[PATCH AUTOSEL for 4.15 119/124] signal/metag: Document a conflict with SI_USER with SIGFPE

Sasha Levin
2018-03-19T15:52:13Z

From: "Eric W. Biederman" <ebiederm@xmission.com>

[ Upstream commit b80328be53c215346b153769267b38f531d89b4f ]

Setting si_code to 0 results in a userspace seeing an si_code of 0.
This is the same si_code as SI_USER. Posix and common sense requires
that SI_USER not be a signal specific si_code. As such this use of 0
for the si_code is a pretty horribly broken ABI.

Further use of si_code == 0 guaranteed that copy_siginfo_to_user saw a
value of __SI_KILL and now sees a value of SIL_KILL with the result
hat uid and pid fields are copied and which might copying the si_addr
field by accident but certainly not by design. Making this a very
flakey implementation.

Utilizing FPE_FIXME siginfo_layout will now return SIL_FAULT and the
appropriate fields will reliably be copied.

Possible ABI fixes includee:
- Send the signal without siginfo
- Don't generate a signal
- Possibly assign and use an appropriate si_code
- Don't handle cases which can't happen

Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-metag@vger.kernel.org
Ref: ac919f0883e5 ("metag: Traps")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
arch/metag/include/uapi/asm/siginfo.h | 7 +++++++
arch/metag/kernel/traps.c | 2 +-
2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/metag/include/uapi/asm/siginfo.h b/arch/metag/include/uapi/asm/siginfo.h
index b54ef7186ca3..9a3f6cde9487 100644
--- a/arch/metag/include/uapi/asm/siginfo.h
+++ b/arch/metag/include/uapi/asm/siginfo.h
@@ -6,4 +6,11 @@

#include <asm-generic/siginfo.h>

+/*
+ * SIGFPE si_codes
+ */
+#ifdef __KERNEL__
+#define FPE_FIXME 0 /* Broken dup of SI_USER */
+#endif /* __KERNEL__ */
+
#endif
diff --git a/arch/metag/kernel/traps.c b/arch/metag/kernel/traps.c
index 444851e510d5..3b62b1b0c0b5 100644
--- a/arch/metag/kernel/traps.c
+++ b/arch/metag/kernel/traps.c
@@ -735,7 +735,7 @@ TBIRES fpe_handler(TBIRES State, int SigNum, int Triggers, int Inst, PTBI pTBI)
else if (error_state & TXSTAT_FPE_INEXACT_BIT)
info.si_code = FPE_FLTRES;
else
- info.si_code = 0;
+ info.si_code = FPE_FIXME;
info.si_errno = 0;
info.si_addr = (__force void __user *)regs->ctx.CurrPC;
force_sig_info(SIGFPE, &info, current);
--
2.14.1