Hello, this should be probably adjusted to match
https://lore.kernel.org/linuxppc-dev/[email protected]/ Thanks Michal 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 >
