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From:   Mao Bibo <maobibo@loongson.cn>
To:     Huacai Chen <chenhuacai@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Christian Brauner <brauner@kernel.org>
Cc:     WANG Xuerui <kernel@xen0n.name>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        loongarch@lists.linux.dev, linux-kernel@vger.kernel.org,
        Bibo Mao <maobibo@loongson.cn>
Subject: [PATCH v3] LoongArch: Add safer signal handler for TLS access
Date:   Fri,  2 Sep 2022 17:59:58 +0800
Message-Id: <20220902095958.3875126-1-maobibo@loongson.cn>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

LoongArch uses general purpose register R2 as thread pointer(TP)
register, signal hanlder also uses TP register to access variables
in TLS area, such as errno and variable in TLS.

If GPR R2 is modified with wrong value, signal handler still uses
the wrong TP register, so signal hanlder is insafe to access TLS
variable.

This patch adds one arch specific syscall set_thread_area, and
restore previoud TP value before signal handler, so that signal
handler is safe to access TLS variable.

It passes to run with the following test case.
=======8<======
 #define _GNU_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <string.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
 #include <signal.h>
 #include <pthread.h>
 #include <asm/ucontext.h>
 #include <asm/sigcontext.h>

 #define ILL_INSN ".word 0x000001f0"
static inline long test_sigill(unsigned long fid)
{
        register long ret __asm__("$r4");
        register unsigned long fun __asm__("$r4") = fid;

        __asm__ __volatile__("move $r2, $r0 \r\n");
        __asm__ __volatile__(
                        ILL_INSN
                        : "=r" (ret)
                        : "r" (fun)
                        : "memory"
                        );

        return ret;
}

static void set_sigill_handler(void (*fn) (int, siginfo_t *, void *))
{
        struct sigaction sa;
        memset(&sa, 0, sizeof(struct sigaction));

        sa.sa_sigaction = fn;
        sa.sa_flags = SA_SIGINFO;
        sigemptyset(&sa.sa_mask);
        if (sigaction(SIGILL, &sa, 0) != 0) {
                perror("sigaction");
        }
}

void catch_sig(int sig, siginfo_t *si, void *vuc)
{
        struct ucontext *uc = vuc;
        register unsigned long tls  __asm__("$r2");

        uc->uc_mcontext.sc_pc +=4;
        uc->uc_mcontext.sc_regs[2] = tls;
        printf("catched signal %d\n", sig);
}

void *print_message_function( void *ptr )
{
        char *message;
        message = (char *) ptr;
        printf("%s \n", message);
        test_sigill(1);
}

void pthread_test(void)
{
        pthread_t thread1, thread2;
        char *message1 = "Thread 1";
        char *message2 = "Thread 2";
        int  iret1, iret2;

        iret1 = pthread_create( &thread1, NULL, print_message_function,
				(void*) message1);
        iret2 = pthread_create( &thread2, NULL, print_message_function,
				(void*) message2);
        pthread_join( thread1, NULL);
        pthread_join( thread2, NULL);
        printf("Thread 1 returns: %d\n",iret1);
        printf("Thread 2 returns: %d\n",iret2);
        exit(0);
}

void exec_test(void) {
        test_sigill(1);
}

void main() {
        register unsigned long tls  __asm__("$r2");
        int val;

        val = syscall(244, tls);
        set_sigill_handler(&catch_sig);
        pthread_test();
        //exec_test();
        return;
}
=======8<======

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
---
v2->v3:
 - Use current_thread_info rather than task_thread_info(current)
v1->v2:
 - Clear TP value in clone function if CLONE_SETTLS is not set
---
 arch/loongarch/include/asm/unistd.h      |  1 +
 arch/loongarch/include/uapi/asm/unistd.h |  2 ++
 arch/loongarch/kernel/process.c          | 10 +++++++++-
 arch/loongarch/kernel/signal.c           |  5 +++++
 arch/loongarch/kernel/syscall.c          |  9 +++++++++
 5 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/arch/loongarch/include/asm/unistd.h b/arch/loongarch/include/asm/unistd.h
index cfddb0116a8c..1581624f0115 100644
--- a/arch/loongarch/include/asm/unistd.h
+++ b/arch/loongarch/include/asm/unistd.h
@@ -9,3 +9,4 @@
 #include <uapi/asm/unistd.h>
 
 #define NR_syscalls (__NR_syscalls)
+__SYSCALL(__NR_set_thread_area, sys_set_thread_area)
diff --git a/arch/loongarch/include/uapi/asm/unistd.h b/arch/loongarch/include/uapi/asm/unistd.h
index b344b1f91715..ceb8c4cec505 100644
--- a/arch/loongarch/include/uapi/asm/unistd.h
+++ b/arch/loongarch/include/uapi/asm/unistd.h
@@ -4,3 +4,5 @@
 #define __ARCH_WANT_SYS_CLONE3
 
 #include <asm-generic/unistd.h>
+
+#define __NR_set_thread_area	(__NR_arch_specific_syscall + 0)
diff --git a/arch/loongarch/kernel/process.c b/arch/loongarch/kernel/process.c
index 660492f064e7..488bbe2e8c99 100644
--- a/arch/loongarch/kernel/process.c
+++ b/arch/loongarch/kernel/process.c
@@ -88,6 +88,7 @@ void start_thread(struct pt_regs *regs, unsigned long pc, unsigned long sp)
 	clear_used_math();
 	regs->csr_era = pc;
 	regs->regs[3] = sp;
+	current_thread_info()->tp_value = 0;
 }
 
 void exit_thread(struct task_struct *tsk)
@@ -152,6 +153,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		childregs->csr_crmd = p->thread.csr_crmd;
 		childregs->csr_prmd = p->thread.csr_prmd;
 		childregs->csr_ecfg = p->thread.csr_ecfg;
+		task_thread_info(p)->tp_value = 0;
 		return 0;
 	}
 
@@ -176,8 +178,14 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	clear_tsk_thread_flag(p, TIF_LSX_CTX_LIVE);
 	clear_tsk_thread_flag(p, TIF_LASX_CTX_LIVE);
 
-	if (clone_flags & CLONE_SETTLS)
+	/*
+	 * record tls val for cloned threads
+	 * clear it for forked process
+	 */
+	if (clone_flags & CLONE_SETTLS) {
 		childregs->regs[2] = tls;
+		task_thread_info(p)->tp_value = tls;
+	}
 
 	return 0;
 }
diff --git a/arch/loongarch/kernel/signal.c b/arch/loongarch/kernel/signal.c
index 7f4889df4a17..2d88630e2a05 100644
--- a/arch/loongarch/kernel/signal.c
+++ b/arch/loongarch/kernel/signal.c
@@ -480,6 +480,9 @@ static int setup_rt_frame(void *sig_return, struct ksignal *ksig,
 	 *
 	 * c0_era point to the signal handler, $r3 (sp) points to
 	 * the struct rt_sigframe.
+	 *
+	 * Use recorded TP to signal handler if exists since $r2 may be
+	 * corrupted already.
 	 */
 	regs->regs[4] = ksig->sig;
 	regs->regs[5] = (unsigned long) &frame->rs_info;
@@ -487,6 +490,8 @@ static int setup_rt_frame(void *sig_return, struct ksignal *ksig,
 	regs->regs[3] = (unsigned long) frame;
 	regs->regs[1] = (unsigned long) sig_return;
 	regs->csr_era = (unsigned long) ksig->ka.sa.sa_handler;
+	if (current_thread_info()->tp_value)
+		regs->regs[2] = current_thread_info()->tp_value;
 
 	DEBUGP("SIG deliver (%s:%d): sp=0x%p pc=0x%lx ra=0x%lx\n",
 	       current->comm, current->pid,
diff --git a/arch/loongarch/kernel/syscall.c b/arch/loongarch/kernel/syscall.c
index 3fc4211db989..330d2aeadc02 100644
--- a/arch/loongarch/kernel/syscall.c
+++ b/arch/loongarch/kernel/syscall.c
@@ -29,6 +29,15 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len, unsigned long,
 	return ksys_mmap_pgoff(addr, len, prot, flags, fd, offset >> PAGE_SHIFT);
 }
 
+SYSCALL_DEFINE1(set_thread_area, unsigned long, addr)
+{
+	struct pt_regs     *regs = current_pt_regs();
+
+	regs->regs[2] = addr;
+	current_thread_info()->tp_value  = addr;
+	return 0;
+}
+
 void *sys_call_table[__NR_syscalls] = {
 	[0 ... __NR_syscalls - 1] = sys_ni_syscall,
 #include <asm/unistd.h>
-- 
2.31.1