From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.223.130]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 520073B3BF5 for ; Fri, 3 Jul 2026 09:54:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=195.135.223.130 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783072496; cv=none; b=PF7pXWA8fdXDPioabbpgVwCMi5LsweNpH796qtf8Nv9QOtW8fmJrRw75UTuPRB8N/iSu7eNxE/yO6YQhUtYbaTtZfT3lyxi6vXLxoLUkhOm0L5pqFHaCLSfmcTbnyuqmOOBcB4BRJ6xZXDTouJxNGrOQN6qzrjKxYBGiwrjHz4Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783072496; c=relaxed/simple; bh=kKTeRKlQNa0bkpPaNtId7lmdJr4H0+onp5rGiFODbtM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=SC0jP7DcM9HL/6ZxP6lwsM3U8BiHf+pyS+xZ61v47DfLpXipWj9TmjX9d9lG23JFJIu3wq+su/aJEj8pK6umIn/2HFSrVEWI79qcib8NcHyF7QtJUIj1AIQv7RJIDOAsiIIm5fyM1y1CkBxmRHWuZebWPyr2AsXoJ62Sy0jtOZg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=suse.de; spf=pass smtp.mailfrom=suse.de; arc=none smtp.client-ip=195.135.223.130 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=suse.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=suse.de Received: from kunlun.suse.cz (unknown [IPv6:2a07:de40:b306:2000::2]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (prime256v1) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id B85CF74879; Fri, 3 Jul 2026 09:54:44 +0000 (UTC) Authentication-Results: smtp-out1.suse.de; none Date: Fri, 3 Jul 2026 11:54:43 +0200 From: Michal =?iso-8859-1?Q?Such=E1nek?= To: Mukesh Kumar Chaurasiya Cc: oleg@redhat.com, maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com, chleroy@kernel.org, sshegde@linux.ibm.com, mchauras@linux.ibm.com, thuth@redhat.com, segher@kernel.crashing.org, ryan.roberts@arm.com, ruanjinjie@huawei.com, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH V3] powerpc/syscall: Fix syscall skip handling for seccomp and ptrace Message-ID: References: <20260703081100.1681924-1-mkchauras@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Pre-Result: action=no action; module=replies; Message is reply to one we originated X-Rspamd-Queue-Id: B85CF74879 X-Spam-Flag: NO X-Spam-Score: -2.00 X-Spam-Level: X-Rspamd-Action: no action X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Rspamd-Pre-Result: action=no action; module=replies; Message is reply to one we originated X-Spamd-Result: default: False [-2.00 / 50.00]; REPLY(-4.00)[]; RDNS_NONE(2.00)[] 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 > > > Closes: https://lore.kernel.org/all/ajpp-_XnbF3UTM_E@kunlun.suse.cz/ > > > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) > > > --- > > > 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 Reviewed-by: Michal Suchánek > > Regards, > Mukesh > > > + > > > if (unlikely(r0 >= NR_syscalls)) { > > > if (unlikely(trap_is_unsupported_scv(regs))) { > > > /* Unsupported scv vector */ > > > -- > > > 2.55.0 > > > >