Re: [PATCH v2 1/2] powerpc: Restore all registers after a traced syscall again

From: Bernhard M. Wiedemann <redacted>

syscall_exit_prepare() asks the exit assembly to reload all GPRs, CTR
and XER from pt_regs only if the thread flags contain any of the flags
in _TIF_SYSCALL_DOTRACE. With the new entry code based GENERIC_ENTRY the
flags are no longer set.

This breaks tracers that modify a stopped syscall, for example ones that
re-execute the syscall instruction by moving the NIP back by 4 at the
syscall-exit stop with the syscall number put back into r0: the sc then
runs with r0 = 0, which is __NR_restart_syscall. dettrace fails like
this with "No filter rule for system call: restart_syscall". A tracer
that writes r4 at the syscall-exit stop of an sc sees the tracee
continue with r4 = 0 instead; with a 7.1 kernel both work.

Check the equivalent syscall_work bits instead, and drop the now unused
_TIF_SYSCALL_DOTRACE.

Fixes: bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
Cc: stable@vger.kernel.org # 7.2.x: c7585b8e99ad: powerpc: Don't drop _TIF_RESTOREALL on syscall restart
Cc: stable@vger.kernel.org # 7.2.x
Assisted-by: LLM
Signed-off-by: Bernhard M. Wiedemann <redacted>
[ms: reword commit message]
Signed-off-by: Michal Suchánek <redacted>
---
A backport also needs commit c7585b8e99ad ("powerpc: Don't drop
_TIF_RESTOREALL on syscall restart"), without which the flag can still
be lost.

v2: rename PPC_SYSCALL_WORK_RESTOREALL to SYSCALL_WORK_RESTOREALL and
move it from interrupt.c to thread_info.h

v1: https://lore.kernel.org/linuxppc-dev/4c1da7f31a6603455ec452960af529e4d77f5334.1791826411.git.msuchanek@suse.de/ (local)

Testing: a small tracer (pokes r4 at the syscall-exit stop of an sc,
and replays a syscall by moving NIP back with a new r0) shows both
writes lost on a ppc64le and both working on 7.1.8.
------------[ cut here ]------------
// SPDX-License-Identifier: GPL-2.0
/*
 * Does a register a tracer writes at the syscall-exit stop of an "sc"
 * reach the tracee? Checks r4, and a replay: NIP moved back onto the sc
 * with r0 set to another syscall number.
 * Build: gcc -O2 -o ppc-exitregs ppc-exitregs.c (powerpc64 only)
 */

/* word offsets in struct pt_regs, arch/powerpc/include/uapi/asm/ptrace.h */

static long do_sc(long nr, long *r4io)
{
	register long r0 __asm__("r0") = nr;
	register long r3 __asm__("r3") = 0;
	register long r4 __asm__("r4") = *r4io;

	__asm__ volatile("sc"
			 : "+r"(r0), "+r"(r3), "+r"(r4)
			 :
			 : "r5", "r6", "r7", "r8", "r9", "r10", "r11", "r12",
			   "cr0", "ctr", "xer", "memory");
	*r4io = r4;
	return r3;
}

static long peek(pid_t pid, int w)
{
	errno = 0;
	long v = ptrace(PTRACE_PEEKUSER, pid, (void *)(w * sizeof(long)), 0);
	if (errno) {
		perror("PEEKUSER");
		exit(2);
	}
	return v;
}

static void poke(pid_t pid, int w, long v)
{
	if (ptrace(PTRACE_POKEUSER, pid, (void *)(w * sizeof(long)), (void *)v)) {
		perror("POKEUSER");
		exit(2);
	}
}

/* Resume to the next syscall stop, which must be one. */
static void next_stop(pid_t pid)
{
	int st;

	ptrace(PTRACE_SYSCALL, pid, 0, 0);
	if (waitpid(pid, &st, 0) != pid || !WIFSTOPPED(st) ||
	    WSTOPSIG(st) != (SIGTRAP | 0x80)) {
		fprintf(stderr, "unexpected wait status %#x\n", st);
		exit(2);
	}
}

int main(void)
{
	pid_t pid = fork();
	int st, fail = 0;

	if (pid == 0) {
		long r4 = 0x1111, r4a, ret;

		ptrace(PTRACE_TRACEME, 0, 0, 0);
		raise(SIGSTOP);
		do_sc(__NR_getpid, &r4);
		r4a = r4;
		ret = do_sc(__NR_getpid, &r4);
		/* bit 0: the tracer's r4 was lost; bit 1: the replay of
		 * the second getpid as getppid returned something else */
		_exit((r4a != 0x2222) | (ret != getppid()) << 1);
	}

	waitpid(pid, &st, 0);
	ptrace(PTRACE_SETOPTIONS, pid, 0, (void *)PTRACE_O_TRACESYSGOOD);

	/* 1: write r4 at the exit stop of getpid */
	next_stop(pid);				/* entry */
	next_stop(pid);				/* exit */
	poke(pid, W_R4, 0x2222);
	next_stop(pid);				/* entry of the second getpid */

	/* 2: replay the second getpid as getppid */
	next_stop(pid);				/* exit */
	poke(pid, W_NIP, peek(pid, W_NIP) - 4);
	poke(pid, W_R0, __NR_getppid);
	next_stop(pid);				/* entry of the replay */
	long nr = peek(pid, W_R0);
	printf("replayed syscall arrives as nr %ld (%s)\n", nr,
	       nr == __NR_getppid ? "getppid, OK" :
	       nr == __NR_restart_syscall ? "restart_syscall, BUG" : "?");
	if (nr != __NR_getppid) {
		fail = 1;
		poke(pid, W_R0, __NR_getppid);	/* entry writes do work */
	}
	next_stop(pid);				/* exit */
	ptrace(PTRACE_CONT, pid, 0, 0);
	waitpid(pid, &st, 0);
	if (!WIFEXITED(st)) {
		printf("child did not exit normally: %#x\n", st);
		return 2;
	}
	printf("r4 written at the exit stop: %s\n",
	       WEXITSTATUS(st) & 1 ? "lost, BUG" : "kept, OK");
	printf("replay result: %s\n",
	       WEXITSTATUS(st) & 2 ? "wrong" : "getppid(), OK");
	return fail || WEXITSTATUS(st);
}
---
 arch/powerpc/include/asm/thread_info.h | 15 ++++++++++++---
 arch/powerpc/kernel/interrupt.c        |  3 +--
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/thread_info.h b/arch/powerpc/include/asm/thread_info.h
index 1e069a2e7ce87..31a7a157bc902 100644
--- a/arch/powerpc/include/asm/thread_info.h
+++ b/arch/powerpc/include/asm/thread_info.h

@@ -142,9 +142,6 @@ void arch_setup_new_exec(void);
 #define _TIF_SYSCALL_TRACEPOINT	(1<<TIF_SYSCALL_TRACEPOINT)
 #define _TIF_EMULATE_STACK_STORE	(1<<TIF_EMULATE_STACK_STORE)
 #define _TIF_SYSCALL_EMU	(1<<TIF_SYSCALL_EMU)
-#define _TIF_SYSCALL_DOTRACE	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
-				 _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT | \
-				 _TIF_SYSCALL_EMU)
 
 #define _TIF_USER_WORK_MASK	(_TIF_SIGPENDING | _TIF_NEED_RESCHED | \
 				 _TIF_NEED_RESCHED_LAZY | _TIF_NOTIFY_RESUME | \

@@ -153,6 +150,18 @@ void arch_setup_new_exec(void);
 
 #define _TIF_PERSYSCALL_MASK	(_TIF_RESTOREALL|_TIF_NOERROR)
 
+/*
+ * Syscall work after which the exit must reload all registers from
+ * pt_regs: a tracer may have changed any of them at a syscall or seccomp
+ * stop, or under PTRACE_SYSEMU. Audit and the tracepoint are included to
+ * keep the pre-GENERIC_ENTRY behaviour.
+ */
+#define SYSCALL_WORK_RESTOREALL (SYSCALL_WORK_SECCOMP |			\
+				 SYSCALL_WORK_SYSCALL_TRACEPOINT |	\
+				 SYSCALL_WORK_SYSCALL_TRACE |		\
+				 SYSCALL_WORK_SYSCALL_EMU |		\
+				 SYSCALL_WORK_SYSCALL_AUDIT)
+
 /* Bits in local_flags */
 /* Don't move TLF_NAPPING without adjusting the code in entry_32.S */
 #define TLF_NAPPING		0	/* idle thread enabled NAP mode */

diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index 55f9c0c9922ac..d6a7b6d2b4bd2 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c

@@ -117,9 +117,8 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
 		regs->gpr[3] = r3;
 	}
 
-	if (unlikely(ti_flags & _TIF_SYSCALL_DOTRACE)) {
+	if (unlikely(READ_ONCE(current_thread_info()->syscall_work) & SYSCALL_WORK_RESTOREALL))
 		ret |= _TIF_RESTOREALL;
-	}
 
 	syscall_exit_to_user_mode(regs);
 

-- 
2.51.0