[PATCH v9 3/6] signal: clear non-uapi flag bits when passing/returning sa_flags
Peter Collingbourne
pcc at google.com
Wed Aug 19 19:39:53 EDT 2020
On Wed, Aug 19, 2020 at 3:39 AM Dave Martin <Dave.Martin at arm.com> wrote:
>
> On Mon, Aug 17, 2020 at 08:33:48PM -0700, Peter Collingbourne wrote:
>
> Nit: please say what the patch does. Subject line should summarise
> what is done, but should not add new information that is not present in
> the description proper.
>
> (Same for all the other patches.)
Will do.
> > This allows userspace to detect missing support for flag bits and
> > allows the kernel to use non-uapi bits internally, as we are already
> > doing in arch/x86 for two flag bits. Now that this change is in
> > place, we no longer need the code in arch/x86 that was hiding these
> > bits from userspace, so remove it.
> >
> > Signed-off-by: Peter Collingbourne <pcc at google.com>
> > ---
> > View this change in Gerrit: https://linux-review.googlesource.com/q/I35aab6f5be932505d90f3b3450c083b4db1eca86
> >
> > arch/arm/include/asm/signal.h | 4 ++++
> > arch/parisc/include/asm/signal.h | 4 ++++
> > arch/x86/kernel/signal_compat.c | 7 -------
> > include/linux/signal_types.h | 12 ++++++++++++
> > kernel/signal.c | 10 ++++++++++
> > 5 files changed, 30 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/arm/include/asm/signal.h b/arch/arm/include/asm/signal.h
> > index 65530a042009..d1070a783993 100644
> > --- a/arch/arm/include/asm/signal.h
> > +++ b/arch/arm/include/asm/signal.h
> > @@ -17,6 +17,10 @@ typedef struct {
> > unsigned long sig[_NSIG_WORDS];
> > } sigset_t;
> >
> > +#define SA_UAPI_FLAGS \
> > + (SA_NOCLDSTOP | SA_NOCLDWAIT | SA_SIGINFO | SA_THIRTYTWO | \
> > + SA_RESTORER | SA_ONSTACK | SA_RESTART | SA_NODEFER | SA_RESETHAND)
> > +
>
> I wonder whether all these per-arch definitions will tend to bitrot when
> people add new common flags.
>
> Can we have a common definition for the common bits, and just add the
> extra arch-specific ones here?
I think so. We could have something like:
#define ARCH_UAPI_SA_FLAGS SA_THIRTYTWO
here. Then in signal_types.h we can do:
#ifndef ARCH_UAPI_SA_FLAGS
#define ARCH_UAPI_SA_FLAGS 0
#endif
#define UAPI_SA_FLAGS (... | ARCH_UAPI_SA_FLAGS)
I'll do that in v10.
> Also, I wonder whether we should avoid the "SA_" prefix here. Maybe
> UAPI_SA_FLAGS?
Sounds good to me.
>
> > #define __ARCH_HAS_SA_RESTORER
> >
> > #include <asm/sigcontext.h>
> > diff --git a/arch/parisc/include/asm/signal.h b/arch/parisc/include/asm/signal.h
> > index 715c96ba2ec8..ad06e14f6e8a 100644
> > --- a/arch/parisc/include/asm/signal.h
> > +++ b/arch/parisc/include/asm/signal.h
> > @@ -21,6 +21,10 @@ typedef struct {
> > unsigned long sig[_NSIG_WORDS];
> > } sigset_t;
> >
> > +#define SA_UAPI_FLAGS \
> > + (SA_ONSTACK | SA_RESETHAND | SA_NOCLDSTOP | SA_SIGINFO | SA_NODEFER | \
> > + SA_RESTART | SA_NOCLDWAIT | _SA_SIGGFAULT)
> > +
> > #include <asm/sigcontext.h>
> >
> > #endif /* !__ASSEMBLY */
> > diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
> > index 9ccbf0576cd0..c599013ae8cb 100644
> > --- a/arch/x86/kernel/signal_compat.c
> > +++ b/arch/x86/kernel/signal_compat.c
> > @@ -165,16 +165,9 @@ void sigaction_compat_abi(struct k_sigaction *act, struct k_sigaction *oact)
> > {
> > signal_compat_build_tests();
> >
> > - /* Don't leak in-kernel non-uapi flags to user-space */
> > - if (oact)
> > - oact->sa.sa_flags &= ~(SA_IA32_ABI | SA_X32_ABI);
> > -
> > if (!act)
> > return;
> >
> > - /* Don't let flags to be set from userspace */
> > - act->sa.sa_flags &= ~(SA_IA32_ABI | SA_X32_ABI);
> > -
> > if (in_ia32_syscall())
> > act->sa.sa_flags |= SA_IA32_ABI;
> > if (in_x32_syscall())
> > diff --git a/include/linux/signal_types.h b/include/linux/signal_types.h
> > index f8a90ae9c6ec..e792f29b5846 100644
> > --- a/include/linux/signal_types.h
> > +++ b/include/linux/signal_types.h
> > @@ -68,4 +68,16 @@ struct ksignal {
> > int sig;
> > };
> >
> > +#ifndef SA_UAPI_FLAGS
> > +#ifdef SA_RESTORER
> > +#define SA_UAPI_FLAGS \
> > + (SA_NOCLDSTOP | SA_NOCLDWAIT | SA_SIGINFO | SA_ONSTACK | SA_RESTART | \
> > + SA_NODEFER | SA_RESETHAND | SA_RESTORER)
> > +#else
> > +#define SA_UAPI_FLAGS \
> > + (SA_NOCLDSTOP | SA_NOCLDWAIT | SA_SIGINFO | SA_ONSTACK | SA_RESTART | \
> > + SA_NODEFER | SA_RESETHAND)
> > +#endif
> > +#endif
> > +
> > #endif /* _LINUX_SIGNAL_TYPES_H */
> > diff --git a/kernel/signal.c b/kernel/signal.c
> > index 42b67d2cea37..348b7981f1ff 100644
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -3984,6 +3984,16 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
> > if (oact)
> > *oact = *k;
> >
> > + /*
> > + * Clear unknown flag bits in order to allow userspace to detect missing
> > + * support for flag bits and to allow the kernel to use non-uapi bits
> > + * internally.
> > + */
> > + if (act)
> > + act->sa.sa_flags &= SA_UAPI_FLAGS;
> > + if (oact)
> > + oact->sa.sa_flags &= SA_UAPI_FLAGS;
> > +
>
> Seems reasonable.
Thanks. I also decided to check how other operating systems handle
unknown flag bits in sigaction.sa_flags. It looks like OpenBSD and
illumos also accept unknown bits but (implicitly, as a result of using
a different internal representation) end up clearing them in oldact:
https://github.com/openbsd/src/blob/f634a6a4b5bf832e9c1de77f7894ae2625e74484/sys/kern/kern_sig.c#L278
https://github.com/illumos/illumos-gate/blob/76f19f5fdc974fe5be5c82a556e43a4df93f1de1/usr/src/uts/common/syscall/sigaction.c#L86
and FreeBSD and NetBSD fail the syscall if unknown bits are set:
https://github.com/freebsd/freebsd/blob/eded70c37057857c6e23fae51f86b8f8f43cd2d0/sys/kern/kern_sig.c#L699
https://github.com/NetBSD/src/blob/3365779becdcedfca206091a645a0e8e22b2946e/sys/kern/sys_sig.c#L473
So there is some precedent for doing what we're planning to do here,
which makes it yet more likely that we'll be okay doing this.
Peter
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