> What do you think about this diff?
> This seems much cleaner.
> 
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index a9da2af6efa8..a6c89052e8c5 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -20,8 +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))) {
>                         /* Unsupported scv vector */
> @@ -30,6 +28,11 @@ 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);
>  

This will skip the check for NR_syscalls for whatever is returned from
syscall_enter_from_user_mode other than -1. To me it is not clear if
invalid syscall can be generated by one of the modifications done in
syscall_enter_from_user_mode.

Thanks

Michal

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

[Mukesh Kumar Chaurasiya]

On Mon, Jun 29, 2026 at 07:07:21PM +0200, Michal Suchánek wrote:
> On Mon, Jun 29, 2026 at 10:32:47PM +0530, Mukesh Kumar Chaurasiya wrote:
> > On Mon, Jun 29, 2026 at 03:31:36PM +0200, Michal Suchánek wrote:
> > > Hello,
> > > 
> > > there is yet another bug identified.
> > > 
> > > When the initial syscall number is -1 the new condition bypasses setting
> > > the ENOSYS below in if (unlikely(r0 >= NR_syscalls)) and returns 0.
> > > 
> > > perl -MPOSIX -e '$!=0; my $r = syscall(-1, 0); print "ret=$r errno=".($!+0)." ($!)\n"'
> > > 
> > > Normally the result is
> > > 
> > > ret=-1 errno=38 (Function not implemented)
> > > 
> > > but with this patch the result is
> > > 
> > > ret=0 errno=0 ()
> > > 
> > > fixup below.
> > > 
> > > On Wed, Jun 24, 2026 at 10:45:20PM +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 (when cast to unsigned long) 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.
> > > > 
> > > > 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>
> > > > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> > > > ---
> > > >  arch/powerpc/kernel/syscall.c | 4 ++++
> > > >  1 file changed, 4 insertions(+)
> > > > 
> > > > diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> > > > index a9da2af6efa8..5b58c8d396c8 100644
> > > > --- a/arch/powerpc/kernel/syscall.c
> > > > +++ b/arch/powerpc/kernel/syscall.c
> > > > @@ -22,6 +22,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
> > > 	unsigned long r0_initial = r0;
> > > >  	add_random_kstack_offset();
> > > >  	r0 = syscall_enter_from_user_mode(regs, r0);
> > > >  
> > > > +	/* Seccomp or ptrace may have set return value, skip syscall */
> > > > +	if (unlikely(r0 == -1L)
> > > 				&& (r0_initial != -1L))
> > > > +		return regs->gpr[3];
> > > > +
> > > >  	if (unlikely(r0 >= NR_syscalls)) {
> > > >  		if (unlikely(trap_is_unsupported_scv(regs))) {
> > > >  			/* Unsupported scv vector */
> > > 
> > > Thanks
> > > 
> > > Michal
> > 
> > What do you think about this diff?
> > This seems much cleaner.
> > 
> > diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> > index a9da2af6efa8..a6c89052e8c5 100644
> > --- a/arch/powerpc/kernel/syscall.c
> > +++ b/arch/powerpc/kernel/syscall.c
> > @@ -20,8 +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))) {
> >                         /* Unsupported scv vector */
> > @@ -30,6 +28,11 @@ 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);
> >  
> 
> This will skip the check for NR_syscalls for whatever is returned from
> syscall_enter_from_user_mode other than -1. To me it is not clear if
> invalid syscall can be generated by one of the modifications done in
> syscall_enter_from_user_mode.
> 
> Thanks
> 
> Michal

The possible return values for syscall_enter_from_user_mode are either
`gpr[0]` or -1. If we have an invalid syscall it'll be handled by the
`r0 >= NR_syscalls` check. I think functionally there should be no
issues.

Regards,
Mukesh