Re: [PATCH V3] powerpc/syscall: Fix syscall skip handling for seccomp and ptrace

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

On Fri, Jul 03, 2026 at 02:06:41PM +0530, Mukesh Kumar Chaurasiya wrote:
> On Fri, Jul 03, 2026 at 10:33:44AM +0200, Michal Suchánek wrote:
> > On Fri, Jul 03, 2026 at 01:41:00PM +0530, Mukesh Kumar Chaurasiya (IBM) wrote:
> > > After enabling GENERIC_ENTRY on PowerPC, syscall_enter_from_user_mode()
> > > returns -1 as a sentinel to signal that seccomp or ptrace has intercepted
> > > the syscall and already set a return value via syscall_set_return_value().
> > > system_call_exception() was not handling this sentinel, and since -1UL
> > > is >= NR_syscalls, the code fell into the out-of-range path and returned
> > > -ENOSYS, overwriting the errno already placed in regs->gpr[3].
> > > 
> > > The naive fix of checking r0 == -1L before the NR_syscalls bounds check
> > > is ambiguous: a user legitimately calling syscall(-1) also produces r0 ==
> > > -1L, and a tracer intercepting such a call would have its injected return
> > > value silently discarded.
> > > 
> > > Fix this properly by introducing regs->entry_flags, a kernel-internal
> > > field in struct pt_regs (consuming one slot of the existing __pt_regs_pad
> > > so the ABI is preserved), with SYSCALL_ENTRY_RET_SET as an out-of-band
> > > flag. syscall_set_return_value() sets this flag whenever seccomp or ptrace
> > > injects a return value. system_call_exception() zeros entry_flags before
> > > calling syscall_enter_from_user_mode(), then checks and clears the flag
> > > afterwards: if set, it returns regs->gpr[3] directly regardless of what
> > > syscall number the user originally requested.
> > > 
> > > This handles all seccomp actions correctly:
> > > 
> > >   - SECCOMP_RET_ERRNO, SECCOMP_RET_TRACE (no tracer), SECCOMP_RET_USER_NOTIF:
> > >     all call syscall_set_return_value(), flag is set, injected value returned.
> > >   - SECCOMP_RET_TRAP, SECCOMP_RET_KILL: call syscall_rollback() and deliver
> > >     a signal; flag is not set, but the process is dying so the return value
> > >     is irrelevant.
> > > 
> > > The fix covers both ppc32 and ppc64 with no #ifdefs.
> > > 
> > > 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>
> > > ---
> > > v2 -> v3:
> > >  - Last fix is not working for -1 syscall. Fixed that with this.
> > > v2: https://lore.kernel.org/all/20260629182946.419552-1-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/include/asm/ptrace.h      | 22 +++++++++++++++++++++-
> > >  arch/powerpc/include/asm/syscall.h     |  6 ++++++
> > >  arch/powerpc/include/uapi/asm/ptrace.h |  6 ++++--
> > >  arch/powerpc/kernel/ptrace/ptrace.c    |  2 ++
> > >  arch/powerpc/kernel/syscall.c          | 18 ++++++++++++++++++
> > >  5 files changed, 51 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/arch/powerpc/include/asm/ptrace.h b/arch/powerpc/include/asm/ptrace.h
> > > index fdeb97421785..1a53d5cfa8db 100644
> > > --- a/arch/powerpc/include/asm/ptrace.h
> > > +++ b/arch/powerpc/include/asm/ptrace.h
> > > @@ -54,8 +54,9 @@ struct pt_regs
> > >  			};
> > >  			unsigned long result;
> > >  			unsigned long exit_flags;
> > > +			unsigned long entry_flags;
> > >  			/* Maintain 16 byte interrupt stack alignment */
> > > -			unsigned long __pt_regs_pad[3];
> > > +			unsigned long __pt_regs_pad[2];
> > >  		};
> > >  	};
> > >  #if defined(CONFIG_PPC64) || defined(CONFIG_PPC_KUAP)
> > > @@ -233,6 +234,25 @@ static inline unsigned long frame_pointer(struct pt_regs *regs)
> > >  #define current_pt_regs() \
> > >  	((struct pt_regs *)((unsigned long)task_stack_page(current) + THREAD_SIZE) - 1)
> > >  
> > > +/*
> > > + * SYSCALL_ENTRY_RET_SET: seccomp or ptrace called syscall_set_return_value()
> > > + * and wants the syscall skipped; regs->gpr[3] already holds the return value.
> > > + */
> > > +#define SYSCALL_ENTRY_RET_SET	BIT(0)
> > > +
> > > +static inline void set_syscall_entry_ret(struct pt_regs *regs)
> > > +{
> > > +	regs->entry_flags |= SYSCALL_ENTRY_RET_SET;
> > > +}
> > > +
> > > +static inline bool test_and_clear_syscall_entry_ret(struct pt_regs *regs)
> > > +{
> > > +	bool set = !!(regs->entry_flags & SYSCALL_ENTRY_RET_SET);
> > > +
> > > +	regs->entry_flags &= ~SYSCALL_ENTRY_RET_SET;
> > > +	return set;
> > > +}
> > > +
> > >  /*
> > >   * The 4 low bits (0xf) are available as flags to overload the trap word,
> > >   * because interrupt vectors have minimum alignment of 0x10. TRAP_FLAGS_MASK
> > > diff --git a/arch/powerpc/include/asm/syscall.h b/arch/powerpc/include/asm/syscall.h
> > > index 834fcc4f7b54..9ae79326abe3 100644
> > > --- a/arch/powerpc/include/asm/syscall.h
> > > +++ b/arch/powerpc/include/asm/syscall.h
> > > @@ -98,6 +98,12 @@ static inline void syscall_set_return_value(struct task_struct *task,
> > >  			regs->gpr[3] = val;
> > >  		}
> > >  	}
> > > +	/*
> > > +	 * Mark that a return value has been explicitly set by seccomp or
> > > +	 * ptrace so that system_call_exception() can skip the syscall
> > > +	 * unconditionally, even when the user requested syscall(-1).
> > > +	 */
> > > +	set_syscall_entry_ret(regs);
> > >  }
> > >  
> > >  static inline void syscall_get_arguments(struct task_struct *task,
> > > diff --git a/arch/powerpc/include/uapi/asm/ptrace.h b/arch/powerpc/include/uapi/asm/ptrace.h
> > > index a393b7f2760a..2f2a43414fe6 100644
> > > --- a/arch/powerpc/include/uapi/asm/ptrace.h
> > > +++ b/arch/powerpc/include/uapi/asm/ptrace.h
> > > @@ -56,7 +56,8 @@ struct pt_regs
> > >  	unsigned long dsisr;		/* on 4xx/Book-E used for ESR */
> > >  	unsigned long result;		/* Result of a system call */
> > >  	unsigned long exit_flags;	/* System call exit flags */
> > > -	unsigned long __pt_regs_pad[3];	/* Maintain 16 byte interrupt stack alignment */
> > > +	unsigned long entry_flags;	/* System call entry flags */
> > > +	unsigned long __pt_regs_pad[2];	/* Maintain 16 byte interrupt stack alignment */
> > >  };
> > >  
> > >  #endif /* __ASSEMBLER__ */
> > > @@ -117,7 +118,8 @@ struct pt_regs
> > >  #define PT_DSISR 42
> > >  #define PT_RESULT 43
> > >  #define PT_EXIT_FLAGS 44
> > > -#define PT_PAD 47 /* 3 times */
> > > +#define PT_ENTRY_FLAGS 45
> > > +#define PT_PAD 46 /* 2 times */
> > >  #define PT_DSCR 48
> > >  #define PT_REGS_COUNT 48
> > >  
> > > diff --git a/arch/powerpc/kernel/ptrace/ptrace.c b/arch/powerpc/kernel/ptrace/ptrace.c
> > > index 316d4f5ead8e..440d00690cf2 100644
> > > --- a/arch/powerpc/kernel/ptrace/ptrace.c
> > > +++ b/arch/powerpc/kernel/ptrace/ptrace.c
> > > @@ -235,6 +235,8 @@ void __init pt_regs_check(void)
> > >  		     offsetof(struct user_pt_regs, dsisr));
> > >  	BUILD_BUG_ON(offsetof(struct pt_regs, result) !=
> > >  		     offsetof(struct user_pt_regs, result));
> > > +	BUILD_BUG_ON(offsetof(struct pt_regs, entry_flags) !=
> > > +		     offsetof(struct user_pt_regs, entry_flags));
> > >  
> > >  	BUILD_BUG_ON(sizeof(struct user_pt_regs) > sizeof(struct pt_regs));
> > >  
> > > diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> > > index a9da2af6efa8..c7458aae199d 100644
> > > --- a/arch/powerpc/kernel/syscall.c
> > > +++ b/arch/powerpc/kernel/syscall.c
> > > @@ -19,9 +19,27 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
> > >  	long ret;
> > >  	syscall_fn f;
> > >  
> > > +	/*
> > > +	 * Zero entry_flags before syscall_enter_from_user_mode() so that
> > > +	 * syscall_set_return_value() can set SYSCALL_ENTRY_RET_SET as an
> > > +	 * unambiguous out-of-band signal.  The field is not initialised by
> > > +	 * the entry assembly.
> > > +	 */
> > > +	regs->entry_flags = 0;
> > >  	add_random_kstack_offset();
> > >  	r0 = syscall_enter_from_user_mode(regs, r0);
> > >  
> > > +	/*
> > > +	 * Seccomp or ptrace may have set a return value and requested that
> > > +	 * the syscall be skipped. syscall_set_return_value() sets
> > > +	 * SYSCALL_ENTRY_RET_SET in regs->entry_flags as an
> > > +	 * unambiguous out-of-band signal. This avoids the ambiguity of
> > > +	 * using r0 == -1 as the skip sentinel when the user themselves
> > > +	 * called syscall(-1).
> > > +	 */
> > > +	if (unlikely(test_and_clear_syscall_entry_ret(regs)))
> > > +		return regs->gpr[3];
> > 
> > Hello,
> > 
> > shouldn't this use the getter to correctly decode the value for both scv
> > and non-scv case?
> > 
> > Thanks
> > 
> > Michal
> > 
> Oh yeah,
> Correct. Lemme fix that.

Other than that it works as expected (note: a.out is the code adapted from
seccomp man page, attached at
https://lore.kernel.org/linuxppc-dev/akTExSO3ZT7iRtBa@kunlun.suse.cz/ )

kwork2:~ # ./a.out -1 55 /usr/bin/perl -MPOSIX -e '$!=0; my $r = syscall(-1, 0); print "ret=$r errno=".($!+0)." ($!)\n"'
ret=-1 errno=55 (No anode)
kwork2:~ # ./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)
kwork2:~ # ./a.out -2 55 /usr/bin/perl -MPOSIX -e '$!=0; my $r = syscall(-1, 0); print "ret=$r errno=".($!+0)." ($!)\n"'
ret=-1 errno=38 (Function not implemented)
kwork2:~ # podman run --rm --security-opt seccomp=<(echo '{"defaultAction":"SCMP_ACT_ALLOW","syscalls":[{"names":["mkdir"],"action":"SCMP_ACT_ERRNO"}]}') docker.io/library/alpine mkdir /tmp/test
mkdir: can't create directory '/tmp/test': Operation not permitted
kwork2:~ # podman run -it opensuse/leap groupadd -r www

All known problematic cases are covered.

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


> 
> Regards,
> Mukesh
> > > +
> > >  	if (unlikely(r0 >= NR_syscalls)) {
> > >  		if (unlikely(trap_is_unsupported_scv(regs))) {
> > >  			/* Unsupported scv vector */
> > > -- 
> > > 2.55.0
> > > 
>