diff mbox series

[v16,4/6] signal: clear non-uapi flag bits when passing/returning sa_flags

Message ID 878dbcb5f47bc9b11881c81f745c0bef5c23f97f.1605235762.git.pcc@google.com (mailing list archive)
State New, archived
Headers show
Series arm64: expose FAR_EL1 tag bits in siginfo | expand

Commit Message

Peter Collingbourne Nov. 13, 2020, 2:53 a.m. UTC 
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
---
v10:
- rename SA_UAPI_FLAGS -> UAPI_SA_FLAGS
- refactor how we define it to avoid mentioning flags more
  than once

 arch/arm/include/asm/signal.h    |  2 ++
 arch/parisc/include/asm/signal.h |  2 ++
 arch/x86/kernel/signal_compat.c  |  7 -------
 include/linux/signal_types.h     | 12 ++++++++++++
 kernel/signal.c                  | 10 ++++++++++
 5 files changed, 26 insertions(+), 7 deletions(-)
diff mbox series

Patch

diff --git a/arch/arm/include/asm/signal.h b/arch/arm/include/asm/signal.h
index 65530a042009..430be7774402 100644
--- a/arch/arm/include/asm/signal.h
+++ b/arch/arm/include/asm/signal.h
@@ -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>
diff --git a/arch/parisc/include/asm/signal.h b/arch/parisc/include/asm/signal.h
index 715c96ba2ec8..30dd1e43ef88 100644
--- a/arch/parisc/include/asm/signal.h
+++ b/arch/parisc/include/asm/signal.h
@@ -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 */
diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
index a7f3e12cfbdb..ddfd919be46c 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..a7887ad84d36 100644
--- a/include/linux/signal_types.h
+++ b/include/linux/signal_types.h
@@ -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 */
diff --git a/kernel/signal.c b/kernel/signal.c
index ef8f2a28d37c..8f5bd12ee41b 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -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) {