Re: [PATCH V5] powerpc/syscall: Fix syscall skip handling for seccomp and ptrace

On Mon, Jul 27, 2026 at 10:03:48AM +0200, Michal Suchánek wrote:
> Hello,
> 
> this should be probably adjusted to match
> 
> https://lore.kernel.org/linuxppc-dev/[email protected]/
> 
> Thanks
> 
> Michal
> 
Hey Michal,

Yeah, i am working towards this.

Regards,
Mukesh
> On Tue, Jul 14, 2026 at 01:29:35PM +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 <[email protected]>
> > Closes: https://lore.kernel.org/all/[email protected]/
> > Tested-by: Michal Suchánek <[email protected]>
> > Reviewed-by: Michal Suchánek <[email protected]>
> > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <[email protected]>
> > ---
> > v4 -> v5:
> >  - Fixed build failure where BITS macro is not available
> > v3 -> v4:
> >  - Use syscall_get_error (Michal)
> > v2 -> v3:
> >  - Last fix is not working for -1 syscall. Fixed that with this.
> > v2: https://lore.kernel.org/all/[email protected]
> > 
> > v1 -> v2:
> >  - Fix issues in the previous fix (Michal)
> > v1: https://lore.kernel.org/all/[email protected]
> > 
> > 
> >  arch/powerpc/include/asm/ptrace.h      | 23 ++++++++++++++++++++++-
> >  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, 52 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/ptrace.h b/arch/powerpc/include/asm/ptrace.h
> > index fdeb97421785..363a89787f1c 100644
> > --- a/arch/powerpc/include/asm/ptrace.h
> > +++ b/arch/powerpc/include/asm/ptrace.h
> > @@ -19,6 +19,7 @@
> >  #ifndef _ASM_POWERPC_PTRACE_H
> >  #define _ASM_POWERPC_PTRACE_H
> >  
> > +#include <linux/bits.h>
> >  #include <linux/err.h>
> >  #include <uapi/asm/ptrace.h>
> >  #include <asm/asm-const.h>
> > @@ -54,8 +55,9 @@ 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 +235,25 @@ 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..a43d3a9428cc 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 syscall_get_error(current, regs);
> > +
> >  	if (unlikely(r0 >= NR_syscalls)) {
> >  		if (unlikely(trap_is_unsupported_scv(regs))) {
> >  			/* Unsupported scv vector */
> > -- 
> > 2.55.0
> >