signal: define the SA_EXPOSE_TAGBITS bit in sa_flags

Architectures that support address tagging, such as arm64, may want to
expose fault address tag bits to the signal handler to help diagnose
memory errors. However, these bits have not been previously set,
and their presence may confuse unaware user applications. Therefore,
introduce a SA_EXPOSE_TAGBITS flag bit in sa_flags that a signal
handler may use to explicitly request that the bits are set.

The generic signal handler APIs expect to receive tagged addresses.
Architectures may specify how to untag addresses in the case where
SA_EXPOSE_TAGBITS is clear by defining the arch_untagged_si_addr
function.

Signed-off-by: Peter Collingbourne <[email protected]>
Acked-by: "Eric W. Biederman" <[email protected]>
Link: https://linux-review.googlesource.com/id/I16dd0ed2081f091fce97be0190cb8caa874c26cb
Link: https://lkml.kernel.org/r/13cf24d00ebdd8e1f55caf1821c7c29d54100191.1605904350.git.pcc@google.com
Signed-off-by: Eric W. Biederman <[email protected]>

diff --git a/include/linux/signal.h b/include/linux/signal.h
index b256f9c65661cca7a5314af58aadac76b2a10f07..205526c4003aa10dba00560af3820b748928afe2 100644 (file)
--- a/include/linux/signal.h
+++ b/include/linux/signal.h
@@ -469,4 +469,18 @@ struct seq_file;
 extern void render_sigset_t(struct seq_file *, const char *, sigset_t *);
 #endif
 
+#ifndef arch_untagged_si_addr
+/*
+ * Given a fault address and a signal and si_code which correspond to the
+ * _sigfault union member, returns the address that must appear in si_addr if
+ * the signal handler does not have SA_EXPOSE_TAGBITS enabled in sa_flags.
+ */
+static inline void __user *arch_untagged_si_addr(void __user *addr,
+                                                unsigned long sig,
+                                                unsigned long si_code)
+{
+       return addr;
+}
+#endif
+
 #endif /* _LINUX_SIGNAL_H */

diff --git a/include/linux/signal_types.h b/include/linux/signal_types.h
index a7887ad84d36ea28bae36647406399731a22b834..68e06c75c5b22ac9d8d5e5c5c45792b60f5d4958 100644 (file)
--- a/include/linux/signal_types.h
+++ b/include/linux/signal_types.h
@@ -78,6 +78,6 @@ struct ksignal {
 
 #define UAPI_SA_FLAGS                                                          \
        (SA_NOCLDSTOP | SA_NOCLDWAIT | SA_SIGINFO | SA_ONSTACK | SA_RESTART |  \
-        SA_NODEFER | SA_RESETHAND | __ARCH_UAPI_SA_FLAGS)
+        SA_NODEFER | SA_RESETHAND | SA_EXPOSE_TAGBITS | __ARCH_UAPI_SA_FLAGS)
 
 #endif /* _LINUX_SIGNAL_TYPES_H */

diff --git a/include/uapi/asm-generic/signal-defs.h b/include/uapi/asm-generic/signal-defs.h
index c790f67304ba864c0d4b10e9b529fc50b907357b..fe929e7b77ca12fc19b84716331293109278d348 100644 (file)
--- a/include/uapi/asm-generic/signal-defs.h
+++ b/include/uapi/asm-generic/signal-defs.h
@@ -20,6 +20,8 @@
  * so this bit allows flag bit support to be detected from userspace while
  * allowing an old kernel to be distinguished from a kernel that supports every
  * flag bit.
+ * SA_EXPOSE_TAGBITS exposes an architecture-defined set of tag bits in
+ * siginfo.si_addr.
  *
  * SA_ONESHOT and SA_NOMASK are the historical Linux names for the Single
  * Unix names RESETHAND and NODEFER respectively.
@@ -41,6 +43,7 @@
 /* 0x00000100 used on sparc */
 /* 0x00000200 used on sparc */
 #define SA_UNSUPPORTED 0x00000400
+#define SA_EXPOSE_TAGBITS      0x00000800
 /* 0x00010000 used on mips */
 /* 0x01000000 used on x86 */
 /* 0x02000000 used on x86 */

diff --git a/kernel/signal.c b/kernel/signal.c
index 8f34819e80de1cdd692dfeec946a568ac5021464..26018c59821dfb7ff35714747dd22ef286289c2c 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2524,6 +2524,26 @@ static int ptrace_signal(int signr, kernel_siginfo_t *info)
        return signr;
 }
 
+static void hide_si_addr_tag_bits(struct ksignal *ksig)
+{
+       switch (siginfo_layout(ksig->sig, ksig->info.si_code)) {
+       case SIL_FAULT:
+       case SIL_FAULT_MCEERR:
+       case SIL_FAULT_BNDERR:
+       case SIL_FAULT_PKUERR:
+               ksig->info.si_addr = arch_untagged_si_addr(
+                       ksig->info.si_addr, ksig->sig, ksig->info.si_code);
+               break;
+       case SIL_KILL:
+       case SIL_TIMER:
+       case SIL_POLL:
+       case SIL_CHLD:
+       case SIL_RT:
+       case SIL_SYS:
+               break;
+       }
+}
+
 bool get_signal(struct ksignal *ksig)
 {
        struct sighand_struct *sighand = current->sighand;
@@ -2761,6 +2781,10 @@ relock:
        spin_unlock_irq(&sighand->siglock);
 
        ksig->sig = signr;
+
+       if (!(ksig->ka.sa.sa_flags & SA_EXPOSE_TAGBITS))
+               hide_si_addr_tag_bits(ksig);
+
        return ksig->sig > 0;
 }