Commit 23acdc76 authored Nov 12, 2020 by Peter Collingbourne Committed by Eric W. Biederman Nov 23, 2020

signal: clear non-uapi flag bits when passing/returning sa_flags

Previously we were not clearing non-uapi flag bits in
sigaction.sa_flags when storing the userspace-provided sa_flags or
when returning them via oldact. Start doing so.

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.

This is technically a userspace-visible behavior change for sigaction, as
the unknown bits returned via oldact.sa_flags are no longer set. However,
we are free to define the behavior for unknown bits exactly because
their behavior is currently undefined, so for now we can define the
meaning of each of them to be "clear the bit in oldact.sa_flags unless
the bit becomes known in the future". Furthermore, this behavior is
consistent with OpenBSD [1], illumos [2] and XNU [3] (FreeBSD [4] and
NetBSD [5] fail the syscall if unknown bits are set). So there is some
precedent for this behavior in other kernels, and in particular in XNU,
which is probably the most popular kernel among those that I looked at,
which means that this change is less likely to be a compatibility issue.

Link: [1] https://github.com/openbsd/src/blob/f634a6a4b5bf832e9c1de77f7894ae2625e74484/sys/kern/kern_sig.c#L278
Link: [2] https://github.com/illumos/illumos-gate/blob/76f19f5fdc974fe5be5c82a556e43a4df93f1de1/usr/src/uts/common/syscall/sigaction.c#L86
Link: [3] https://github.com/apple/darwin-xnu/blob/a449c6a3b8014d9406c2ddbdc81795da24aa7443/bsd/kern/kern_sig.c#L480
Link: [4] https://github.com/freebsd/freebsd/blob/eded70c37057857c6e23fae51f86b8f8f43cd2d0/sys/kern/kern_sig.c#L699
Link: [5] https://github.com/NetBSD/src/blob/3365779becdcedfca206091a645a0e8e22b2946e/sys/kern/sys_sig.c#L473

Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://linux-review.googlesource.com/id/I35aab6f5be932505d90f3b3450c083b4db1eca86
Link: https://lkml.kernel.org/r/878dbcb5f47bc9b11881c81f745c0bef5c23f97f.1605235762.git.pcc@google.com

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

parent 1d82b789

arch/arm/include/asm/signal.h

+2 −0

Original line number	Diff line number	Diff line
		@@ -17,6 +17,8 @@ typedef struct {
		unsigned long sig[_NSIG_WORDS];
		} sigset_t;

		#define __ARCH_UAPI_SA_FLAGS (SA_THIRTYTWO \| SA_RESTORER)

		#define __ARCH_HAS_SA_RESTORER

		#include <asm/sigcontext.h>

+2 −0

Original line number	Diff line number	Diff line
		@@ -21,6 +21,8 @@ typedef struct {
		unsigned long sig[_NSIG_WORDS];
		} sigset_t;

		#define __ARCH_UAPI_SA_FLAGS _SA_SIGGFAULT

		#include <asm/sigcontext.h>

		#endif /* !__ASSEMBLY */

+0 −7

Original line number	Diff line number	Diff line
		@@ -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())

+12 −0

Original line number	Diff line number	Diff line
		@@ -68,4 +68,16 @@ struct ksignal {
		int sig;
		};

		#ifndef __ARCH_UAPI_SA_FLAGS
		#ifdef SA_RESTORER
		#define __ARCH_UAPI_SA_FLAGS SA_RESTORER
		#else
		#define __ARCH_UAPI_SA_FLAGS 0
		#endif
		#endif

		#define UAPI_SA_FLAGS \
		(SA_NOCLDSTOP \| SA_NOCLDWAIT \| SA_SIGINFO \| SA_ONSTACK \| SA_RESTART \| \
		SA_NODEFER \| SA_RESETHAND \| __ARCH_UAPI_SA_FLAGS)

		#endif /* _LINUX_SIGNAL_TYPES_H */

+10 −0

Original line number	Diff line number	Diff line
		@@ -3985,6 +3985,16 @@ 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 &= UAPI_SA_FLAGS;
		if (oact)
		oact->sa.sa_flags &= UAPI_SA_FLAGS;

		sigaction_compat_abi(act, oact);

		if (act) {