linux-kernel - [PATCH 4.14 050/157] arm64: Fix syscall restarting around signal suppressed by tracer

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Message-Id: <20180701160855.155799663@linuxfoundation.org>
Date:   Sun,  1 Jul 2018 18:22:02 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Dave Martin <Dave.Martin@....com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: [PATCH 4.14 050/157] arm64: Fix syscall restarting around signal suppressed by tracer

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dave Martin <Dave.Martin@....com>

commit 0fe42512b2f03f9e5a20b9f55ef1013a68b4cd48 upstream.

Commit 17c2895 ("arm64: Abstract syscallno manipulation") abstracts
out the pt_regs.syscallno value for a syscall cancelled by a tracer
as NO_SYSCALL, and provides helpers to set and check for this
condition.  However, the way this was implemented has the
unintended side-effect of disabling part of the syscall restart
logic.

This comes about because the second in_syscall() check in
do_signal() re-evaluates the "in a syscall" condition based on the
updated pt_regs instead of the original pt_regs.  forget_syscall()
is explicitly called prior to the second check in order to prevent
restart logic in the ret_to_user path being spuriously triggered,
which means that the second in_syscall() check always yields false.

This triggers a failure in
tools/testing/selftests/seccomp/seccomp_bpf.c, when using ptrace to
suppress a signal that interrups a nanosleep() syscall.

Misbehaviour of this type is only expected in the case where a
tracer suppresses a signal and the target process is either being
single-stepped or the interrupted syscall attempts to restart via
-ERESTARTBLOCK.

This patch restores the old behaviour by performing the
in_syscall() check only once at the start of the function.

Fixes: 17c289586009 ("arm64: Abstract syscallno manipulation")
Signed-off-by: Dave Martin <Dave.Martin@....com>
Reported-by: Sumit Semwal <sumit.semwal@...aro.org>
Cc: Will Deacon <will.deacon@....com>
Cc: <stable@...r.kernel.org> # 4.14.x-
Signed-off-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/arm64/kernel/signal.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -676,11 +676,12 @@ static void do_signal(struct pt_regs *re
 	unsigned long continue_addr = 0, restart_addr = 0;
 	int retval = 0;
 	struct ksignal ksig;
+	bool syscall = in_syscall(regs);

 	/*
 	 * If we were from a system call, check for system call restarting...
 	 */
-	if (in_syscall(regs)) {
+	if (syscall) {
 		continue_addr = regs->pc;
 		restart_addr = continue_addr - (compat_thumb_mode(regs) ? 2 : 4);
 		retval = regs->regs[0];
@@ -732,7 +733,7 @@ static void do_signal(struct pt_regs *re
 	 * Handle restarting a different system call. As above, if a debugger
 	 * has chosen to restart at a different PC, ignore the restart.
 	 */
-	if (in_syscall(regs) && regs->pc == restart_addr) {
+	if (syscall && regs->pc == restart_addr) {
 		if (retval == -ERESTART_RESTARTBLOCK)
 			setup_restart_syscall(regs);
 		user_rewind_single_step(current);